[New-bugs-announce] [issue25484] Operator issue with "in" on same level and preceding ==

[Sapphire Becker]

New submission from Sapphire Becker:

So I found this while using == as xnor, here's a summary of the issue:

>>> "h" in ["h"] == True
False
>>> ("h" in ["h"]) == True
True
>>> "h" in (["h"] == True)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: argument of type 'bool' is not iterable

Now obviously #3 should error but I would expect #1 and #2 to be the same. I checked it out with dis (in Py 3.4) and it seems like a bug. In detail:

#2's is very clean, exactly what I would expect:
>>> dis.dis('("h" in ["h"]) == True')
  1           0 LOAD_CONST               0 ('h')
              3 LOAD_CONST               2 (('h',))
              6 COMPARE_OP               6 (in)
              9 LOAD_CONST               1 (True)
             12 COMPARE_OP               2 (==)
             15 RETURN_VALUE

#1's, however, looks like a mess:
>>> dis.dis('"h" in ["h"] == True')
  1           0 LOAD_CONST               0 ('h')
              3 LOAD_CONST               0 ('h')
              6 BUILD_LIST               1
              9 DUP_TOP
             10 ROT_THREE
             11 COMPARE_OP               6 (in)
             14 JUMP_IF_FALSE_OR_POP    24
             17 LOAD_CONST               1 (True)
             20 COMPARE_OP               2 (==)
             23 RETURN_VALUE
        >>   24 ROT_TWO
             25 POP_TOP
             26 RETURN_VALUE

I had to write out the stack to understand this:
1. ['h']
2. ['h', 'h']
3. ['h', ['h']]
4. ['h', ['h'], ['h']]
5. [['h'], 'h', ['h']]
6. [['h'], True] # True = result of in operation
7. [['h']]
8. [['h'], True] # True = =='s right-hand value
9. [False] # False = result of == operation

Therefore the way to actually get true in the first case is:

>>> "h" in ["h"] == ["h"]
True


I won't pretend to know how simple it is in Python's architecture but I imagine it's just setting a higher precedence for "in" than ==?

Not a huge deal, it's workaroundable, just awkward that there wasn't really any indication it was failing so it took a bit to debug.

----------
components: Interpreter Core
messages: 253478
nosy: Sapphire Becker
priority: normal
severity: normal
status: open
title: Operator issue with "in" on same level and preceding ==
type: behavior
versions: Python 2.7, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25484>
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