__init__ return value

[Alex Martelli]

Skip Montanaro wrote:

> 
>     >> No, __init__ method return values are always ignored.
> 
>     Bob> Oh?
> 
>     >>>> class A:
>     Bob> ...     def __init__(self):
>     Bob> ...             return 'asdf'
>     Bob> ...
>     >>>> A()
>     Bob> Traceback (most recent call last):
>     Bob>    File "<interactive input>", line 1, in ?
>     Bob> TypeError: __init__() should return None
> 
> My apologies.  It is checked and an exception raised if it's not None.
> That's an implementation detail, not a property of the language.

Actually...:

>>> import this
The Zen of Python, by Tim Peters
   [snip]
Errors should never pass silently.
Unless explicitly silenced.

I _would_ call this "a property of the language".  Returning not-None
from __init__ is most likely an error: ignoring that error silently by
default would be very un-Pythonic indeed.

As a point of contrast, look at Ruby -- a pretty neat language with a
semantic level and application niche QUITE close to Python's, but most
definitely NOT having "errors should never pass silently" as a
property of the language.  Most fundamental errors, say indexing an
array with an index outside of the array's bounds, DO pass silently in
Ruby -- returning nil (like Python's None) for indexing-reads out of
bounds, silently and implicitly padding the array with nil's for
indexing-writes out of bounds.  I think this language property and its
practical implications make an ENORMOUS difference to the approach
one takes when programming in the two different languages...


Alex