[Tutor] The trap of the year

[Steven D'Aprano]

Karim wrote:
> 
> Hello Bob,
> 
> I know this fact for function but in this case this is not a function 
> but a constructor method of a class.

Methods *are* functions. (Technically, they are lightweight wrappers 
around functions.) They are treated exactly the same by Python. The 
"constructor method" __init__ is treated not special. You can easily see 
this by printing the function from *inside* the class, before it gets 
wrapped by the class machinery:

 >>> class Test:  # From Python 3, Python 2 may be a bit different.
...     def __init__(myname, arg=[]):
...         pass
...     print(__init__)
...     print(__init__.__defaults__)
...
<function __init__ at 0xb7c1de2c>
([],)

And there you can clearly see the list used as a default value.

It is a little bit harder from outside, because the function is wrapped 
in a method-wrapper, but not that hard:

 >>> instance = Test()
 >>> instance.__init__.__func__
<function __init__ at 0xb7c1de2c>
 >>> instance.__init__.__func__.__defaults__
([],)




-- 
Steven