text: from __future__ import absolute_import
  
  import logging
  import sys
  from email.parser import FeedParser
  
  from pip._internal import exceptions
  from pip._internal.utils.misc import display_path
  from pip._internal.utils.typing import MYPY_CHECK_RUNNING
  from pip._vendor import pkg_resources
  from pip._vendor.packaging import specifiers, version
  
  if MYPY_CHECK_RUNNING:
      from typing import Optional  # noqa: F401
      from email.message import Message  # noqa: F401
      from pip._vendor.pkg_resources import Distribution  # noqa: F401
  
  
  logger = logging.getLogger(__name__)
  
  
  def check_requires_python(requires_python):
      # type: (Optional[str]) -> bool
      """
      Check if the python version in use match the `requires_python` specifier.
  
      Returns `True` if the version of python in use matches the requirement.
      Returns `False` if the version of python in use does not matches the
      requirement.
  
      Raises an InvalidSpecifier if `requires_python` have an invalid format.
      """
      if requires_python is None:
          # The package provides no information
          return True
      requires_python_specifier = specifiers.SpecifierSet(requires_python)
  
      # We only use major.minor.micro
      python_version = version.parse('.'.join(map(str, sys.version_info[:3])))
      return python_version in requires_python_specifier
  
  
  def get_metadata(dist):
      # type: (Distribution) -> Message
      if (isinstance(dist, pkg_resources.DistInfoDistribution) and
              dist.has_metadata('METADATA')):
          metadata = dist.get_metadata('METADATA')
      elif dist.has_metadata('PKG-INFO'):
          metadata = dist.get_metadata('PKG-INFO')
      else:
          logger.warning("No metadata found in %s", display_path(dist.location))
          metadata = ''
  
      feed_parser = FeedParser()
      feed_parser.feed(metadata)
      return feed_parser.close()
  
  
  def check_dist_requires_python(dist):
      pkg_info_dict = get_metadata(dist)
      requires_python = pkg_info_dict.get('Requires-Python')
      try:
          if not check_requires_python(requires_python):
              raise exceptions.UnsupportedPythonVersion(
                  "%s requires Python '%s' but the running Python is %s" % (
                      dist.project_name,
                      requires_python,
                      '.'.join(map(str, sys.version_info[:3])),)
              )
      except specifiers.InvalidSpecifier as e:
          logger.warning(
              "Package %s has an invalid Requires-Python entry %s - %s",
              dist.project_name, requires_python, e,
          )
          return
  
  
  def get_installer(dist):
      # type: (Distribution) -> str
      if dist.has_metadata('INSTALLER'):
          for line in dist.get_metadata_lines('INSTALLER'):
              if line.strip():
                  return line.strip()
      return ''
  