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

def spam(a, b): .... return a+b
.... class Spam(spam): .... 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?
class Report: .... 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?
class Report(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.
class ABCD: .... 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) class ABCD: .... 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....
class XYZZY: .... 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
da***@dalkescientific.com

[Thomas Heller]

"Andrew Dalke" <ad****@mindspring.com> writes:

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

def spam(a, b): ... return a+b
... class Spam(spam): ... 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?

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:

def spam(a, b): .... return a + b
.... print spam.__class__ <type 'function'> 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]

Thomas Heller <th*****@python.net> writes:

"Andrew Dalke" <ad****@mindspring.com> writes:

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

> def spam(a, b):

... return a+b
...

> class Spam(spam):

... 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?

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

--
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