Why does “not(True) in [False, True]” return False?

StackOverflow

If I do this:

>>> False in [False, True]
True

That returns True. Simply because False is in the list.

But if I do:

>>> not(True) in [False, True]
False

That returns False. Whereas not(True) is equal to False:

>>> not(True)
False

Why?

Answer rating: 737

Operator precedence 2.x, 3.x. The precedence of not is lower than that of in. So it is equivalent to:

>>> not ((True) in [False, True])
False

This is what you want:

>>> (not True) in [False, True]
True

As @Ben points out: It"s recommended to never write not(True), prefer not True. The former makes it look like a function call, while not is an operator, not a function.

Answer rating: 76

not x in y is evaluated as x not in y

You can see exactly what"s happening by disassembling the code. The first case works as you expect:

>>> x = lambda: False in [False, True]
>>> dis.dis(x)
  1           0 LOAD_GLOBAL              0 (False)
              3 LOAD_GLOBAL              0 (False)
              6 LOAD_GLOBAL              1 (True)
              9 BUILD_LIST               2
             12 COMPARE_OP               6 (in)
             15 RETURN_VALUE

The second case, evaluates to True not in [False, True], which is False clearly:

>>> x = lambda: not(True) in [False, True]
>>> dis.dis(x)
  1           0 LOAD_GLOBAL              0 (True)
              3 LOAD_GLOBAL              1 (False)
              6 LOAD_GLOBAL              0 (True)
              9 BUILD_LIST               2
             12 COMPARE_OP               7 (not in)
             15 RETURN_VALUE        
>>> 

What you wanted to express instead was (not(True)) in [False, True], which as expected is True, and you can see why:

>>> x = lambda: (not(True)) in [False, True]
>>> dis.dis(x)
  1           0 LOAD_GLOBAL              0 (True)
              3 UNARY_NOT           
              4 LOAD_GLOBAL              1 (False)
              7 LOAD_GLOBAL              0 (True)
             10 BUILD_LIST               2
             13 COMPARE_OP               6 (in)
             16 RETURN_VALUE        




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