How to check if a function/method/class is built-in Python?

Question

If I do something like this :

from mailbox import Mailbox
Mailbox.get = 'dummy'

There is no error, I just replaced the get method. But if I do a :

from datetime import datetime
datetime.now = 'dummy'

I get :

TypeError: can't set attributes of built-in/extension type 'datetime.datetime'

This is strange because the source code is readable at /usr/lib/python3.6/datetime.py. I guess this library has been compiled for performance reason, that is why it cannot be modified. But, then, the question is : how I can know a class is immutable because it is built-in or an extension (without just testing like I did) ? why can I patch Mailbox class and not datetime class ?

Answer 1

It won't tell you directly, but for your case, you can test the method you're trying to replace to see if it's an instance of types.BuiltinMethodType:

>>> isinstance(Mailbox.get, types.BuiltinMethodType)
False
>>> isinstance(datetime.now, types.BuiltinMethodType)
True

inspect.isbuiltin provides the same info:

>>> inspect.isbuiltin(Mailbox.get)
False
>>> inspect.isbuiltin(datetime.now)
True

To be clear, this is a limitation of the CPython reference interpreter; nothing requires attributes/methods of built-in types to be immutable, and in other interpreters (e.g. PyPy) they may be mutable.