code: def which(cmd, mode=os.F_OK | os.X_OK, path=None):
      """Given a command, mode, and a PATH string, return the path which
      conforms to the given mode on the PATH, or None if there is no such
      file.
      `mode` defaults to os.F_OK | os.X_OK. `path` defaults to the result
      of os.environ.get("PATH"), or can be overridden with a custom search
      path.
      """
  
      # Check that a given file can be accessed with the correct mode.
      # Additionally check that `file` is not a directory, as on Windows
      # directories pass the os.access check.
      def _access_check(fn, mode):
          return (
              os.path.exists(fn)
              and os.access(fn, mode)
              and not os.path.isdir(fn)
          )
  
      # Short circuit. If we're given a full path which matches the mode
      # and it exists, we're done here.
      if _access_check(cmd, mode):
          return cmd
  
      path = (path or os.environ.get("PATH", os.defpath)).split(
          os.pathsep
      )
  
      if sys.platform == "win32":
          # The current directory takes precedence on Windows.
          if os.curdir not in path:
              path.insert(0, os.curdir)
  
          # PATHEXT is necessary to check on Windows.
          pathext = os.environ.get("PATHEXT", "").split(os.pathsep)
          # See if the given file matches any of the expected path extensions.
          # This will allow us to short circuit when given "python.exe".
          matches = [
              cmd
              for ext in pathext
              if cmd.lower().endswith(ext.lower())
          ]
          # If it does match, only test that one, otherwise we have to try
          # others.
          files = (
              [cmd]
              if matches
              else [cmd + ext.lower() for ext in pathext]
          )
      else:
          # On other platforms you don't have things like PATHEXT to tell you
          # what file suffixes are executable, so just pass on cmd as-is.
          files = [cmd]
  
      seen = set()
      for dir in path:
          dir = os.path.normcase(dir)
          if dir not in seen:
              seen.add(dir)
              for thefile in files:
                  name = os.path.join(dir, thefile)
                  if _access_check(name, mode):
                      return name
      return None
  
imports: from collections import defaultdict
  from os.path import join
  import io
  import os
  import functools
  import pipes
  import re
  import shlex
  import subprocess
  import sys
  from setuptools import Command
  from setuptools.command.build_py import build_py
  from setuptools.command.sdist import sdist
  from distutils import log
  from setuptools.command.develop import develop
  from setuptools.command.bdist_egg import bdist_egg
label_desc: Use image node_id 15 for calling a global function with example usage: which(cmd, mode, path) and returns: None, unknown, cmd, name