Overriding 'to boolean' operator in python?

Question

I'm using a class that is inherited from list as a data structure:

class CItem( list ) : pass oItem = CItem() oItem.m_something = 10 oItem += [ 1, 2, 3 ]

All is perfect, but if I use my object of my class inside of an 'if', python evaluates it to False if underlying the list has no elements. Since my class is not just list, I really want it to evaluate False only if it's None, and evaluate to True otherwise:

a = None if a : print "this is not called, as expected" a = CItem() if a : print "and this is not called too, since CItem is empty list. How to fix it?"

+1 since you appear to uncover yourself another wonderful Python gem :) — ilya n.
– ilya n., Commented Aug 14, 2009 at 21:35

Elias Zamaria · Accepted Answer · 2015-02-05 18:13:21Z

47

In 2.x: override __nonzero__(). In 3.x, override __bool__().

edited Feb 5, 2015 at 18:13

Elias Zamaria

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answered Apr 17, 2009 at 18:21

John Millikin

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