Python Equality Check Difference

Question

Suppose we want some block of code to be executed when both \'a\' and \'b\' are equal to say 5. Then we can write like :

if a == 5 and b == 5:
    # do something         


        

Answer 1

It depends. You could write your own custom __eq__ which allows you to compare yourself to ints and things:

 class NonNegativeInt(object):
   def __init__(self, value):
     if value < 0:
       raise Exception("Hey, what the...")
     self.value = value

   def __eq__(self, that):
     if isinstance(that, int):
       return self.value == that
     elif isinstance(that, NonNegativeInt):
       return self.value == that.value
     else:
       raise ArgumentError("Not an acceptible argument", "__eq__", that)

which would work different depending on comparing "b" to "a" and "b" to an "int." Hence, a == b could be false while a == 5 and b == 5 could be True.

Answer 2

If you have more variables to test, using all might be slightly more readable:

if all(i==5 for i in [a,b,c,d]):
    # do something

Answer 3

Since they are basically equivalent, you could also consider the way you read/think about the code:

if a == 5 and b == 5:
  # do something

can be read as "if a equals 5 and b equals 5, then do ...". You have to think/conclude, that then also a will be equal to b.

This is opposite to the next example:

if a == b and b == 5:
  # do something 

This reads as "if a is equal to b and b equal to 5" and you have to conclude that then also a will be equal to 5

This is why I prefer the last example:

if a == b == 5:
  # do something

If you are familiar with Python (thanks to Itzkata) it is immediately clear that all three things must be equal (to 5). If however people with less experience in Python (but programming skills in other languages) see this, they might evaluate this to

if (a == b) == 5:

which would compare the boolean result of the first comparison with the integer 5, which is not what Python does and might lead to different results (consider for example with a=0, b=0: a==b==0 is true while (a==b) == 0 is not!

The manual says:

There are eight comparison operations in Python. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).

There might even be a difference, for example if evaulating b in your example would have a side effect.

Regarding transitivity, you are right.

Answer 4

As far as integers are concerned, there is no difference, in terms of sheer performance, between the first two comparisons.

The third comparison is different, though; since a little more fiddling with the stack gets involved. Indeed, the code

import dis

def comparison_1(a, b):
    if a == 5 and b == 5:
        pass

def comparison_2(a, b):
    if a == b and b == 5:
        pass

def comparison_3(a, b):
    if a == b == 5:
        pass

print("*** First comparison ***")
dis.dis(comparison_1)

print("\n*** Second comparison ***")
dis.dis(comparison_2)

print("\n*** Third comparison ***")
dis.dis(comparison_3)

returns

*** First comparison ***
  4           0 LOAD_FAST                0 (a)
              3 LOAD_CONST               1 (5)
              6 COMPARE_OP               2 (==)
              9 POP_JUMP_IF_FALSE       27
             12 LOAD_FAST                1 (b)
             15 LOAD_CONST               1 (5)
             18 COMPARE_OP               2 (==)
             21 POP_JUMP_IF_FALSE       27

  5          24 JUMP_FORWARD             0 (to 27)
        >>   27 LOAD_CONST               0 (None)
             30 RETURN_VALUE        

*** Second comparison ***
  8           0 LOAD_FAST                0 (a)
              3 LOAD_FAST                1 (b)
              6 COMPARE_OP               2 (==)
              9 POP_JUMP_IF_FALSE       27
             12 LOAD_FAST                1 (b)
             15 LOAD_CONST               1 (5)
             18 COMPARE_OP               2 (==)
             21 POP_JUMP_IF_FALSE       27

  9          24 JUMP_FORWARD             0 (to 27)
        >>   27 LOAD_CONST               0 (None)
             30 RETURN_VALUE        

*** Third comparison ***
 12           0 LOAD_FAST                0 (a)
              3 LOAD_FAST                1 (b)
              6 DUP_TOP             
              7 ROT_THREE           
              8 COMPARE_OP               2 (==)
             11 JUMP_IF_FALSE_OR_POP    23
             14 LOAD_CONST               1 (5)
             17 COMPARE_OP               2 (==)
             20 JUMP_FORWARD             2 (to 25)
        >>   23 ROT_TWO             
             24 POP_TOP             
        >>   25 POP_JUMP_IF_FALSE       31

 13          28 JUMP_FORWARD             0 (to 31)
        >>   31 LOAD_CONST               0 (None)
             34 RETURN_VALUE