What are the rules regarding chaining of "==" and "!=" in Python

Question

This morning, I find myself writing something like:

if (a == b == c):
  # do something

And was surprised that it gave me the expected result.

I thought it would behave as:

if ((a == b) == c):
  # do something

But it obviously didn't. It seems Python is treating the first statement differently from the second, which is nice but I couldn't find any documentation or explanation regarding this.

I tested and got this:

In [1]: 2 == 2 == 2
Out[1]: True

In [2]: (2 == 2) == 2
Out[2]: False

Would someone care to explain me what are the rules regarding such "chaining" of == (or !=) ?

Thank you very much.

Answer 1

As far as I know the example you point out isn't chaining.

2 == 2 == 2 is like (2 == 2) and ( 2 == 2) which turns out to be True and True

while

(2 == 2) == 2 is like (True) == 2

Answer 2

This works with all comparison operators - eg, you can also do:

>>> 4 < 5 < 6
True
>>> 4 < 5 !=2
True

In general, according to the documentation, a op1 b op2 c where op1 and op2 are any of: <, >, !=, ==, <=, >=, is , is not, in or not in will give the same result as:

a op1 b and b op2 c

The docs also say that this can work with arbitrarily many comparisons, so:

>>> 5 != '5' != 'five' != (3+2)
True

Which can be a slightly confusing result sometimes since it seems to say 5 != (3+2) - each operand is only compared with the ones immediately adjacent to it, rather than doing all possible combinations (which mightn't be clear from examples using only ==, since it won't affect the answer if everything defines __eq__ sanely).

Answer 3

Check here: http://docs.python.org/reference/expressions.html#not-in