# -*- coding: utf-8 -*-
# Copyright (C) 2012 Anaconda, Inc
# SPDX-License-Identifier: BSD-3-Clause
from __future__ import absolute_import, division, print_function, unicode_literals

from collections import OrderedDict

from errno import ENOENT
from logging import getLogger
import os
from os.path import abspath, basename, expanduser, isdir, isfile, join, split as path_split
import platform
import sys
import struct

from .constants import (APP_NAME, ChannelPriority, DEFAULTS_CHANNEL_NAME, REPODATA_FN,
                        DEFAULT_AGGRESSIVE_UPDATE_PACKAGES, DEFAULT_CHANNELS,
                        DEFAULT_CHANNEL_ALIAS, DEFAULT_CUSTOM_CHANNELS, DepsModifier,
                        ERROR_UPLOAD_URL, KNOWN_SUBDIRS, PREFIX_MAGIC_FILE, PathConflict,
                        ROOT_ENV_NAME, SEARCH_PATH, SafetyChecks, SatSolverChoice, UpdateModifier)
from .. import __version__ as CONDA_VERSION
from .._vendor.appdirs import user_data_dir
from .._vendor.auxlib.decorators import memoize, memoizedproperty
from .._vendor.auxlib.ish import dals
from .._vendor.boltons.setutils import IndexedSet
from .._vendor.frozendict import frozendict
from .._vendor.toolz import concat, concatv, unique
from ..common.compat import NoneType, iteritems, itervalues, odict, on_win, string_types
from ..common.configuration import (Configuration, ConfigurationLoadError, MapParameter,
                                    ParameterLoader, PrimitiveParameter, SequenceParameter,
                                    ValidationError)
from ..common._os.linux import linux_get_libc_version
from ..common.path import expand, paths_equal
from ..common.url import has_scheme, path_to_url, split_scheme_auth_token
from ..common.decorators import env_override

from .. import CONDA_PACKAGE_ROOT

try:
    os.getcwd()
except (IOError, OSError) as e:
    if e.errno == ENOENT:
        # FileNotFoundError can occur when cwd has been deleted out from underneath the process.
        # To resolve #6584, let's go with setting cwd to sys.prefix, and see how far we get.
        os.chdir(sys.prefix)
    else:
        raise

log = getLogger(__name__)

_platform_map = {
    'linux2': 'linux',
    'linux': 'linux',
    'darwin': 'osx',
    'win32': 'win',
    'zos': 'zos',
}
non_x86_linux_machines = {
    'armv6l',
    'armv7l',
    'aarch64',
    'ppc64',
    'ppc64le',
    's390x',
}
_arch_names = {
    32: 'x86',
    64: 'x86_64',
}

user_rc_path = abspath(expanduser('~/.condarc'))
sys_rc_path = join(sys.prefix, '.condarc')


def mockable_context_envs_dirs(root_writable, root_prefix, _envs_dirs):
    if root_writable:
        fixed_dirs = (
            join(root_prefix, 'envs'),
            join('~', '.conda', 'envs'),
        )
    else:
        fixed_dirs = (
            join('~', '.conda', 'envs'),
            join(root_prefix, 'envs'),
        )
    if on_win:
        fixed_dirs += join(user_data_dir(APP_NAME, APP_NAME), 'envs'),
    return tuple(IndexedSet(expand(p) for p in concatv(_envs_dirs, fixed_dirs)))


def channel_alias_validation(value):
    if value and not has_scheme(value):
        return "channel_alias value '%s' must have scheme/protocol." % value
    return True


def default_python_default():
    ver = sys.version_info
    return '%d.%d' % (ver.major, ver.minor)


def default_python_validation(value):
    if value:
        if len(value) == 3 and value[1] == '.':
            try:
                value = float(value)
                if 2.0 <= value < 4.0:
                    return True
            except ValueError:  # pragma: no cover
                pass
    else:
        # Set to None or '' meaning no python pinning
        return True

    return "default_python value '%s' not of the form '[23].[0-9]' or ''" % value


def ssl_verify_validation(value):
    if isinstance(value, string_types):
        if not isfile(value) and not isdir(value):
            return ("ssl_verify value '%s' must be a boolean, a path to a "
                    "certificate bundle file, or a path to a directory containing "
                    "certificates of trusted CAs." % value)
    return True


class Context(Configuration):

    add_pip_as_python_dependency = ParameterLoader(PrimitiveParameter(True))
    allow_conda_downgrades = ParameterLoader(PrimitiveParameter(False))
    # allow cyclical dependencies, or raise
    allow_cycles = ParameterLoader(PrimitiveParameter(True))
    allow_softlinks = ParameterLoader(PrimitiveParameter(False))
    auto_update_conda = ParameterLoader(PrimitiveParameter(True), aliases=('self_update',))
    auto_activate_base = ParameterLoader(PrimitiveParameter(True))
    auto_stack = ParameterLoader(PrimitiveParameter(0))
    notify_outdated_conda = ParameterLoader(PrimitiveParameter(True))
    clobber = ParameterLoader(PrimitiveParameter(False))
    changeps1 = ParameterLoader(PrimitiveParameter(True))
    env_prompt = ParameterLoader(PrimitiveParameter("({default_env}) "))
    create_default_packages = ParameterLoader(
        SequenceParameter(PrimitiveParameter("", element_type=string_types)))
    default_python = ParameterLoader(
        PrimitiveParameter(default_python_default(),
                           element_type=string_types + (NoneType,),
                           validation=default_python_validation))
    download_only = ParameterLoader(PrimitiveParameter(False))
    enable_private_envs = ParameterLoader(PrimitiveParameter(False))
    force_32bit = ParameterLoader(PrimitiveParameter(False))
    non_admin_enabled = ParameterLoader(PrimitiveParameter(True))

    pip_interop_enabled = ParameterLoader(PrimitiveParameter(False))

    # multithreading in various places
    _default_threads = ParameterLoader(PrimitiveParameter(0, element_type=int),
                                       aliases=('default_threads',))
    _repodata_threads = ParameterLoader(PrimitiveParameter(0, element_type=int),
                                        aliases=('repodata_threads',))
    _verify_threads = ParameterLoader(PrimitiveParameter(0, element_type=int),
                                      aliases=('verify_threads',))
    # this one actually defaults to 1 - that is handled in the property below
    _execute_threads = ParameterLoader(PrimitiveParameter(0, element_type=int),
                                       aliases=('execute_threads',))

    # Safety & Security
    _aggressive_update_packages = ParameterLoader(
        SequenceParameter(
            PrimitiveParameter("", element_type=string_types),
            DEFAULT_AGGRESSIVE_UPDATE_PACKAGES),
        aliases=('aggressive_update_packages',))
    safety_checks = ParameterLoader(PrimitiveParameter(SafetyChecks.warn))
    extra_safety_checks = ParameterLoader(PrimitiveParameter(False))
    path_conflict = ParameterLoader(PrimitiveParameter(PathConflict.clobber))

    pinned_packages = ParameterLoader(SequenceParameter(
        PrimitiveParameter("", element_type=string_types),
        string_delimiter='&'))  # TODO: consider a different string delimiter  # NOQA
    disallowed_packages = ParameterLoader(
        SequenceParameter(
            PrimitiveParameter("", element_type=string_types), string_delimiter='&'),
        aliases=('disallow',))
    rollback_enabled = ParameterLoader(PrimitiveParameter(True))
    track_features = ParameterLoader(
        SequenceParameter(PrimitiveParameter("", element_type=string_types)))
    use_index_cache = ParameterLoader(PrimitiveParameter(False))

    separate_format_cache = ParameterLoader(PrimitiveParameter(False))

    _root_prefix = ParameterLoader(PrimitiveParameter(""), aliases=('root_dir', 'root_prefix'))
    _envs_dirs = ParameterLoader(
        SequenceParameter(PrimitiveParameter("", element_type=string_types),
                          string_delimiter=os.pathsep),
        aliases=('envs_dirs', 'envs_path'),
        expandvars=True)
    _pkgs_dirs = ParameterLoader(SequenceParameter(PrimitiveParameter("", string_types)),
                                 aliases=('pkgs_dirs',),
                                 expandvars=True)
    _subdir = ParameterLoader(PrimitiveParameter(''), aliases=('subdir',))
    _subdirs = ParameterLoader(
        SequenceParameter(PrimitiveParameter("", string_types)), aliases=('subdirs',))

    local_repodata_ttl = ParameterLoader(PrimitiveParameter(1, element_type=(bool, int)))
    # number of seconds to cache repodata locally
    #   True/1: respect Cache-Control max-age header
    #   False/0: always fetch remote repodata (HTTP 304 responses respected)

    # remote connection details
    ssl_verify = ParameterLoader(
        PrimitiveParameter(True,
                           element_type=string_types + (bool,),
                           validation=ssl_verify_validation),
        aliases=('verify_ssl',),
        expandvars=True)
    client_ssl_cert = ParameterLoader(
        PrimitiveParameter(None, element_type=string_types + (NoneType,)),
        aliases=('client_cert',),
        expandvars=True)
    client_ssl_cert_key = ParameterLoader(
        PrimitiveParameter(None, element_type=string_types + (NoneType,)),
        aliases=('client_cert_key',),
        expandvars=True)
    proxy_servers = ParameterLoader(
        MapParameter(PrimitiveParameter(None, string_types + (NoneType,))),
        expandvars=True)
    remote_connect_timeout_secs = ParameterLoader(PrimitiveParameter(9.15))
    remote_read_timeout_secs = ParameterLoader(PrimitiveParameter(60.))
    remote_max_retries = ParameterLoader(PrimitiveParameter(3))
    remote_backoff_factor = ParameterLoader(PrimitiveParameter(1))

    add_anaconda_token = ParameterLoader(PrimitiveParameter(True), aliases=('add_binstar_token',))

    # #############################
    # channels
    # #############################
    allow_non_channel_urls = ParameterLoader(PrimitiveParameter(False))
    _channel_alias = ParameterLoader(
        PrimitiveParameter(DEFAULT_CHANNEL_ALIAS,
                           validation=channel_alias_validation),
        aliases=('channel_alias',))
    channel_priority = ParameterLoader(PrimitiveParameter(ChannelPriority.FLEXIBLE))
    _channels = ParameterLoader(
        SequenceParameter(PrimitiveParameter(
            "", element_type=string_types), default=(DEFAULTS_CHANNEL_NAME,)),
        aliases=('channels', 'channel',),
        expandvars=True)  # channel for args.channel
    _custom_channels = ParameterLoader(
        MapParameter(PrimitiveParameter("", element_type=string_types), DEFAULT_CUSTOM_CHANNELS),
        aliases=('custom_channels',),
        expandvars=True)
    _custom_multichannels = ParameterLoader(
        MapParameter(SequenceParameter(PrimitiveParameter("", element_type=string_types))),
        aliases=('custom_multichannels',),
        expandvars=True)
    _default_channels = ParameterLoader(
        SequenceParameter(PrimitiveParameter("", element_type=string_types), DEFAULT_CHANNELS),
        aliases=('default_channels',),
        expandvars=True)
    _migrated_channel_aliases = ParameterLoader(
        SequenceParameter(PrimitiveParameter("", element_type=string_types)),
        aliases=('migrated_channel_aliases',))
    migrated_custom_channels = ParameterLoader(
        MapParameter(PrimitiveParameter("", element_type=string_types)),
        expandvars=True)  # TODO: also take a list of strings
    override_channels_enabled = ParameterLoader(PrimitiveParameter(True))
    show_channel_urls = ParameterLoader(PrimitiveParameter(None, element_type=(bool, NoneType)))
    use_local = ParameterLoader(PrimitiveParameter(False))
    whitelist_channels = ParameterLoader(
        SequenceParameter(PrimitiveParameter("", element_type=string_types)),
        expandvars=True)
    restore_free_channel = ParameterLoader(PrimitiveParameter(False))
    repodata_fns = ParameterLoader(
        SequenceParameter(
            PrimitiveParameter("", element_type=string_types),
            ("current_repodata.json", REPODATA_FN)))
    _use_only_tar_bz2 = ParameterLoader(PrimitiveParameter(None, element_type=(bool, NoneType)),
                                        aliases=('use_only_tar_bz2',))

    always_softlink = ParameterLoader(PrimitiveParameter(False), aliases=('softlink',))
    always_copy = ParameterLoader(PrimitiveParameter(False), aliases=('copy',))
    always_yes = ParameterLoader(
        PrimitiveParameter(None, element_type=(bool, NoneType)), aliases=('yes',))
    debug = ParameterLoader(PrimitiveParameter(False))
    dev = ParameterLoader(PrimitiveParameter(False))
    dry_run = ParameterLoader(PrimitiveParameter(False))
    error_upload_url = ParameterLoader(PrimitiveParameter(ERROR_UPLOAD_URL))
    force = ParameterLoader(PrimitiveParameter(False))
    json = ParameterLoader(PrimitiveParameter(False))
    offline = ParameterLoader(PrimitiveParameter(False))
    quiet = ParameterLoader(PrimitiveParameter(False))
    ignore_pinned = ParameterLoader(PrimitiveParameter(False))
    report_errors = ParameterLoader(PrimitiveParameter(None, element_type=(bool, NoneType)))
    shortcuts = ParameterLoader(PrimitiveParameter(True))
    _verbosity = ParameterLoader(
        PrimitiveParameter(0, element_type=int), aliases=('verbose', 'verbosity'))

    # ######################################################
    # ##               Solver Configuration               ##
    # ######################################################
    deps_modifier = ParameterLoader(PrimitiveParameter(DepsModifier.NOT_SET))
    update_modifier = ParameterLoader(PrimitiveParameter(UpdateModifier.UPDATE_SPECS))
    sat_solver = ParameterLoader(PrimitiveParameter(SatSolverChoice.PYCOSAT))
    solver_ignore_timestamps = ParameterLoader(PrimitiveParameter(False))

    # # CLI-only
    # no_deps = ParameterLoader(PrimitiveParameter(NULL, element_type=(type(NULL), bool)))
    # # CLI-only
    # only_deps = ParameterLoader(PrimitiveParameter(NULL, element_type=(type(NULL), bool)))
    #
    # freeze_installed = ParameterLoader(PrimitiveParameter(False))
    # update_deps = ParameterLoader(PrimitiveParameter(False), aliases=('update_dependencies',))
    # update_specs = ParameterLoader(PrimitiveParameter(False))
    # update_all = ParameterLoader(PrimitiveParameter(False))

    force_remove = ParameterLoader(PrimitiveParameter(False))
    force_reinstall = ParameterLoader(PrimitiveParameter(False))

    target_prefix_override = ParameterLoader(PrimitiveParameter(''))

    unsatisfiable_hints = ParameterLoader(PrimitiveParameter(True))
    unsatisfiable_hints_check_depth = ParameterLoader(PrimitiveParameter(2))

    # conda_build
    bld_path = ParameterLoader(PrimitiveParameter(''))
    anaconda_upload = ParameterLoader(
        PrimitiveParameter(None, element_type=(bool, NoneType)), aliases=('binstar_upload',))
    _croot = ParameterLoader(PrimitiveParameter(''), aliases=('croot',))
    _conda_build = ParameterLoader(
        MapParameter(PrimitiveParameter("", element_type=string_types)),
        aliases=('conda-build', 'conda_build'))

    def __init__(self, search_path=None, argparse_args=None):
        if search_path is None:
            search_path = SEARCH_PATH

        if argparse_args:
            # This block of code sets CONDA_PREFIX based on '-n' and '-p' flags, so that
            # configuration can be properly loaded from those locations
            func_name = ('func' in argparse_args and argparse_args.func or '').rsplit('.', 1)[-1]
            if func_name in ('create', 'install', 'update', 'remove', 'uninstall', 'upgrade'):
                if 'prefix' in argparse_args and argparse_args.prefix:
                    os.environ['CONDA_PREFIX'] = argparse_args.prefix
                elif 'name' in argparse_args and argparse_args.name:
                    # Currently, usage of the '-n' flag is inefficient, with all configuration
                    # files being loaded/re-loaded at least two times.
                    target_prefix = determine_target_prefix(context, argparse_args)
                    if target_prefix != context.root_prefix:
                        os.environ['CONDA_PREFIX'] = determine_target_prefix(context,
                                                                             argparse_args)

        super(Context, self).__init__(search_path=search_path, app_name=APP_NAME,
                                      argparse_args=argparse_args)

    def post_build_validation(self):
        errors = []
        if self.client_ssl_cert_key and not self.client_ssl_cert:
            error = ValidationError('client_ssl_cert', self.client_ssl_cert, "<<merged>>",
                                    "'client_ssl_cert' is required when 'client_ssl_cert_key' "
                                    "is defined")
            errors.append(error)
        if self.always_copy and self.always_softlink:
            error = ValidationError('always_copy', self.always_copy, "<<merged>>",
                                    "'always_copy' and 'always_softlink' are mutually exclusive. "
                                    "Only one can be set to 'True'.")
            errors.append(error)
        return errors

    @property
    def conda_build_local_paths(self):
        # does file system reads to make sure paths actually exist
        return tuple(unique(full_path for full_path in (
            expand(d) for d in (
                self._croot,
                self.bld_path,
                self.conda_build.get('root-dir'),
                join(self.root_prefix, 'conda-bld'),
                '~/conda-bld',
            ) if d
        ) if isdir(full_path)))

    @property
    def conda_build_local_urls(self):
        return tuple(path_to_url(p) for p in self.conda_build_local_paths)

    @property
    def croot(self):
        """This is where source caches and work folders live"""
        if self._croot:
            return abspath(expanduser(self._croot))
        elif self.bld_path:
            return abspath(expanduser(self.bld_path))
        elif 'root-dir' in self.conda_build:
            return abspath(expanduser(self.conda_build['root-dir']))
        elif self.root_writable:
            return join(self.root_prefix, 'conda-bld')
        else:
            return expand('~/conda-bld')

    @property
    def local_build_root(self):
        return self.croot

    @property
    def conda_build(self):
        # conda-build needs its config map to be mutable
        try:
            return self.__conda_build
        except AttributeError:
            self.__conda_build = __conda_build = dict(self._conda_build)
            return __conda_build

    @property
    def arch_name(self):
        m = platform.machine()
        if self.platform == 'linux' and m in non_x86_linux_machines:
            return m
        else:
            return _arch_names[self.bits]

    @property
    def conda_private(self):
        return conda_in_private_env()

    @property
    def platform(self):
        return _platform_map.get(sys.platform, 'unknown')

    @property
    def default_threads(self):
        return self._default_threads if self._default_threads else None

    @property
    def repodata_threads(self):
        return self._repodata_threads if self._repodata_threads else self.default_threads

    @property
    def verify_threads(self):
        if self._verify_threads:
            threads = self._verify_threads
        elif self.default_threads:
            threads = self.default_threads
        else:
            threads = 1
        return threads

    @property
    def execute_threads(self):
        if self._execute_threads:
            threads = self._execute_threads
        elif self.default_threads:
            threads = self.default_threads
        else:
            threads = 1
        return threads

    @property
    def subdir(self):
        if self._subdir:
            return self._subdir
        m = platform.machine()
        if m in non_x86_linux_machines:
            return 'linux-%s' % m
        elif self.platform == 'zos':
            return 'zos-z'
        else:
            return '%s-%d' % (self.platform, self.bits)

    @property
    def subdirs(self):
        return self._subdirs if self._subdirs else (self.subdir, 'noarch')

    @memoizedproperty
    def known_subdirs(self):
        return frozenset(concatv(KNOWN_SUBDIRS, self.subdirs))

    @property
    def bits(self):
        if self.force_32bit:
            return 32
        else:
            return 8 * struct.calcsize("P")

    @property
    def root_dir(self):
        # root_dir is an alias for root_prefix, we prefer the name "root_prefix"
        # because it is more consistent with other names
        return self.root_prefix

    @property
    def root_writable(self):
        # rather than using conda.gateways.disk.test.prefix_is_writable
        # let's shortcut and assume the root prefix exists
        path = join(self.root_prefix, PREFIX_MAGIC_FILE)
        if isfile(path):
            try:
                fh = open(path, 'a+')
            except (IOError, OSError) as e:
                log.debug(e)
                return False
            else:
                fh.close()
                return True
        return False

    @property
    def envs_dirs(self):
        return mockable_context_envs_dirs(self.root_writable, self.root_prefix, self._envs_dirs)

    @property
    def pkgs_dirs(self):
        if self._pkgs_dirs:
            return tuple(IndexedSet(expand(p) for p in self._pkgs_dirs))
        else:
            cache_dir_name = 'pkgs32' if context.force_32bit else 'pkgs'
            fixed_dirs = (
                self.root_prefix,
                join('~', '.conda'),
            )
            if on_win:
                fixed_dirs += user_data_dir(APP_NAME, APP_NAME),
            return tuple(IndexedSet(expand(join(p, cache_dir_name)) for p in (fixed_dirs)))

    @memoizedproperty
    def trash_dir(self):
        # TODO: this inline import can be cleaned up by moving pkgs_dir write detection logic
        from ..core.package_cache_data import PackageCacheData
        pkgs_dir = PackageCacheData.first_writable().pkgs_dir
        trash_dir = join(pkgs_dir, '.trash')
        from ..gateways.disk.create import mkdir_p
        mkdir_p(trash_dir)
        return trash_dir

    @property
    def default_prefix(self):
        if self.active_prefix:
            return self.active_prefix
        _default_env = os.getenv('CONDA_DEFAULT_ENV')
        if _default_env in (None, ROOT_ENV_NAME, 'root'):
            return self.root_prefix
        elif os.sep in _default_env:
            return abspath(_default_env)
        else:
            for envs_dir in self.envs_dirs:
                default_prefix = join(envs_dir, _default_env)
                if isdir(default_prefix):
                    return default_prefix
        return join(self.envs_dirs[0], _default_env)

    @property
    def active_prefix(self):
        return os.getenv('CONDA_PREFIX')

    @property
    def shlvl(self):
        return int(os.getenv('CONDA_SHLVL', -1))

    @property
    def aggressive_update_packages(self):
        from ..models.match_spec import MatchSpec
        return tuple(MatchSpec(s) for s in self._aggressive_update_packages)

    @property
    def target_prefix(self):
        # used for the prefix that is the target of the command currently being executed
        # different from the active prefix, which is sometimes given by -p or -n command line flags
        return determine_target_prefix(self)

    @memoizedproperty
    def root_prefix(self):
        if self._root_prefix:
            return abspath(expanduser(self._root_prefix))
        elif conda_in_private_env():
            return abspath(join(self.conda_prefix, '..', '..'))
        else:
            return self.conda_prefix

    @property
    def conda_prefix(self):
        return abspath(sys.prefix)

    @property
    # This is deprecated, please use conda_exe_vars_dict instead.
    def conda_exe(self):
        bin_dir = 'Scripts' if on_win else 'bin'
        exe = 'conda.exe' if on_win else 'conda'
        return join(self.conda_prefix, bin_dir, exe)

    @property
    def conda_exe_vars_dict(self):
        '''
        An OrderedDict so the vars can refer to each other if necessary.
        None means unset it.
        '''

        if context.dev:
            return OrderedDict([('CONDA_EXE', sys.executable),
                                ('PYTHONPATH', os.path.dirname(CONDA_PACKAGE_ROOT) + '{}{}'.format(
                                    os.pathsep, os.environ.get('PYTHONPATH', ''))),
                                ('_CE_M', '-m'),
                                ('_CE_CONDA', 'conda'),
                                ('CONDA_PYTHON_EXE', sys.executable)])
        else:
            bin_dir = 'Scripts' if on_win else 'bin'
            exe = 'conda.exe' if on_win else 'conda'
            # I was going to use None to indicate a variable to unset, but that gets tricky with
            # error-on-undefined.
            return OrderedDict([('CONDA_EXE', os.path.join(sys.prefix, bin_dir, exe)),
                                ('_CE_M', ''),
                                ('_CE_CONDA', ''),
                                ('CONDA_PYTHON_EXE', sys.executable)])

    @memoizedproperty
    def channel_alias(self):
        from ..models.channel import Channel
        location, scheme, auth, token = split_scheme_auth_token(self._channel_alias)
        return Channel(scheme=scheme, auth=auth, location=location, token=token)

    @property
    def migrated_channel_aliases(self):
        from ..models.channel import Channel
        return tuple(Channel(scheme=scheme, auth=auth, location=location, token=token)
                     for location, scheme, auth, token in
                     (split_scheme_auth_token(c) for c in self._migrated_channel_aliases))

    @property
    def prefix_specified(self):
        return (self._argparse_args.get("prefix") is not None
                or self._argparse_args.get("name") is not None)

    @memoizedproperty
    def default_channels(self):
        # the format for 'default_channels' is a list of strings that either
        #   - start with a scheme
        #   - are meant to be prepended with channel_alias
        return self.custom_multichannels[DEFAULTS_CHANNEL_NAME]

    @memoizedproperty
    def custom_multichannels(self):
        from ..models.channel import Channel

        default_channels = list(self._default_channels)
        if self.restore_free_channel:
            default_channels.insert(1, 'https://repo.anaconda.com/pkgs/free')

        reserved_multichannel_urls = odict((
            (DEFAULTS_CHANNEL_NAME, default_channels),
            ('local', self.conda_build_local_urls),
        ))
        reserved_multichannels = odict(
            (name, tuple(
                Channel.make_simple_channel(self.channel_alias, url) for url in urls)
             ) for name, urls in iteritems(reserved_multichannel_urls)
        )
        custom_multichannels = odict(
            (name, tuple(
                Channel.make_simple_channel(self.channel_alias, url) for url in urls)
             ) for name, urls in iteritems(self._custom_multichannels)
        )
        all_multichannels = odict(
            (name, channels)
            for name, channels in concat(map(iteritems, (
                custom_multichannels,
                reserved_multichannels,  # reserved comes last, so reserved overrides custom
            )))
        )
        return all_multichannels

    @memoizedproperty
    def custom_channels(self):
        from ..models.channel import Channel
        custom_channels = (Channel.make_simple_channel(self.channel_alias, url, name)
                           for name, url in iteritems(self._custom_channels))
        channels_from_multichannels = concat(channel for channel
                                             in itervalues(self.custom_multichannels))
        all_channels = odict((x.name, x) for x in (ch for ch in concatv(
            channels_from_multichannels,
            custom_channels,
        )))
        return all_channels

    @property
    def channels(self):
        local_add = ('local',) if self.use_local else ()
        if (self._argparse_args and 'override_channels' in self._argparse_args
                and self._argparse_args['override_channels']):
            if not self.override_channels_enabled:
                from ..exceptions import OperationNotAllowed
                raise OperationNotAllowed(dals("""
                Overriding channels has been disabled.
                """))
            elif not (self._argparse_args and 'channel' in self._argparse_args
                      and self._argparse_args['channel']):
                from ..exceptions import CommandArgumentError
                raise CommandArgumentError(dals("""
                At least one -c / --channel flag must be supplied when using --override-channels.
                """))
            else:
                return tuple(IndexedSet(concatv(local_add, self._argparse_args['channel'])))

        # add 'defaults' channel when necessary if --channel is given via the command line
        if self._argparse_args and 'channel' in self._argparse_args:
            # TODO: it's args.channel right now, not channels
            argparse_channels = tuple(self._argparse_args['channel'] or ())
            if argparse_channels and argparse_channels == self._channels:
                return tuple(IndexedSet(concatv(local_add, argparse_channels,
                                                (DEFAULTS_CHANNEL_NAME,))))
        return tuple(IndexedSet(concatv(local_add, self._channels)))

    @property
    def use_only_tar_bz2(self):
        from ..models.version import VersionOrder
        # we avoid importing this at the top to avoid PATH issues.  Ensure that this
        #    is only called when use_only_tar_bz2 is first called.
        import conda_package_handling.api
        use_only_tar_bz2 = False
        if self._use_only_tar_bz2 is None:
            try:
                import conda_build
                use_only_tar_bz2 = VersionOrder(conda_build.__version__) < VersionOrder("3.18.3")

            except ImportError:
                pass
            if self._argparse_args and 'use_only_tar_bz2' in self._argparse_args:
                use_only_tar_bz2 &= self._argparse_args['use_only_tar_bz2']
        return ((hasattr(conda_package_handling.api, 'libarchive_enabled') and
                 not conda_package_handling.api.libarchive_enabled) or
                self._use_only_tar_bz2 or
                use_only_tar_bz2)

    @property
    def binstar_upload(self):
        # backward compatibility for conda-build
        return self.anaconda_upload

    @property
    def verbosity(self):
        return 2 if self.debug else self._verbosity

    @memoizedproperty
    def user_agent(self):
        builder = ["conda/%s requests/%s" % (CONDA_VERSION, self.requests_version)]
        builder.append("%s/%s" % self.python_implementation_name_version)
        builder.append("%s/%s" % self.platform_system_release)
        builder.append("%s/%s" % self.os_distribution_name_version)
        if self.libc_family_version[0]:
            builder.append("%s/%s" % self.libc_family_version)
        return " ".join(builder)

    @memoizedproperty
    def requests_version(self):
        try:
            from requests import __version__ as REQUESTS_VERSION
        except ImportError:  # pragma: no cover
            try:
                from pip._vendor.requests import __version__ as REQUESTS_VERSION
            except ImportError:
                REQUESTS_VERSION = "unknown"
        return REQUESTS_VERSION

    @memoizedproperty
    def python_implementation_name_version(self):
        # CPython, Jython
        # '2.7.14'
        return platform.python_implementation(), platform.python_version()

    @memoizedproperty
    def platform_system_release(self):
        # tuple of system name and release version
        #
        # `uname -s` Linux, Windows, Darwin, Java
        #
        # `uname -r`
        # '17.4.0' for macOS
        # '10' or 'NT' for Windows
        return platform.system(), platform.release()

    @memoizedproperty
    def os_distribution_name_version(self):
        # tuple of os distribution name and version
        # e.g.
        #   'debian', '9'
        #   'OSX', '10.13.6'
        #   'Windows', '10.0.17134'
        platform_name = self.platform_system_release[0]
        if platform_name == 'Linux':
            from .._vendor.distro import id, version
            try:
                distinfo = id(), version(best=True)
            except Exception as e:
                log.debug('%r', e, exc_info=True)
                distinfo = ('Linux', 'unknown')
            distribution_name, distribution_version = distinfo[0], distinfo[1]
        elif platform_name == 'Darwin':
            distribution_name = 'OSX'
            distribution_version = platform.mac_ver()[0]
        else:
            distribution_name = platform.system()
            distribution_version = platform.version()
        return distribution_name, distribution_version

    @memoizedproperty
    def libc_family_version(self):
        # tuple of lic_family and libc_version
        # None, None if not on Linux
        libc_family, libc_version = linux_get_libc_version()
        return libc_family, libc_version

    @memoizedproperty
    def cpu_flags(self):
        # DANGER: This is rather slow
        info = _get_cpu_info()
        return info['flags']

    @memoizedproperty
    @env_override('CONDA_OVERRIDE_CUDA', convert_empty_to_none=True)
    def cuda_version(self):
        from conda.common.cuda import cuda_detect
        return cuda_detect()

    @property
    def category_map(self):
        return odict((
            ('Channel Configuration', (
                'channels',
                'channel_alias',
                'default_channels',
                'override_channels_enabled',
                'whitelist_channels',
                'custom_channels',
                'custom_multichannels',
                'migrated_channel_aliases',
                'migrated_custom_channels',
                'add_anaconda_token',
                'allow_non_channel_urls',
                'restore_free_channel',
                'repodata_fns',
                'use_only_tar_bz2',
                'repodata_threads',
            )),
            ('Basic Conda Configuration', (  # TODO: Is there a better category name here?
                'envs_dirs',
                'pkgs_dirs',
                'default_threads',
            )),
            ('Network Configuration', (
                'client_ssl_cert',
                'client_ssl_cert_key',
                'local_repodata_ttl',
                'offline',
                'proxy_servers',
                'remote_connect_timeout_secs',
                'remote_max_retries',
                'remote_backoff_factor',
                'remote_read_timeout_secs',
                'ssl_verify',
            )),
            ('Solver Configuration', (
                'aggressive_update_packages',
                'auto_update_conda',
                'channel_priority',
                'create_default_packages',
                'disallowed_packages',
                'force_reinstall',
                'pinned_packages',
                'pip_interop_enabled',
                'track_features',
            )),
            ('Package Linking and Install-time Configuration', (
                'allow_softlinks',
                'always_copy',
                'always_softlink',
                'path_conflict',
                'rollback_enabled',
                'safety_checks',
                'extra_safety_checks',
                'shortcuts',
                'non_admin_enabled',
                'separate_format_cache',
                'verify_threads',
                'execute_threads',
            )),
            ('Conda-build Configuration', (
                'bld_path',
                'croot',
                'anaconda_upload',
                'conda_build',
            )),
            ('Output, Prompt, and Flow Control Configuration', (
                'always_yes',
                'auto_activate_base',
                'auto_stack',
                'changeps1',
                'env_prompt',
                'json',
                'notify_outdated_conda',
                'quiet',
                'report_errors',
                'show_channel_urls',
                'verbosity',
                'unsatisfiable_hints',
                'unsatisfiable_hints_check_depth'
            )),
            ('CLI-only', (
                'deps_modifier',
                'update_modifier',

                'force',
                'force_remove',
                'clobber',

                'dry_run',
                'download_only',
                'ignore_pinned',
                'use_index_cache',
                'use_local',
            )),
            ('Hidden and Undocumented', (
                'allow_cycles',  # allow cyclical dependencies, or raise
                'allow_conda_downgrades',
                'add_pip_as_python_dependency',
                'debug',
                'dev',
                'default_python',
                'enable_private_envs',
                'error_upload_url',  # should remain undocumented
                'force_32bit',
                'root_prefix',
                'sat_solver',
                'solver_ignore_timestamps',
                'subdir',
                'subdirs',
                # https://conda.io/docs/config.html#disable-updating-of-dependencies-update-dependencies # NOQA
                # I don't think this documentation is correct any longer. # NOQA
                'target_prefix_override',
                # used to override prefix rewriting, for e.g. building docker containers or RPMs  # NOQA
            )),
        ))

    def get_descriptions(self):
        return self.description_map

    @memoizedproperty
    def description_map(self):
        return frozendict({
            'add_anaconda_token': dals("""
                In conjunction with the anaconda command-line client (installed with
                `conda install anaconda-client`), and following logging into an Anaconda
                Server API site using `anaconda login`, automatically apply a matching
                private token to enable access to private packages and channels.
                """),
            # 'add_pip_as_python_dependency': dals("""
            #     Add pip, wheel and setuptools as dependencies of python. This ensures pip,
            #     wheel and setuptools will always be installed any time python is installed.
            #     """),
            'aggressive_update_packages': dals("""
                A list of packages that, if installed, are always updated to the latest possible
                version.
                """),
            'allow_non_channel_urls': dals("""
                Warn, but do not fail, when conda detects a channel url is not a valid channel.
                """),
            'allow_softlinks': dals("""
                When allow_softlinks is True, conda uses hard-links when possible, and soft-links
                (symlinks) when hard-links are not possible, such as when installing on a
                different filesystem than the one that the package cache is on. When
                allow_softlinks is False, conda still uses hard-links when possible, but when it
                is not possible, conda copies files. Individual packages can override
                this setting, specifying that certain files should never be soft-linked (see the
                no_link option in the build recipe documentation).
                """),
            'always_copy': dals("""
                Register a preference that files be copied into a prefix during install rather
                than hard-linked.
                """),
            'always_softlink': dals("""
                Register a preference that files be soft-linked (symlinked) into a prefix during
                install rather than hard-linked. The link source is the 'pkgs_dir' package cache
                from where the package is being linked. WARNING: Using this option can result in
                corruption of long-lived conda environments. Package caches are *caches*, which
                means there is some churn and invalidation. With this option, the contents of
                environments can be switched out (or erased) via operations on other environments.
                """),
            'always_yes': dals("""
                Automatically choose the 'yes' option whenever asked to proceed with a conda
                operation, such as when running `conda install`.
                """),
            'anaconda_upload': dals("""
                Automatically upload packages built with conda build to anaconda.org.
                """),
            'auto_activate_base': dals("""
                Automatically activate the base environment during shell initialization.
                """),
            'auto_update_conda': dals("""
                Automatically update conda when a newer or higher priority version is detected.
                """),
            'auto_stack': dals("""
                Implicitly use --stack when using activate if current level of nesting
                (as indicated by CONDA_SHLVL environment variable) is less than or equal to
                specified value. 0 or false disables automatic stacking, 1 or true enables
                it for one level.
                """),
            'bld_path': dals("""
                The location where conda-build will put built packages. Same as 'croot', but
                'croot' takes precedence when both are defined. Also used in construction of the
                'local' multichannel.
                """),
            'changeps1': dals("""
                When using activate, change the command prompt ($PS1) to include the
                activated environment.
                """),
            'channel_alias': dals("""
                The prepended url location to associate with channel names.
                """),
            'channel_priority': dals("""
                Accepts values of 'strict', 'flexible', and 'disabled'. The default value
                is 'flexible'. With strict channel priority, packages in lower priority channels
                are not considered if a package with the same name appears in a higher
                priority channel. With flexible channel priority, the solver may reach into
                lower priority channels to fulfill dependencies, rather than raising an
                unsatisfiable error. With channel priority disabled, package version takes
                precedence, and the configured priority of channels is used only to break ties.
                In previous versions of conda, this parameter was configured as either True or
                False. True is now an alias to 'flexible'.
                """),
            'channels': dals("""
                The list of conda channels to include for relevant operations.
                """),
            'client_ssl_cert': dals("""
                A path to a single file containing a private key and certificate (e.g. .pem
                file). Alternately, use client_ssl_cert_key in conjuction with client_ssl_cert
                for individual files.
                """),
            'client_ssl_cert_key': dals("""
                Used in conjunction with client_ssl_cert for a matching key file.
                """),
            # 'clobber': dals("""
            #     Allow clobbering of overlapping file paths within packages, and suppress
            #     related warnings. Overrides the path_conflict configuration value when
            #     set to 'warn' or 'prevent'.
            #     """),
            'conda_build': dals("""
                General configuration parameters for conda-build.
                """),
            # TODO: add shortened link to docs for conda_build at See https://conda.io/docs/user-guide/configuration/use-condarc.html#conda-build-configuration  # NOQA
            'create_default_packages': dals("""
                Packages that are by default added to a newly created environments.
                """),  # TODO: This is a bad parameter name. Consider an alternate.
            'croot': dals("""
                The location where conda-build will put built packages. Same as 'bld_path', but
                'croot' takes precedence when both are defined. Also used in construction of the
                'local' multichannel.
                """),
            'custom_channels': dals("""
                A map of key-value pairs where the key is a channel name and the value is
                a channel location. Channels defined here override the default
                'channel_alias' value. The channel name (key) is not included in the channel
                location (value).  For example, to override the location of the 'conda-forge'
                channel where the url to repodata is
                https://anaconda-repo.dev/packages/conda-forge/linux-64/repodata.json, add an
                entry 'conda-forge: https://anaconda-repo.dev/packages'.
                """),
            'custom_multichannels': dals("""
                A multichannel is a metachannel composed of multiple channels. The two reserved
                multichannels are 'defaults' and 'local'. The 'defaults' multichannel is
                customized using the 'default_channels' parameter. The 'local'
                multichannel is a list of file:// channel locations where conda-build stashes
                successfully-built packages.  Other multichannels can be defined with
                custom_multichannels, where the key is the multichannel name and the value is
                a list of channel names and/or channel urls.
                """),
            'default_channels': dals("""
                The list of channel names and/or urls used for the 'defaults' multichannel.
                """),
            # 'default_python': dals("""
            #     specifies the default major & minor version of Python to be used when
            #     building packages with conda-build. Also used to determine the major
            #     version of Python (2/3) to be used in new environments. Defaults to
            #     the version used by conda itself.
            #     """),
            'default_threads': dals("""
                Threads to use by default for parallel operations.  Default is None,
                which allows operations to choose themselves.  For more specific
                control, see the other *_threads parameters:
                    * repodata_threads - for fetching/loading repodata
                    * verify_threads - for verifying package contents in transactions
                    * execute_threads - for carrying out the unlinking and linking steps
            """),
            'disallowed_packages': dals("""
                Package specifications to disallow installing. The default is to allow
                all packages.
                """),
            'download_only': dals("""
                Solve an environment and ensure package caches are populated, but exit
                prior to unlinking and linking packages into the prefix
                """),
            'envs_dirs': dals("""
                The list of directories to search for named environments. When creating a new
                named environment, the environment will be placed in the first writable
                location.
                """),
            'env_prompt': dals("""
                Template for prompt modification based on the active environment. Currently
                supported template variables are '{prefix}', '{name}', and '{default_env}'.
                '{prefix}' is the absolute path to the active environment. '{name}' is the
                basename of the active environment prefix. '{default_env}' holds the value
                of '{name}' if the active environment is a conda named environment ('-n'
                flag), or otherwise holds the value of '{prefix}'. Templating uses python's
                str.format() method.
                """),
            'execute_threads': dals("""
                Threads to use when performing the unlink/link transaction.  When not set,
                defaults to 1.  This step is pretty strongly I/O limited, and you may not
                see much benefit here.
            """),
            'force_reinstall': dals("""
                Ensure that any user-requested package for the current operation is uninstalled
                and reinstalled, even if that package already exists in the environment.
                """),
            # 'force': dals("""
            #     Override any of conda's objections and safeguards for installing packages and
            #     potentially breaking environments. Also re-installs the package, even if the
            #     package is already installed. Implies --no-deps.
            #     """),
            # 'force_32bit': dals("""
            #     CONDA_FORCE_32BIT should only be used when running conda-build (in order
            #     to build 32-bit packages on a 64-bit system).  We don't want to mention it
            #     in the documentation, because it can mess up a lot of things.
            #     """),
            'json': dals("""
                Ensure all output written to stdout is structured json.
                """),
            'local_repodata_ttl': dals("""
                For a value of False or 0, always fetch remote repodata (HTTP 304 responses
                respected). For a value of True or 1, respect the HTTP Cache-Control max-age
                header. Any other positive integer values is the number of seconds to locally
                cache repodata before checking the remote server for an update.
                """),
            'migrated_channel_aliases': dals("""
                A list of previously-used channel_alias values. Useful when switching between
                different Anaconda Repository instances.
                """),
            'migrated_custom_channels': dals("""
                A map of key-value pairs where the key is a channel name and the value is
                the previous location of the channel.
                """),
            # 'no_deps': dals("""
            #     Do not install, update, remove, or change dependencies. This WILL lead to broken
            #     environments and inconsistent behavior. Use at your own risk.
            #     """),
            'non_admin_enabled': dals("""
                Allows completion of conda's create, install, update, and remove operations, for
                non-privileged (non-root or non-administrator) users.
                """),
            'notify_outdated_conda': dals("""
                Notify if a newer version of conda is detected during a create, install, update,
                or remove operation.
                """),
            'offline': dals("""
                Restrict conda to cached download content and file:// based urls.
                """),
            'override_channels_enabled': dals("""
                Permit use of the --overide-channels command-line flag.
                """),
            'path_conflict': dals("""
                The method by which conda handle's conflicting/overlapping paths during a
                create, install, or update operation. The value must be one of 'clobber',
                'warn', or 'prevent'. The '--clobber' command-line flag or clobber
                configuration parameter overrides path_conflict set to 'prevent'.
                """),
            'pinned_packages': dals("""
                A list of package specs to pin for every environment resolution.
                This parameter is in BETA, and its behavior may change in a future release.
                """),
            'pip_interop_enabled': dals("""
                Allow the conda solver to interact with non-conda-installed python packages.
                """),
            'pkgs_dirs': dals("""
                The list of directories where locally-available packages are linked from at
                install time. Packages not locally available are downloaded and extracted
                into the first writable directory.
                """),
            'proxy_servers': dals("""
                A mapping to enable proxy settings. Keys can be either (1) a scheme://hostname
                form, which will match any request to the given scheme and exact hostname, or
                (2) just a scheme, which will match requests to that scheme. Values are are
                the actual proxy server, and are of the form
                'scheme://[user:password@]host[:port]'. The optional 'user:password' inclusion
                enables HTTP Basic Auth with your proxy.
                """),
            'quiet': dals("""
                Disable progress bar display and other output.
                """),
            'remote_connect_timeout_secs': dals("""
                The number seconds conda will wait for your client to establish a connection
                to a remote url resource.
                """),
            'remote_max_retries': dals("""
                The maximum number of retries each HTTP connection should attempt.
                """),
            'remote_backoff_factor': dals("""
                The factor determines the time HTTP connection should wait for attempt.
                """),
            'remote_read_timeout_secs': dals("""
                Once conda has connected to a remote resource and sent an HTTP request, the
                read timeout is the number of seconds conda will wait for the server to send
                a response.
                """),
            'repodata_threads': dals("""
                Threads to use when downloading and reading repodata.  When not set,
                defaults to None, which uses the default ThreadPoolExecutor behavior.
            """),
            'report_errors': dals("""
                Opt in, or opt out, of automatic error reporting to core maintainers. Error
                reports are anonymous, with only the error stack trace and information given
                by `conda info` being sent.
                """),
            'restore_free_channel': dals(""""
                Add the "free" channel back into defaults, behind "main" in priority. The "free"
                channel was removed from the collection of default channels in conda 4.7.0.
            """),
            'rollback_enabled': dals("""
                Should any error occur during an unlink/link transaction, revert any disk
                mutations made to that point in the transaction.
                """),
            'safety_checks': dals("""
                Enforce available safety guarantees during package installation.
                The value must be one of 'enabled', 'warn', or 'disabled'.
                """),
            'separate_format_cache': dals("""
                Treat .tar.bz2 files as different from .conda packages when
                filenames are otherwise similar. This defaults to False, so
                that your package cache doesn't churn when rolling out the new
                package format. If you'd rather not assume that a .tar.bz2 and
                .conda from the same place represent the same content, set this
                to True.
            """),
            'extra_safety_checks': dals("""
                Spend extra time validating package contents.  Currently, runs sha256 verification
                on every file within each package during installation.
                """),
            'shortcuts': dals("""
                Allow packages to create OS-specific shortcuts (e.g. in the Windows Start
                Menu) at install time.
                """),
            'show_channel_urls': dals("""
                Show channel URLs when displaying what is going to be downloaded.
                """),
            'ssl_verify': dals("""
                Conda verifies SSL certificates for HTTPS requests, just like a web
                browser. By default, SSL verification is enabled, and conda operations will
                fail if a required url's certificate cannot be verified. Setting ssl_verify to
                False disables certification verification. The value for ssl_verify can also
                be (1) a path to a CA bundle file, or (2) a path to a directory containing
                certificates of trusted CA.
                """),
            'track_features': dals("""
                A list of features that are tracked by default. An entry here is similar to
                adding an entry to the create_default_packages list.
                """),
            'repodata_fns': dals("""
                Specify filenames for repodata fetching. The default is ('current_repodata.json',
                'repodata.json'), which tries a subset of the full index containing only the
                latest version for each package, then falls back to repodata.json.  You may
                want to specify something else to use an alternate index that has been reduced
                somehow.
                """),
            'use_index_cache': dals("""
                Use cache of channel index files, even if it has expired.
                """),
            'use_only_tar_bz2': dals("""
                A boolean indicating that only .tar.bz2 conda packages should be downloaded.
                This is forced to True if conda-build is installed and older than 3.18.3,
                because older versions of conda break when conda feeds it the new file format.
                """),
            'verbosity': dals("""
                Sets output log level. 0 is warn. 1 is info. 2 is debug. 3 is trace.
                """),
            'verify_threads': dals("""
                Threads to use when performing the transaction verification step.  When not set,
                defaults to 1.
            """),
            'whitelist_channels': dals("""
                The exclusive list of channels allowed to be used on the system. Use of any
                other channels will result in an error. If conda-build channels are to be
                allowed, along with the --use-local command line flag, be sure to include the
                'local' channel in the list. If the list is empty or left undefined, no
                channel exclusions will be enforced.
                """),
            'unsatisfiable_hints': dals("""
                A boolean to determine if conda should find conflicting packages in the case
                of a failed install.
                """),
            'unsatisfiable_hints_check_depth': dals("""
                An integer that specifies how many levels deep to search for unsatisfiable
                dependencies. If this number is 1 it will complete the unsatisfiable hints
                fastest (but perhaps not the most complete). The higher this number, the
                longer the generation of the unsat hint will take. Defaults to 3.
                """),

        })


def conda_in_private_env():
    # conda is located in its own private environment named '_conda_'
    envs_dir, env_name = path_split(sys.prefix)
    return env_name == '_conda_' and basename(envs_dir) == 'envs'


def reset_context(search_path=SEARCH_PATH, argparse_args=None):
    global context
    context.__init__(search_path, argparse_args)
    context.__dict__.pop('_Context__conda_build', None)
    from ..models.channel import Channel
    Channel._reset_state()
    # need to import here to avoid circular dependency
    return context


class ContextStackObject(object):

    def __init__(self, search_path=SEARCH_PATH, argparse_args=None):
        self.set_value(search_path, argparse_args)

    def set_value(self, search_path=SEARCH_PATH, argparse_args=None):
        self.search_path = search_path
        self.argparse_args = argparse_args

    def apply(self):
        reset_context(self.search_path, self.argparse_args)


class ContextStack(object):

    def __init__(self):
        self._stack = [ContextStackObject() for _ in range(3)]
        self._stack_idx = 0
        self._last_search_path = None
        self._last_argparse_args = None

    def push(self, search_path, argparse_args):
        self._stack_idx += 1
        old_len = len(self._stack)
        if self._stack_idx >= old_len:
            self._stack.extend([ContextStackObject() for _ in range(old_len)])
        self._stack[self._stack_idx].set_value(search_path, argparse_args)
        self.apply()

    def apply(self):
        if self._last_search_path != self._stack[self._stack_idx].search_path or \
           self._last_argparse_args != self._stack[self._stack_idx].argparse_args:
            # Expensive:
            self._stack[self._stack_idx].apply()
            self._last_search_path = self._stack[self._stack_idx].search_path
            self._last_argparse_args = self._stack[self._stack_idx].argparse_args

    def pop(self):
        self._stack_idx -= 1
        self._stack[self._stack_idx].apply()

    def replace(self, search_path, argparse_args):
        self._stack[self._stack_idx].set_value(search_path, argparse_args)
        self._stack[self._stack_idx].apply()


context_stack = ContextStack()


def stack_context(pushing, search_path=SEARCH_PATH, argparse_args=None):
    if pushing:
        # Fast
        context_stack.push(search_path, argparse_args)
    else:
        # Slow
        context_stack.pop()


# Default means "The configuration when there are no condarc files present". It is
# all the settings and defaults that are built in to the code and *not* the default
# value of search_path=SEARCH_PATH. It means search_path=().
def stack_context_default(pushing, argparse_args=None):
    return stack_context(pushing, search_path=(), argparse_args=argparse_args)


def replace_context(pushing=None, search_path=SEARCH_PATH, argparse_args=None):
    # pushing arg intentionally not used here, but kept for API compatibility
    return context_stack.replace(search_path, argparse_args)


def replace_context_default(pushing=None, argparse_args=None):
    # pushing arg intentionally not used here, but kept for API compatibility
    return context_stack.replace(search_path=(), argparse_args=argparse_args)


# Tests that want to only declare 'I support the project-wide default for how to
# manage stacking of contexts'. Tests that are known to be careful with context
# can use `replace_context_default` which might be faster, though it should
# be a stated goal to set conda_tests_ctxt_mgmt_def_pol to replace_context_default
# and not to stack_context_default.
conda_tests_ctxt_mgmt_def_pol = replace_context_default

@memoize
def _get_cpu_info():
    # DANGER: This is rather slow
    from .._vendor.cpuinfo import get_cpu_info
    return frozendict(get_cpu_info())


def env_name(prefix):
    # counter part to `locate_prefix_by_name()` below
    if not prefix:
        return None
    if paths_equal(prefix, context.root_prefix):
        return ROOT_ENV_NAME
    maybe_envs_dir, maybe_name = path_split(prefix)
    for envs_dir in context.envs_dirs:
        if paths_equal(envs_dir, maybe_envs_dir):
            return maybe_name
    return prefix


def locate_prefix_by_name(name, envs_dirs=None):
    """Find the location of a prefix given a conda env name.  If the location does not exist, an
    error is raised.
    """
    assert name
    if name in (ROOT_ENV_NAME, 'root'):
        return context.root_prefix
    if envs_dirs is None:
        envs_dirs = context.envs_dirs
    for envs_dir in envs_dirs:
        if not isdir(envs_dir):
            continue
        prefix = join(envs_dir, name)
        if isdir(prefix):
            return abspath(prefix)

    from ..exceptions import EnvironmentNameNotFound
    raise EnvironmentNameNotFound(name)


def determine_target_prefix(ctx, args=None):
    """Get the prefix to operate in.  The prefix may not yet exist.

    Args:
        ctx: the context of conda
        args: the argparse args from the command line

    Returns: the prefix
    Raises: CondaEnvironmentNotFoundError if the prefix is invalid
    """

    argparse_args = args or ctx._argparse_args
    try:
        prefix_name = argparse_args.name
    except AttributeError:
        prefix_name = None
    try:
        prefix_path = argparse_args.prefix
    except AttributeError:
        prefix_path = None

    if prefix_name is not None and not prefix_name.strip():  # pragma: no cover
        from ..exceptions import ArgumentError
        raise ArgumentError("Argument --name requires a value.")

    if prefix_path is not None and not prefix_path.strip():  # pragma: no cover
        from ..exceptions import ArgumentError
        raise ArgumentError("Argument --prefix requires a value.")

    if prefix_name is None and prefix_path is None:
        return ctx.default_prefix
    elif prefix_path is not None:
        return expand(prefix_path)
    else:
        disallowed_chars = ('/', ' ', ':', '#')
        if any(_ in prefix_name for _ in disallowed_chars):
            from ..exceptions import CondaValueError
            builder = ["Invalid environment name: '" + prefix_name + "'"]
            builder.append("  Characters not allowed: {}".format(disallowed_chars))
            raise CondaValueError("\n".join(builder))
        if prefix_name in (ROOT_ENV_NAME, 'root'):
            return ctx.root_prefix
        else:
            from ..exceptions import EnvironmentNameNotFound
            try:
                return locate_prefix_by_name(prefix_name)
            except EnvironmentNameNotFound:
                return join(_first_writable_envs_dir(), prefix_name)


def _first_writable_envs_dir():
    # Calling this function will *create* an envs directory if one does not already
    # exist. Any caller should intend to *use* that directory for *writing*, not just reading.
    for envs_dir in context.envs_dirs:

        if envs_dir == os.devnull:
            continue

        # The magic file being used here could change in the future.  Don't write programs
        # outside this code base that rely on the presence of this file.
        # This value is duplicated in conda.gateways.disk.create.create_envs_directory().
        envs_dir_magic_file = join(envs_dir, '.conda_envs_dir_test')

        if isfile(envs_dir_magic_file):
            try:
                open(envs_dir_magic_file, 'a').close()
                return envs_dir
            except (IOError, OSError):
                log.trace("Tried envs_dir but not writable: %s", envs_dir)
        else:
            from ..gateways.disk.create import create_envs_directory
            was_created = create_envs_directory(envs_dir)
            if was_created:
                return envs_dir

    from ..exceptions import NoWritableEnvsDirError
    raise NoWritableEnvsDirError(context.envs_dirs)


# backward compatibility for conda-build
def get_prefix(ctx, args, search=True):  # pragma: no cover
    return determine_target_prefix(ctx or context, args)


try:
    context = Context((), None)
except ConfigurationLoadError as e:  # pragma: no cover
    print(repr(e), file=sys.stderr)
    # Exception handler isn't loaded so use sys.exit
    sys.exit(1)
