def which(cmd, mode=(os.F_OK | os.X_OK), executable_search_paths=None):
    'Given a command, mode, and executable search paths list, return the paths which\n    conforms to the given mode on the PATH with the prepended additional search paths,\n    or None if there is no such file.\n    `mode` defaults to os.F_OK | os.X_OK. the default search `path` defaults\n    to the result of os.environ.get("PATH")\n    Note: This function was backported from the Python 3 source code.\n\n    :type cmd: str\n    :param cmd:\n        Executable to be looked up in PATH.\n\n    :type mode: str\n    :param mode:\n        Modes of access for the executable.\n\n    :type executable_search_paths: list\n    :param executable_search_paths:\n        List of paths to look for `cmd` in preference order.\n    '

    def _access_check(fn, mode):
        return (os.path.exists(fn) and os.access(fn, mode) and (not os.path.isdir(fn)))
    if os.path.dirname(cmd):
        if _access_check(cmd, mode):
            return cmd
        return None
    path = os.environ.get('PATH', os.defpath)
    if (not path):
        return None
    path = path.split(os.pathsep)
    if executable_search_paths:
        path = (executable_search_paths + path)
    if (sys.platform == 'win32'):
        if (os.curdir not in path):
            path.insert(0, os.curdir)
        pathext = os.environ.get('PATHEXT', '').split(os.pathsep)
        if any((cmd.lower().endswith(ext.lower()) for ext in pathext)):
            files = [cmd]
        else:
            files = [(cmd + ext) for ext in pathext]
    else:
        files = [cmd]
    seen = set()
    paths = []
    for dir in path:
        normdir = os.path.normcase(dir)
        if (normdir not in seen):
            seen.add(normdir)
            for thefile in files:
                name = os.path.join(dir, thefile)
                if _access_check(name, mode):
                    paths.append(name)
    return paths
Given a command, mode, and executable search paths list, return the paths which
conforms to the given mode on the PATH with the prepended additional search paths,
or None if there is no such file.
`mode` defaults to os.F_OK | os.X_OK. the default search `path` defaults
to the result of os.environ.get("PATH")
Note: This function was backported from the Python 3 source code.

:type cmd: str
:param cmd:
    Executable to be looked up in PATH.

:type mode: str
:param mode:
    Modes of access for the executable.

:type executable_search_paths: list
:param executable_search_paths:
    List of paths to look for `cmd` in preference order.
def which(cmd, mode=(os.F_OK | os.X_OK), executable_search_paths=None):
    'Given a command, mode, and executable search paths list, return the paths which\n    conforms to the given mode on the PATH with the prepended additional search paths,\n    or None if there is no such file.\n    `mode` defaults to os.F_OK | os.X_OK. the default search `path` defaults\n    to the result of os.environ.get("PATH")\n    Note: This function was backported from the Python 3 source code.\n\n    :type cmd: str\n    :param cmd:\n        Executable to be looked up in PATH.\n\n    :type mode: str\n    :param mode:\n        Modes of access for the executable.\n\n    :type executable_search_paths: list\n    :param executable_search_paths:\n        List of paths to look for `cmd` in preference order.\n    '

    def _access_check(fn, mode):
        return (os.path.exists(fn) and os.access(fn, mode) and (not os.path.isdir(fn)))
    if os.path.dirname(cmd):
        if _access_check(cmd, mode):
            return cmd
        return None
    path = os.environ.get('PATH', os.defpath)
    if (not path):
        return None
    path = path.split(os.pathsep)
    if executable_search_paths:
        path = (executable_search_paths + path)
    if (sys.platform == 'win32'):
        if (os.curdir not in path):
            path.insert(0, os.curdir)
        pathext = os.environ.get('PATHEXT', ).split(os.pathsep)
        if any((cmd.lower().endswith(ext.lower()) for ext in pathext)):
            files = [cmd]
        else:
            files = [(cmd + ext) for ext in pathext]
    else:
        files = [cmd]
    seen = set()
    paths = []
    for dir in path:
        normdir = os.path.normcase(dir)
        if (normdir not in seen):
            seen.add(normdir)
            for thefile in files:
                name = os.path.join(dir, thefile)
                if _access_check(name, mode):
                    paths.append(name)
    return paths