Return the path to an executable which would be run if the given *cmd* was

called. If no *cmd* would be called, return ``None``.

If *cmd* contains a directory component, *which* will only check the specified path directly

and will not search the directories listed in *path* or in the system's ``PATH`` environment variable.

*mode* is a permission mask passed to:func:`os.access`, by default

determining if the file exists and is executable.

:envvar:`PATH` environment variable is read from:data:`os.environ`,

falling back to:data:`os.defpath` if it is not set.

..note::

If *cmd* contains a directory component,:func:`!which` only checks the

specified path directly and does not search the directories listed in *path*

or in the system's ``PATH`` environment variable.

On Windows, the current directory is prepended to the *path* if *mode* does

not include ``os.X_OK``. When the *mode* does include ``os.X_OK``, the

Windows API ``NeedCurrentDirectoryForExePathW`` will be consulted to