How to define `bool` conversion for custom data type?

Question

I’m extending Python with a new type, and looking New Types section of Python manual I don’t see anything for bool conversion. How do I specify how my type is converted to a bool?

For instance in the example below native numpy type to is interpreted as Python bool somehow

import numpy as np
if (np.array(False)):
  print 'hi'
else:
  print 'hey'

Looking at bytecode, this corresponds to POP_JUMP_IF_FALSE but this doesn’t seem useful for tracking down the conversion logic.

Asked By: Yaroslav Bulatov

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Source

Answer 1

In the Python documentation, under Special Method Names, you can see that the .__bool__() method does exactly this.

class MyType(object):
    def __init__(self, value):
        self.value = value
    def __bool__(self):
        return self.value != 0

If you are still using Python 2.x then the method name is __nonzero__ instead of __bool__ (see Python 2.7 Documentation).

Answered By: Dietrich Epp

How to define `bool` conversion for custom data type?

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