Evaluation of boolean expressions in Python

Question

What truth value do objects evaluate to in Python?

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• Boolean Value of Objects in Python: Discussion about overriding the way it is evaluated

Answer 1

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

• None

• False

• zero of any numeric type, for example, 0, 0L, 0.0, 0j.

• any empty sequence, for example, '', (), [].

• any empty mapping, for example, {}.

• instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False.

All other values are considered true -- so objects of many types are always true. Operations and built-in functions that have a Boolean result always return 0 or False for false and 1 or True for true, unless otherwise stated. (Important exception: the Boolean operations "or" and "and" always return one of their operands.)

https://docs.python.org/2/library/stdtypes.html#truth-value-testing

And as mentioned, you can override with custom objects by modifying nonzero.

Answer 2

Update: Removed all duplicate infomation with Meder's post

For custom objects in Python < 3.0 __nonzero__ to change how it is evaluated. In Python 3.0 this is __bool__ (Reference by e-satis)

It is important to understand what is meant by evaluate. One meaning is when an object is explicitly casting to a bool or implicitly cast by its location (in a if or while loop).

Another is == evalutation. 1==True, 0==False, nothing else is equal via ==.

>>> None==False
False
>>> 1==True
True
>>> 0==False
True
>>> 2==False
False
>>> 2==True
False

Finally, for is, only True or False are themselves.