(cmd, mode=os.F_OK | os.X_OK, path=None)
return 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.
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.
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 .
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
def which ( cmd , mode = os . F_OK | os . X_OK , path = None ) : # 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