python中的is()函数是什么

Question

我已经使用 python 几年了，从未见过is关键字在 if 条件下用作函数。我遇到了这段代码。它按预期工作，但我不知道 is() 的作用。

def face_ext(img):
    gray = frame
    faces = face_cascade.detectMultiScale(gray, scaleFactor=1.5, minNeighbors=5)

    if faces is():
        return None

    for(x, y, w, h) in faces:
        cropped_face = img[y:y+h, x:x+w]

    return cropped_face

删除is()代码时会产生以下错误：

[INFO] Initializing face capture. Look the camera and wait ...
Traceback (most recent call last):
  File "c:\Users\smoke\miniconda3\envs\skyhawk\Scripts\skyhawk-script.py", line 33, in <module>
    sys.exit(load_entry_point('skyhawk', 'console_scripts', 'skyhawk')())
  File "c:\Users\smoke\miniconda3\envs\skyhawk\lib\site-packages\click\core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "c:\Users\smoke\miniconda3\envs\skyhawk\lib\site-packages\click\core.py", line 782, in main
    rv = self.invoke(ctx)
  File "c:\Users\smoke\miniconda3\envs\skyhawk\lib\site-packages\click\core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "c:\Users\smoke\miniconda3\envs\skyhawk\lib\site-packages\click\core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "c:\Users\smoke\miniconda3\envs\skyhawk\lib\site-packages\click\core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "c:\Users\smoke\miniconda3\envs\skyhawk\lib\site-packages\click\core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "c:\Users\smoke\miniconda3\envs\skyhawk\lib\site-packages\click\decorators.py", line 21, in new_func    
    return f(get_current_context(), *args, **kwargs)
  File "c:\users\smoke\documents\dev\python\skyhawk-cli\skyhawk\commands\cmd_capture.py", line 21, in color    
    result = capture.Capture.color()
  File "c:\users\smoke\documents\dev\python\skyhawk-cli\skyhawk\services\capture.py", line 41, in color        
    if face_ext(frame) is not None:
  File "c:\users\smoke\documents\dev\python\skyhawk-cli\skyhawk\services\capture.py", line 31, in face_ext     
    if faces:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()        
[ WARN:0] terminating async callback

完整的代码文件在此链接中给出。

Answer 1

它根本不是一个函数，它是恒等比较运算符。

\n

\n

对象\xe2\x80\x99s 身份的运算符is和测试：当且仅当 x 和 y 是同一对象时才为 true。is notx is y

\n

\n

该声明

\n

if faces is():\n

\n

应格式化为

\n

if faces is ():\n

\n

即“这正是faces这个空元组对象的身份吗？”。

\n

（这可能永远不会；比较应该是if faces == ():，或者可能只是if not faces:）。

\n