Python判斷一個文件夾內哪些文件是圖片

def is_img(ext):
    ext = ext.lower()
    if ext == '.jpg':
        return True
    elif ext == '.png':
        return True
    elif ext == '.jpeg':
        return True
    elif ext == '.bmp':
        return True
    else:
        return False

調用時

for x in os.listdir(directory):
    if is_img(osp.splitext(x)[1]):
        do something