code: def _shutil_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.
  
      This is a backport of shutil.which from Python 3.4
      """
  
      # 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)
          )
  
      # 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
  
      if path is None:
          path = os.environ.get("PATH", os.defpath)
      if not path:
          return None
      path = path.split(os.pathsep)
  
      if sys.platform == "win32":
          # The current directory takes precedence on Windows.
          if not os.curdir 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".
          # 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
  
file_path: catboost/contrib/python/ipython/py2/IPython/utils/py3compat.py
imports: import functools
  import os
  import sys
  import re
  import shutil
  import types
  import platform
  from .encoding import DEFAULT_ENCODING
label_desc: Use image node_id 9 for calling a global function with example usage: _shutil_which(cmd, mode, path) and returns: None, unknown, None, None, cmd, name