Example of modifying a builtin type:

>>> class Sneaky:
...     def __eq__(self, other):
...         other['real'] = 42
... 
>>> int.__dict__ == Sneaky()
>>> (1).real
42

But it can also lead to crash (due to outdated type cache):

>>> class Sneaky:
...     def __eq__(self, other):
...         other['bit_length'] = 42
... 
>>> int.__dict__ == Sneaky()
>>> (1).bit_length
Segmentation fault (core dumped)