Why is bool() an instance of int()?

[voidtrance]

I was surprised to find out that isinstance(bool(1), int) returns True. Why would that be?

If you have the following code:

def a(*args):
   if len(args) != 3:
       return False
   return True

def b(*args):
   return errno.EINVAL

func = <dynamically assign one of a() or b()>
ret = func()

How would you check if either of the returned a failure?

[buran]

Why would that be?

check PEP-285 written by Guido:

Should bool inherit from int?

=> Yes.

In an ideal world, bool might be better implemented as a separate integer type that knows how to perform mixed-mode arithmetic. However, inheriting bool from int eases the implementation enormously (in part since all C code that calls PyInt_Check() will continue to work – this returns true for subclasses of int). Also, I believe this is right in terms of substitutability: code that requires an int can be fed a bool and it will behave the same as 0 or 1. Code that requires a bool may not work when it is given an int; for example, 3 & 4 is 0, but both 3 and 4 are true when considered as truth values.

Also check https://stackoverflow.com/q/8169001/4046632

[voidtrance]

Also check https://stackoverflow.com/q/8169001/4046632

The explanation there does not make sense to me. "... True and False masquerade as 1 and 0, respectively." is perfectly fine. The issue is that the reality is the opposite - 0 and 1 are masquerading as False and True. I always though that bool was a different type than int not just an "alias".

But, I guess, it is what it is. The decision has been made a long time ago and now we have to live with it.

[Gribouillis]

I always though that bool was a different type than int not just an "alias".

bool is not an alias of int, it is a subclass

>>> bool.__mro__
(<class 'bool'>, <class 'int'>, <class 'object'>)
>>> 

[deanhystad]

How would you check if either of the returned a failure?

failed = retval in (errno.EINVAL, False)

Better yet, raise an exception when you have an error. Status return values are for languages without good exception handling support.