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.
  
      """
      # If we're given a path with a directory part, look it up directly rather
      # than referring to PATH directories. This includes checking relative to the
      # current directory, e.g. ./script
      if os.path.dirname(cmd):
          if _access_check(cmd, mode):
              return cmd
          return None
  
      use_bytes = isinstance(cmd, bytes)
  
      if path is None:
          path = os.environ.get("PATH", None)
          if path is None:
              try:
                  path = os.confstr("CS_PATH")
              except (AttributeError, ValueError):
                  # os.confstr() or CS_PATH is not available
                  path = os.defpath
          # bpo-35755: Don't use os.defpath if the PATH environment variable is
          # set to an empty string
  
      # PATH='' doesn't match, whereas PATH=':' looks in the current directory
      if not path:
          return None
  
      if use_bytes:
          path = os.fsencode(path)
          path = path.split(os.fsencode(os.pathsep))
      else:
          path = os.fsdecode(path)
          path = path.split(os.pathsep)
  
      if sys.platform == "win32":
          # The current directory takes precedence on Windows.
          curdir = os.curdir
          if use_bytes:
              curdir = os.fsencode(curdir)
          if curdir not in path:
              path.insert(0, curdir)
  
          # PATHEXT is necessary to check on Windows.
          pathext_source = os.getenv("PATHEXT") or _WIN_DEFAULT_PATHEXT
          pathext = [
              ext for ext in pathext_source.split(os.pathsep) if ext
          ]
  
          if use_bytes:
              pathext = [os.fsencode(ext) for ext in pathext]
          # See if the given file matches any of the expected path extensions.
          # This will allow us to short circuit when given "python.exe".
          # If it does match, only test that one, otherwise we have to try
          # others.
          if any(cmd.lower().endswith(ext.lower()) for ext in pathext):
              files = [cmd]
          else:
              files = [cmd + ext 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:
          normdir = os.path.normcase(dir)
          if not normdir in seen:
              seen.add(normdir)
              for thefile in files:
                  name = os.path.join(dir, thefile)
                  if _access_check(name, mode):
                      return name
      return None
  
imports: import os
  import sys
  import stat
  import fnmatch
  import collections
  import errno
label_desc: Use image node_id 43 for calling a global function with example usage: which(cmd, mode, path) and returns: None, None, None, cmd, name