Python: how if is handled?

Question

Could you please advise how python runs several terms if?

For example:

   a = 0
   b = 0
   c = 0

   if a == 0 and b == 1 and c == 0:
   # test fails

I guess python internally splits the test into 3 if. But then, there are 2 possible cases:

python runs the all 3 if one by one, and one of them is wrong, the test fails
or python runs if one by one, at the first failed if, the test fails and the others if are not run

How does python run this test internally?

Thank you and regards, Hugo

It's short-circuited... ie, the first condition to *not* meet the criteria makes the test fail... — Jon Clements, Sep 17 '13 at 08:32
See http://stackoverflow.com/questions/13960657/does-python-evaluate-ifs-paramaters-lazily — doctorlove, Sep 17 '13 at 08:35
To be exact, this is not a property of the `if` clause, but of the `and` / `or` operators. — glglgl, Sep 17 '13 at 08:35

score 11 · Accepted Answer · answered Sep 17 '13 at 08:33

11

and is a short-circuit operator.

The second argument is evaluated if the first one is True. Similarly, for the subsequent arguments.

answered Sep 17 '13 at 08:33

devnull

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score 5 · Answer 2 · answered Sep 17 '13 at 08:37

This doesn't have anything to do with the conditional clause, but the boolean operators and and or. They are short-circuit operators. If the first value is False, then False is immediately used. Otherwise, the second value is evaluated.

Here's a nice example:

>>> def a():
...     print 'a is running!'
...     return True
... 
>>> def b():
...     print 'b is running!'
...     return False
... 
>>> def c():
...     print 'c is running!'
...     return True
... 
>>> if a() and b() and c():
...     print 'hello!'
... 
a is running!
b is running!

Because b returns False, c does not end up running because there is no need.

score 3 · Answer 3 · answered Sep 17 '13 at 08:32

3

The second. and/or are short-circuit operators - if there is no need, the second argument is not evaluated. See the doc boolean-operations-and-or-not.

answered Sep 17 '13 at 08:32

Roman Bodnarchuk

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doctorlove · Answer 4 · 2013-09-17T08:39:00.630

3

Python uses "lazy evaluation" for if: See the docs

"The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned."

edited Sep 17 '13 at 08:39

answered Sep 17 '13 at 08:32

doctorlove

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5

I'm not sure that it's correct to say that python uses lazy evaluation. OP if statement evaluates conditions lazily, yes. The correct term here would be http://en.wikipedia.org/wiki/Short-circuit_evaluation. – Roman Pekar Sep 17 '13 at 08:34
1

from wiki: In languages that use lazy evaluation by default (like Haskell), all functions are effectively "short-circuit", and special short-circuit operators are unnecessary. – Roman Pekar Sep 17 '13 at 08:36

Python: how if is handled?

4 Answers4