why am I not allowed to redefine a class ?
Terry Reedy
tjreedy at udel.edu
Sun Nov 9 12:59:50 EST 2008
Stef Mientki wrote:
> hello,
>
> although this is not a real problem for me,
> it was caused by a copying, instead of moving, a piece of code.
> But I don't understand at all why the code below gives the error.
> class derived_class, is defined twice,
> the error is cuase by the second instance creation "test2="
> for me even weirder, if I create test2 in another module, everything
> works perfect ???
> Any explanation would be welcome.
A different version of what Arnaud D. said:
> ==== start of code ===
> class base_class ( object ) :
> def __init__ ( self ) :
> pass
>
> class derived_class ( base_class ) :
> def __init__ ( self ) :
> base_class.__init__ ( self )
>
> class final_class_1 ( derived_class ) :
In this line, 'derived_class' is evaluated when the class is defined
> def __init__ ( self ) :
> derived_class.__init__ ( self )
In this line, 'devired_class' is evaluated when the class is
instantiated and __init__ is called.
> test1 = final_class_1 ()
Here, the object corresponding to 'derived_class in f_c_1.__init__is the
same as when f_c_1 was created.
> class derived_class ( base_class ) :
> def __init__ ( self ) :
> base_class.__init__ ( self )
>
> test2 = final_class_1 ()
Here, object corresponding to 'derived_class in f_c_1.__init__is no
longer the same as when f_c_1 was created.
> ==== end of code =====
>
> ==== error meassage =====
> Traceback (most recent call last):
> File "D:\Data_Python_25\PyLab_Works\module1.py", line 19, in <module>
> test2 = final_class_1 ()
> File "D:\Data_Python_25\PyLab_Works\module1.py", line 11, in __init__
> derived_class.__init__ ( self )
> TypeError: unbound method __init__() must be called with derived_class
This refers to the second 'derived_class', and f_c_1 ia *not* a subclass
of *that* class object.
> instance as first argument (got final_class_1 instance instead)
tjr
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