1

I'd expect the following two code blocks to be evaluated the same way, but it would seem that is not the case. For example, with this:

if True or False and False:
    print('True')                                                               
else:                                                                           
    print('False')

True is printed. But with this:

if (True or False) and False:
    print('True')
else:                                                                           
    print('False')

False is printed. Here's my breakdown of the logic:

  • True or False = True
  • True and False = False

By substitution, (True or False) and False = True and False = False.

Why does this happen?

Andrew Li
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James T.
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3 Answers3

3

This is because of operator precedence. Per the Python 2.x and 3.x docs, the and operator has higher precedence than the or operator. Also, have a look at the boolean truth table:

enter image description here

That means in your expression:

if True or False and False:

In the expression, False and False is grouped together because of precedence. That means Python evaluates it as:

if True or (False and False):

Now, it's evaluated left to right. Since the first condition is True, it short-circuits and skips the second condition and evaluates to True printing 'True'. (This short-circuits because if the first side is true, it has to be true.)

Now in your second example:

if (True or False) and False:

This makes True or False evaluate first, which gives True. Then it does True and False which is False, printing 'False'.

>>> print(True or False)
True
>>> print(True and False)
False
Andrew Li
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  • This has nothing to do with short-circuiting. Non-short-circuit evaluation would have produced the same outcome. – user2357112 Sep 19 '16 at 16:49
  • You're still saying nothing about precedence. By the logic of your answer, `False and True or True` would be `False`. – user2357112 Sep 19 '16 at 17:14
  • @user2357112 I've updated the answer to include correct information. Thanks for pointing out the misinformation and criticism, very much appreciated! – Andrew Li Sep 19 '16 at 20:41
  • Well, you fixed the precedence issues, but now your answer is using the wrong evaluation order. Precedence only controls argument grouping, not evaluation order; the expression is still evaluated left-to-right and short-circuited. – user2357112 Sep 19 '16 at 21:08
  • @user2357112 Right again. Updated :) – Andrew Li Sep 19 '16 at 21:31
1

(True or False) evaluates to True. Therefore (True or False) and False evaluates to True and False, which evaluates to False.

This answer explains the rules for boolean evaluation quite well: https://stackoverflow.com/a/16069560/1470749

Community
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1

Standard Order of operations gives and precedence over or, so the first statement True or False and False is logically equivalent to 

True or (False and False)

This evaluates to True.

The second statement is 

(True or False) and False

This evaluates to False.

notovny
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