Bill de hÓra

Python: don't return from __init__()

March 01, 2007 |  co.mments

Returning a value from __init__() will cause an exception in Python:

In [1]: 
In [1]: class Foo:
   ...:     def __init__(self): return self
   ...:     
   ...:     
In [2]: 
In [2]: f = Foo()
----------------------------------------
exceptions.TypeError                                 
Traceback (most recent call last)
TypeError: __init__() should return None
In [3]: 

That's because __init__() is not the call to cons in Python; it's an initializer called after object instantiation, though most people (myself included) will treat intuitively treat __init__() as the construction point, 'cos that what it looks like and it takes the same arguments logically as the constructor would. It's rare enough you see anyone returning from __init__ as the code will fail fast (not swallowing exceptions is a Python tenet). Usually it happens when a programmer has conditional logic in the initializer or is doing factory type stuff, all combinations of which didn't get executed during development and testing.

The construction method in python is __new__(). There's a bit of detail here - python basic customizations - and the cookbook has an example of overriding __new__().

March 1, 2007 11:37 PM

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