On Class namespaces, calling methods
gandalf at shopzeus.com
Sat Apr 10 17:28:31 CEST 2010
> class Uno:
> def m():
> print "mouse"
> Say that I have this "silly" class.
> While I can then write
> print Uno.a
> I cannot write
> I get the following error message:
> TypeError: m() takes no arguments (1 given)
As a workaround, use this pattern:
>>> class Uno(object):
... def m(cls):
... print "mouse"
> Since I have not created any instances of Uno, there is no self
> object, and I do not understand what object is supplied to the
> function call.
The method is not actually called, so nothing is supplied. The error is
raised when it turns out that the number of actual parameters and the
number of formal parameters are different.
This is how you call a method:
#1. you try to call Uno.m() - the 'm' method object is found and taken
#2. argument values are evaluated and taken (in your example, there are
#3. an extra argument is inserted in front of the argument list. This
parameter is the object you where calling the method on. In this case,
it is the 'Uno' class.
#4. actual arguments are assigned to formal parameters. If this fails
for some reason, an exception is raised. In your case - the number of
actual parameters is one, the number of formal parameters is zero. This
is why you get an exception.
#5. If actual and formal parameters are matched, then the implicit self
parameter is checked. If you call a method, then it must be an instance
of the class being called (or an instanc of its subclass). For
classmethods, it must be the same class (or a subclass of it).
#6. Function body executed, value returned
The implicit parameter (the instance, or for classmethods, the class) is
ALWAYS added. So if you change your code:
Then you won't get an exception in #4. But you will in #5, because
instance methods must be called on an instance, not on a class. You can
create a classmethod (e.g. with the @classmethod decorator) as shown
above, and it will work.
> Could anybody explain what argument is being supplied to the method?
> Is ther any workaround to call the m function?
> Thank you
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