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
... return a+b
...
... pass
...
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: function() argument 1 must be code, not str
# What's 'function'? Why is it called?

class Spam(1): pass

...
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: int() takes at most 2 arguments (3 given)
# what were the three given arguments?
# is it something I can redefine?
... def __getattr__(self, name):
... print "Trying to get", repr(name)
... raise AttributeError(name)
...

class Spam(Report()): pass

...
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: this constructor takes no arguments
# doesn't look like it. What if I derive from an instance
# derived from object?
... def __getattr__(self, name):
... print "Trying to get", repr(name)
... raise AttributeError(name)
...

class Spam(Report()): pass

...
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: default __new__ takes no parameters
# Okay.... Don't know what's going on, so I'll
# just fiddle around a bit.
... def __init__(self): pass
...

class Spam(ABCD()): pass

...
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: __init__() takes exactly 1 argument (4 given)
... def __init__(self, a): pass
...

class Spam(ABCD()): pass

...
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: __init__() takes exactly 2 arguments (1 given)
# Which is it; 4 given or 1 given? And
# int had 3 passed to it....
... def __init__(self, **args): print "I have", args
...

class Spam(XYZZY()): pass

...
I have {}
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: __init__() takes exactly 1 argument (4 given)
Comments?

Andrew
***@dalkescientific.com

[Thomas Heller]

Post by 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
... return a+b
...
... pass
...
File "<interactive input>", line 1, in ?
TypeError: function() argument 1 must be code, not str
# What's 'function'? Why is it called?

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
... return a + b
...

Post by Andrew Dalke

print spam.__class__

<type 'function'>

Post by Andrew Dalke

spam.__class__("Spam", (spam,), {})

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: function() argument 1 must be code, not str
Thomas

[Michael Hudson]

Post by Thomas Heller

Post by 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
... return a+b
...
... pass
...
File "<interactive input>", line 1, in ?
TypeError: function() argument 1 must be code, not str
# What's 'function'? Why is it called?

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...

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