Why do 'and' & 'or' return operands in Python?

Question

I\'m going through the LPTHW and I came across something I cannot understand. When will it ever be the case that you want your boolean and or or to

Answer 1

This has to do with the way short circuit effect is implemented in Python.

With and (remember True and X = X), the result of the right expression is pushed into the stack, if it's false, it's get poped immediately, else the second expression is poped:

>>> import dis
>>> 
>>> dis.dis(lambda : True and 0)
  1           0 LOAD_CONST               2 (True)
              3 JUMP_IF_FALSE_OR_POP     9
              6 LOAD_CONST               1 (0)
        >>    9 RETURN_VALUE
>>>
>>> True and 0
0
>>> 0 and True
0
>>>

smiler to:

def exec_and(obj1, obj2):
    if bool(obj1) != False:
        return obj2
    return obj1

With or, if the first expression is true, it's get popped immediately. If not, the second expression is poped, now the result really depends on the second.

>>> dis.dis(lambda : 0 or False)
  1           0 LOAD_CONST               1 (0)
              3 JUMP_IF_TRUE_OR_POP      9
              6 LOAD_CONST               2 (False)
        >>    9 RETURN_VALUE
>>>
>>> True or 0
True
>>> 1 or False
1
>>> False or 1
1
>>>

smiler to:

def exec_or(obj1, obj2):
    if bool(obj1) != True:
        return obj2
    return obj1