text: from __future__ import absolute_import
  
  import io
  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:
          # Although PEP 405 does not specify, the built-in venv module always
          # writes with UTF-8. (pypa/pip#8717)
          with io.open(pyvenv_cfg_file, encoding='utf-8') 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.
      """
      # PEP 405 compliance needs to be checked first since virtualenv >=20 would
      # return True for both checks, but is only able to use the PEP 405 config.
      if _running_under_venv():
          return _no_global_under_venv()
  
      if _running_under_regular_virtualenv():
          return _no_global_under_regular_virtualenv()
  
      return False
  