Vipul Sharma wrote:
> Hello,
>
> 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*
>
> But a few days ago, I just involuntarily wrote a similar condition check
> as
> :
>
> *if a == b and b == 5:*
> * # do something *
>
> which made me think, is there any difference between the two ?
>
> Is there any difference, any difference in the process of evaluation or
> execution ? and also which one is the better ?
>
> I think this is a general programming question and not specifically for
> python.
>
> P.S. : Newbie here :)
In mathematics there is a property called "transitivity" which basically
says that an operation op is transitive if from
(a op b) and (a op c)
follows
b op c
Integer equality is such an operation, and this holds for Python, too. If a
and b are integers you can safely assume that both conditions are
equivalent. There is even a third way to spell this:
if a == b == 5:
...
But for arbitrary objects transitivity is not guaranteed, and you may see
different outcomes. Here is a simple class that implements non-transitive
equality:
>>> class A:
... def __eq__(self, other):
... return isinstance(other, int)
...
>>> a = A()
>>> b = A()
>>> if a == 5 and b == 5:
... print("yes")
... else:
... print("no")
...
yes
>>> if a == b and b == 5:
... print("yes")
... else:
... print("no")
...
no
When you write a == b Python under the hood translates that to
a.__eq__(b), and when you implement your own __eq__() method you are free to
do anything you like. You can even vary the output between calls:
>>> import random
>>> class B:
... def __eq__(self, other):
... return random.choice([True, False])
...
>>> c = B()
>>> c == 42
False
>>> c == 42
False
>>> c == 42
True
>>> c == 42
False
>>> c == 42
True
Of course nobody (I hope) would recommend that you actually write such code.
PS: Can you guess what a == a prints?
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