Usage Of The "==" Operator For Three Objects

Is there any computational difference between these two methods of checking equality between three objects? I have two variables: x and y. Say I do this: >>> x = 5 >>

Solution 1:

Python has chained comparisons, so these two forms are equivalent:

x == y == z
x == y and y == z

except that in the first, y is only evaluated once.

This means you can also write:

0 < x < 10
10 >= z >= 2

etc. You can also write confusing things like:

a < b == c is d   # Don't  dothis

Beginners sometimes get tripped up on this:

a < 100isTrue# Definitely don't do this!

which will always be false since it is the same as:

a < 100and100isTrue# Now we see the violence inherent in the system!

Solution 2:

Adding a little visual demonstration to already accepted answer.

For testing equality of three values or variables. We can either use:

>>> print(1) == print(2) == print(3)
123True>>> print(1) == print(2) andprint(2) == print(3)
1223True

The above statements are equivalent but not equal to, since accesses are only performed once. Python chains relational operators naturally. See this docs:

Comparisons can be chained arbitrarily, e.g., 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).

If the functions called (and you are comparing return values) have no side-effects, then the two ways are same.

In both examples, the second comparison will not be evaluated if the first one evaluates to false. However: beware of adding parentheses. For example:

>>> 1 == 2 == 0False>>> (1 == 2) == 0True

See this answer.