Associativity of “in” in Python?

Question

I\'m making a Python parser, and this is really confusing me:

>>>  1 in  []  in \'a\'
False

>>> (1 in  []) in \'a\'
TypeError: \'in &         


        

Answer 1

1 in [] in 'a' is evaluated as (1 in []) and ([] in 'a').

Since the first condition (1 in []) is False, the whole condition is evaluated as False; ([] in 'a') is never actually evaluated, so no error is raised.

Here are the statement definitions:

In [121]: def func():
   .....:     return 1 in [] in 'a'
   .....: 

In [122]: dis.dis(func)
  2           0 LOAD_CONST               1 (1)
              3 BUILD_LIST               0
              6 DUP_TOP             
              7 ROT_THREE           
              8 COMPARE_OP               6 (in)
             11 JUMP_IF_FALSE            8 (to 22)  #if first comparison is wrong 
                                                    #then jump to 22, 
             14 POP_TOP             
             15 LOAD_CONST               2 ('a')
             18 COMPARE_OP               6 (in)     #this is never executed, so no Error
             21 RETURN_VALUE         
        >>   22 ROT_TWO             
             23 POP_TOP             
             24 RETURN_VALUE        

In [150]: def func1():
   .....:     return (1 in  []) in 'a'
   .....: 

In [151]: dis.dis(func1)
  2           0 LOAD_CONST               1 (1)
              3 LOAD_CONST               3 (())
              6 COMPARE_OP               6 (in)   # perform 1 in []
              9 LOAD_CONST               2 ('a')  # now load 'a'
             12 COMPARE_OP               6 (in)   # compare result of (1 in []) with 'a'
                                                  # throws Error coz (False in 'a') is
                                                  # TypeError
             15 RETURN_VALUE   



In [153]: def func2():
   .....:     return 1 in ([] in 'a')
   .....: 

In [154]: dis.dis(func2)
  2           0 LOAD_CONST               1 (1)
              3 BUILD_LIST               0
              6 LOAD_CONST               2 ('a') 
              9 COMPARE_OP               6 (in)  # perform ([] in 'a'), which is 
                                                 # Incorrect, so it throws TypeError
             12 COMPARE_OP               6 (in)  # if no Error then 
                                                 # compare 1 with the result of ([] in 'a')
             15 RETURN_VALUE