boolean context of a python object

Question

Can I use a python object in a desired boolean context? By default any object is True in boolean context.

>>> class Person():
...           pass
... 
>>> a=Person()
>>> bool(a)
True

Like bool(0) returns False and bool(1) returns True.Can i have any way to define an object to have it's boolean value either True or False. Correct me if i'm wrong anywhere,thanks.

score 8 · Accepted Answer · answered Jan 24 '14 at 12:39

8

class Something(object):
    def __nonzero__(self):
        return False  # Something is False always. 

print bool(Something())

Take note __nonzero__ has been renamed to __bool__ in Python 3.x

As pointed out in a comment, you would be better to define __len__ on container like collections. When __len__ returns 0, bool(x) will evaluate False. Otherwise any positive number above 1 will equate to True.

answered Jan 24 '14 at 12:39

Jakob Bowyer

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The other option is to implement `__len__` (like a container); return `0` will translate to `False` in a boolean context, any integer over 1 is `True`. – Martijn Pieters Jan 24 '14 at 12:41
@MartijnPieters Edited to include – Jakob Bowyer Jan 24 '14 at 12:43
@MartijnPieters thanks for pointing out the __len__ method solution to the above question. – darxtrix Jan 24 '14 at 12:50

score 1 · Answer 2 · edited Jan 24 '14 at 12:46

1

See the documentation on Truth Value Testing. It first checks if your instance has a __nonzero__ method, if not it uses a __len__ method. The instance is False if __nonzero__ is False or its length is 0.

edited Jan 24 '14 at 12:46

Daniel Roseman

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answered Jan 24 '14 at 12:41

RemcoGerlich

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score 0 · Answer 3 · answered Jan 24 '14 at 12:42

0

object.__nonzero__ and object.__bool__

answered Jan 24 '14 at 12:42

Filip Malczak

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boolean context of a python object

3 Answers3