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Monday, December 9, 2013

A few words on boolean casting/mapping in python

So I thought I'd write a few words about truth value testing in python. It's quite common to if some_object conditionals, and I believe that programmers are not always aware of what actually happens when such a line is being evaluated, and tend to perceive it only as len(some_object) > 0 if it's a collection or some_object is not None in other cases. We may refer to the python docs to verify it:

Called to implement truth value testing and the built-in operation bool(); should return False or True, or their integer equivalents 0 or 1. When this method is not defined, __len__() is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither __len__() nor __nonzero__(), all its instances are considered true.
So basicly when we compose something like this:


Is evaluated like this:

some_obj.__nonzero__() if hasattr(some_obj, '__nonzero__') else len(some_obj) <> 0 if hasattr(some_obj, '__len__') else True

So basically conditionals that  made with premeditation not from laziness, especially when using 3rd-party libraries/frameworks. For example, when using python requests,

>>> import requests
>>> res = requests.get('')
>>> res
<Response [404]>
>>> bool(res)
>>> res = requests.get('')
>>> res
<Response [200]>
>>> bool(res)

In case of python-requests http errors are not raised by default, instead if a status code of 4XX or 5XX is returned, the __nonzero__ method returns False. In any case res is not None is always True in the above case.
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