Python class __init__(mandatory arguments?)

Erik Max Francis max at alcyone.com
Mon Jun 16 01:30:32 CEST 2003


Lee John Moore wrote:

> Just for confirmation really: When I create a custom class, is
> it *really* true that def __init__() arguments can only ever be
> optional?

No, you're doing something wrong.  There's nothing special about the
optionality of __init__'s arguments:

>>> class C:
...  def __init__(self, x, y): pass
... 
>>> c = C()

> class MyClass:
>         param1 = None
>         param2 = None

These two are class attributes.

>         def __init__(self, param1, param2):
>                 param1 = self.param1
>                 param2 = self.param2

These assignments are reversed.

> I'm so accustomed to constructor arguments being mandatory in
> C++ & OP classes....and yes, yes, I realise __init__() is not a
> constructor (even though it looks like one), ...

"Constructor" is a perfectly fine name for Python's __init__; it does a
precisely analogous thing in Python that constructors do in C++ and
Java.  Constructors are really initializers, after all.

> ... but if there's a
> way I can *force* arguments upon class instantiation, I'd like
> to know about it. :-)

Maybe if you showed us a precise code example that's exhibiting this
behavior we could help.  As it is, even the code sample you show above
doesn't exhibit this behavior.

-- 
   Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
 __ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
/  \ Sit loosely in the saddle of life.
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