CODEWITHSUNDEEP
pythonpython-3.x
Kidus
Kidus

Reputation: 1833

Comparison operators and 'is' - operator precedence in python?

So I was looking at some code online and I came across a line (at line 286):
if depth > 0 and best <= -MATE_VALUE is None and nullscore > -MATE_VALUE:

The part I had trouble understanding was the best <= -MATE_VALUE is None.

So I fired up the interpreter to see how a statement such as value1 > value2 is value3 work.
So I tried 

>>> 5 > 2 is True
False

>>> (5 > 2) is True 
True

>>> 5 > (2 is True) 
True


My Question

Why is 5 > 2 is True not True? And how do these things generally work?

Thanks.

Upvotes: 9

Views: 365

Answers (2)

Daniel Lenz
Daniel Lenz

Reputation: 3857

First, 5 > 2 is True is equivalent to (5 > 2) and (2 is True) because of operator chaining in python (section 5.9 here).

It's clear that 5 > 2 evaluates to True. However, 2 is True will evaluate to False because it is not implicitly converted to bool. If you force the conversion, you will find that bool(2) is True yields True. Other statements such as the if-statement will do this conversion for you, so if 2: will work.

Second, there is an important difference between the is operator and the == operator (taken from here):

Use is when you want to check against an object's identity (e.g. checking to see if var is None). Use == when you want to check equality (e.g. Is var equal to 3?).

>> [1,2] is [1,2]
False

>> [1,2] == [1,2]
True

While this does not have an immediate impact on this example, you should keep it in mind for the future.

Upvotes: 3

muddyfish
muddyfish

Reputation: 3650

You're seeing python's operator chaining working

5 > 2 is True

Is equivalent to

5>2 and 2 is True

You can see this in that

>>> 5>2 is 2

Returns True.

Upvotes: 8

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