Python Uses Operational Chaining for Boolean Comparisons

A Python feature?

A colleague noticed this behavior:

>>> 'a' in 'b' == 0
False
>>> ('a' in 'b') == 0
True
>>> 'a' in ('b' == 0)
TypeError: argument of type 'bool' is not iterable

offering a bucket of doubloons to anyone who can explain it. The final case is simply a typecasting problem, but the first two cases demonstrate Python’s operating chaining behavior.

Explanation

Using the Dis Module to dig deeper

Using the dis¹ module, we find:

'a' in 'b' == 0

  7           0 LOAD_CONST               1 ('a')
              3 LOAD_CONST               2 ('b')
              6 DUP_TOP
              7 ROT_THREE
              8 COMPARE_OP               6 (in)
             11 JUMP_IF_FALSE_OR_POP    21
             14 LOAD_CONST               3 (0)
             17 COMPARE_OP               2 (==)
             20 RETURN_VALUE
        >>   21 ROT_TWO
             22 POP_TOP
             23 RETURN_VALUE

'a' in 'b' and 'b' == 0
15           0 LOAD_CONST               1 ('a')
              3 LOAD_CONST               2 ('b')
              6 COMPARE_OP               6 (in)
              9 JUMP_IF_FALSE_OR_POP    21
             12 LOAD_CONST               2 ('b')
             15 LOAD_CONST               3 (0)
             18 COMPARE_OP               2 (==)
        >>   21 RETURN_VALUE

'a' in 'a' == 0
  9           0 LOAD_CONST               1 ('a')
              3 LOAD_CONST               1 ('a')
              6 DUP_TOP
              7 ROT_THREE
              8 COMPARE_OP               6 (in)
             11 JUMP_IF_FALSE_OR_POP    21
             14 LOAD_CONST               2 (0)
             17 COMPARE_OP               2 (==)
             20 RETURN_VALUE
        >>   21 ROT_TWO
             22 POP_TOP
             23 RETURN_VALUE

In words

Parenthesis essential forces Python to do ‘something’ first before doing anything else. I often use parenthesis out of habit even when not needed just to be 100% clear. The above behavior shows where that habit might come in handy if Python’s operation chaining isn’t the desired behavior. When we put 'a' in 'b' inside parenthesis, Python is forced to evaluate that independently of the rest of line; this evaluates to True or False, and then the == 0 comparison acts as a Boolean comparison, giving expected behavior since Python sees False and 0 as equivalent here.

In the first instance above, however, Python is operator chaining. For 'a' in 'b' == 0, it first loads a and b strings such that the stack is

[ b, a ]

it then DUP_TOP duplicates b so that it doesn’t have to waste time to load it again for the second evaluation, resulting in the stack:

[ b, b, a ]

ROT_THREE lifts second and third stack item up and the top item down to third place, so now the stack is:

[ b, a, b ]

COMPARE_OP acts on [ b, a], and since it is not true, JUMP_IF_FALSE_OR_POP forces it to jump and just returns False. This makes sense because it sees this as a compound AND statement, and when the first part of an AND statement is false, the entire thing is false and it is a waste of resources to compute further.

What is interesting is if you do 'a' in 'a' == 1, or 'a' in 'a' == 0. Take the former, for example, which after DUP_TOP gives us the stack:

[ a, a, a ]

Here 'a' in 'a' is obviously true, and these two entries of the stack are replaced with the result of that evaluation, which is True. This, now being the top of the stack, is popped off by JUMP_IF_FALSE_OR_POP leaving the duplicated ‘a’ on the top of the stack, where then 1 is loaded for the next comparison, so the stack is now:

[1, a]

and the == comparison is applied which is indeed False. Thus 'a' in 'a' == 0 is chained as 'a' in 'a' and 'a' == 0 and will also be false. Similarly, 'a' in 'b' == 0 is effectively the same as 'a' in 'b' and 'b' == 0 which is false.

The following behavior, then, makes sense:

>>> 'a' in 'a'
True
>>> 'a' in 'a' and 'a' == True
False
>>> 'a' in 'a' == True
False

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