class with invalid base class

[Andrew Dalke]

Out of curiosity, I tried passing using an invalid base
for a class. I can't explain why I got the error messages
I did. Can someone here enlighten me?

# Here I'm just curious
[color=blue][color=green][color=darkred]
>>> def spam(a, b):[/color][/color][/color]
.... return a+b
....[color=blue][color=green][color=darkred]
>>> class Spam(spam):[/color][/color][/color]
.... pass
....
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: function() argument 1 must be code, not str[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

# What's 'function'? Why is it called?
[color=blue][color=green][color=darkred]
>>> class Spam(1): pass[/color][/color][/color]
....
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: int() takes at most 2 arguments (3 given)[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

# what were the three given arguments?
# is it something I can redefine?
[color=blue][color=green][color=darkred]
>>> class Report:[/color][/color][/color]
.... def __getattr__(sel f, name):
.... print "Trying to get", repr(name)
.... raise AttributeError( name)
....[color=blue][color=green][color=darkred]
>>> class Spam(Report()): pass[/color][/color][/color]
....
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: this constructor takes no arguments[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

# doesn't look like it. What if I derive from an instance
# derived from object?
[color=blue][color=green][color=darkred]
>>> class Report(object):[/color][/color][/color]
.... def __getattr__(sel f, name):
.... print "Trying to get", repr(name)
.... raise AttributeError( name)
....[color=blue][color=green][color=darkred]
>>> class Spam(Report()): pass[/color][/color][/color]
....
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: default __new__ takes no parameters[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

# Okay.... Don't know what's going on, so I'll
# just fiddle around a bit.
[color=blue][color=green][color=darkred]
>>> class ABCD:[/color][/color][/color]
.... def __init__(self): pass
....[color=blue][color=green][color=darkred]
>>> class Spam(ABCD()): pass[/color][/color][/color]
....
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: __init__() takes exactly 1 argument (4 given)[color=blue][color=green][color=darkred]
>>> class ABCD:[/color][/color][/color]
.... def __init__(self, a): pass
....[color=blue][color=green][color=darkred]
>>> class Spam(ABCD()): pass[/color][/color][/color]
....
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: __init__() takes exactly 2 arguments (1 given)[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

# Which is it; 4 given or 1 given? And
# int had 3 passed to it....
[color=blue][color=green][color=darkred]
>>> class XYZZY:[/color][/color][/color]
.... def __init__(self, **args): print "I have", args
....[color=blue][color=green][color=darkred]
>>> class Spam(XYZZY()): pass[/color][/color][/color]
....
I have {}
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: __init__() takes exactly 1 argument (4 given)[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Comments?

Andrew
dalke@dalkescie ntific.com

[Thomas Heller]

Re: class with invalid base class

"Andrew Dalke" <adalke@mindspr ing.com> writes:
[color=blue]
> Out of curiosity, I tried passing using an invalid base
> for a class. I can't explain why I got the error messages
> I did. Can someone here enlighten me?
>
> # Here I'm just curious
>[color=green][color=darkred]
>>>> def spam(a, b):[/color][/color]
> ... return a+b
> ...[color=green][color=darkred]
>>>> class Spam(spam):[/color][/color]
> ... pass
> ...
> Traceback (most recent call last):
> File "<interacti ve input>", line 1, in ?
> TypeError: function() argument 1 must be code, not str[color=green][color=darkred]
>>>>[/color][/color]
>
> # What's 'function'? Why is it called?[/color]

It's the Don Beaudry hook ;-). If the base class has callable a
__class__ attribute, this is called with three arguments: The name of
the new class, a tuple of the bases, and a dictionary. Or something like
this, the details may be wrong...

Since now functions have a __class__ attribute, the hook is triggered by
your code above:
[color=blue][color=green][color=darkred]
>>> def spam(a, b):[/color][/color][/color]
.... return a + b
....[color=blue][color=green][color=darkred]
>>> print spam.__class__[/color][/color][/color]
<type 'function'>[color=blue][color=green][color=darkred]
>>> spam.__class__( "Spam", (spam,), {})[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: function() argument 1 must be code, not str[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Thomas

[Michael Hudson]

Re: class with invalid base class

Thomas Heller <theller@python .net> writes:
[color=blue]
> "Andrew Dalke" <adalke@mindspr ing.com> writes:
>[color=green]
> > Out of curiosity, I tried passing using an invalid base
> > for a class. I can't explain why I got the error messages
> > I did. Can someone here enlighten me?
> >
> > # Here I'm just curious
> >[color=darkred]
> >>>> def spam(a, b):[/color]
> > ... return a+b
> > ...[color=darkred]
> >>>> class Spam(spam):[/color]
> > ... pass
> > ...
> > Traceback (most recent call last):
> > File "<interacti ve input>", line 1, in ?
> > TypeError: function() argument 1 must be code, not str[color=darkred]
> >>>>[/color]
> >
> > # What's 'function'? Why is it called?[/color]
>
> It's the Don Beaudry hook ;-). If the base class has callable a
> __class__ attribute, this is called with three arguments: The name of
> the new class, a tuple of the bases, and a dictionary. Or something like
> this, the details may be wrong...[/color]

Actually, I think it's type(baseclass) that gets called -- otherwise
you wouldn't be able to inherit from old-style classes! What's
changed since 2.2 is that type objects are now callable.

(Isn't it amazing how something as simple as metaclasses -- and they
*are* a simple idea -- can be *so* confusing? I wrote and rewrote
that paragraph at least three times...)

Cheers,
mwh

--
But since your post didn't lay out your assumptions, your goals,
or how you view language characteristics as fitting in with
either, you're not a *natural* candidate for embracing Design by
Contract <0.6 wink>. -- Tim Peters, giving Eiffel adoption advice