How is the __contains__ method of the list class in Python implemented?

Question

Suppose I define the following variables:

mode = "access"
allowed_modes = ["access", "read", "write"]

I currently have a type checking statement which is

assert any(mode == allowed_mode for allowed_mode in allowed_modes)

However, it seems that I can replace this simply with

assert mode in allowed_modes

According to ThiefMaster's answer in Python List Class __contains__ Method Functionality, these two should be equivalent. Is this indeed the case? And how could I easily verify this by looking up Python's source code?

Answer 1

No, they're not equivalent. For example:

>>> mode = float('nan')
>>> allowed_modes = [mode]
>>> any(mode == allowed_mode for allowed_mode in allowed_modes)
False
>>> mode in allowed_modes
True

See Membership test operations for more details, including this statement:

For container types such as list, tuple, set, frozenset, dict, or collections.deque, the expression x in y is equivalent to any(x is e or x == e for e in y).