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//usr/lib/python2.7/site-packages/pip/_internal/utils/virtualenv.py

from __future__ import absolute_import

import logging
import os
import re
import site
import sys

from pip._internal.utils.typing import MYPY_CHECK_RUNNING

if MYPY_CHECK_RUNNING:
    from typing import List, Optional

logger = logging.getLogger(__name__)
_INCLUDE_SYSTEM_SITE_PACKAGES_REGEX = re.compile(
    r"include-system-site-packages\s*=\s*(?P<value>true|false)"
)

def _running_under_venv():
    # type: () -> bool
    """Checks if sys.base_prefix and sys.prefix match.

This handles PEP 405 compliant virtual environments.
    """
    return sys.prefix != getattr(sys, "base_prefix", sys.prefix)

def _running_under_regular_virtualenv():
    # type: () -> bool
    """Checks if sys.real_prefix is set.

This handles virtual environments created with pypa's virtualenv.
    """
    # pypa/virtualenv case
    return hasattr(sys, 'real_prefix')

def running_under_virtualenv():
    # type: () -> bool
    """Return True if we're running inside a virtualenv, False otherwise.
    """
    return _running_under_venv() or _running_under_regular_virtualenv()

def _get_pyvenv_cfg_lines():
    # type: () -> Optional[List[str]]
    """Reads {sys.prefix}/pyvenv.cfg and returns its contents as list of lines

Returns None, if it could not read/access the file.
    """
    pyvenv_cfg_file = os.path.join(sys.prefix, 'pyvenv.cfg')
    try:
        with open(pyvenv_cfg_file) as f:
            return f.read().splitlines()  # avoids trailing newlines
    except IOError:
        return None

def _no_global_under_venv():
    # type: () -> bool
    """Check `{sys.prefix}/pyvenv.cfg` for system site-packages inclusion

PEP 405 specifies that when system site-packages are not supposed to be
    visible from a virtual environment, `pyvenv.cfg` must contain the following
    line:

include-system-site-packages = false

Additionally, log a warning if accessing the file fails.
    """
    cfg_lines = _get_pyvenv_cfg_lines()
    if cfg_lines is None:
        # We're not in a "sane" venv, so assume there is no system
        # site-packages access (since that's PEP 405's default state).
        logger.warning(
            "Could not access 'pyvenv.cfg' despite a virtual environment "
            "being active. Assuming global site-packages is not accessible "
            "in this environment."
        )
        return True

for line in cfg_lines:
        match = _INCLUDE_SYSTEM_SITE_PACKAGES_REGEX.match(line)
        if match is not None and match.group('value') == 'false':
            return True
    return False

def _no_global_under_regular_virtualenv():
    # type: () -> bool
    """Check if "no-global-site-packages.txt" exists beside site.py

This mirrors logic in pypa/virtualenv for determining whether system
    site-packages are visible in the virtual environment.
    """
    site_mod_dir = os.path.dirname(os.path.abspath(site.__file__))
    no_global_site_packages_file = os.path.join(
        site_mod_dir, 'no-global-site-packages.txt',
    )
    return os.path.exists(no_global_site_packages_file)

def virtualenv_no_global():
    # type: () -> bool
    """Returns a boolean, whether running in venv with no system site-packages.
    """

if _running_under_regular_virtualenv():
        return _no_global_under_regular_virtualenv()

if _running_under_venv():
        return _no_global_under_venv()

return False