Class factory functions

[Steven D'Aprano]

Here's a simple class-factory function that returns a sub-class of the
old-style class it is passed.

def verbosify_oclass(klass):
"""Returns a verbose sub-class of old-style klass."""
class VClass(klass):
def __init__(self, *args, **kwargs):
print "Calling initializer __init__ ..."
klass.__init__(self, *args, **kwargs)
return VClass


Here it is in action:
.... def __init__(self, colour): self.colour = colour
....'red'


Here's an equivalent for new-style classes. It uses super() because I
understand that super() is preferred to calling the super-class by name.


def verbosify_nclass(klass):
"""Returns a verbose sub-class of new-style klass."""
class VClass(klass):
def __new__(cls, *args, **kwargs):
print "Calling constructor __new__ ..."
return super(klass, cls).__new__(cls, *args, **kwargs)
def __init__(self, *args, **kwargs):
print "Calling initializer __init__ ..."
super(klass, self).__init__(*args, **kwargs)
return VClass



But it doesn't work:
Calling constructor __new__ ...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __new__
TypeError: object.__new__(VClass) is not safe, use int.__new__()


What am I doing wrong?


Here's one solution: dump the call to super() and call the super-class
directly. It seems to work. Are there any problems in not using super()?
What about multiple inheritance?


def verbosify_nclass2(klass):
"""Returns a verbose sub-class of new-style klass."""
class VClass(klass):
def __new__(cls, *args, **kwargs):
print "Calling constructor __new__ ..."
return klass.__new__(klass, *args, **kwargs)
def __init__(self, *args, **kwargs):
print "Calling initializer __init__ ..."
super(klass, self).__init__(*args, **kwargs)
return VClass

 

[Peter Otten]

Here's a simple class-factory function that returns a sub-class of the
old-style class it is passed.

def verbosify_oclass(klass):
"""Returns a verbose sub-class of old-style klass."""
class VClass(klass):
def __init__(self, *args, **kwargs):
print "Calling initializer __init__ ..."
klass.__init__(self, *args, **kwargs)
return VClass


Here it is in action:

... def __init__(self, colour): self.colour = colour
...
'red'


Here's an equivalent for new-style classes. It uses super() because I
understand that super() is preferred to calling the super-class by name.


def verbosify_nclass(klass):
"""Returns a verbose sub-class of new-style klass."""
class VClass(klass):
def __new__(cls, *args, **kwargs):
print "Calling constructor __new__ ..."
return super(klass, cls).__new__(cls, *args, **kwargs)
def __init__(self, *args, **kwargs):
print "Calling initializer __init__ ..."
super(klass, self).__init__(*args, **kwargs)
return VClass



But it doesn't work:

Calling constructor __new__ ...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __new__
TypeError: object.__new__(VClass) is not safe, use int.__new__()


What am I doing wrong?

Why would you skip VClass in the super() calls? This should work:

def verbosify_nclass(klass):
"""Returns a verbose sub-class of new-style klass."""
class VClass(klass):
def __new__(cls, *args, **kwargs):
print "Calling constructor __new__ ..."
return super(VClass, cls).__new__(cls, *args, **kwargs)
def __init__(self, *args, **kwargs):
print "Calling initializer __init__ ..."
super(VClass, self).__init__(*args, **kwargs)
return VClass

Peer

 

[Steven D'Aprano]

Why would you skip VClass in the super() calls?

That's a good question. If I every get a good answer, I'll let you know.

This should work:

[snip]

As indeed it seems like it does.

Thank you,