why am I not allowed to redefine a class ?

[Stef Mientki]

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.
thanks,
Stef Mientki

==== 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 ) :
def __init__ ( self ) :
derived_class.__init__ ( self )

test1 = final_class_1 ()

class derived_class ( base_class ) :
def __init__ ( self ) :
base_class.__init__ ( self )

test2 = final_class_1 ()
==== 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
instance as first argument (got final_class_1 instance instead)

 

[Arnaud Delobelle]

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.
thanks,
Stef Mientki

==== 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 ) :
def __init__ ( self ) :
derived_class.__init__ ( self )

test1 = final_class_1 ()

class derived_class ( base_class ) :
def __init__ ( self ) :
base_class.__init__ ( self )

test2 = final_class_1 ()
==== 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
instance as first argument (got final_class_1 instance instead)

Because after rebinding 'derived_class', the class 'final_class_1' is
not a subclass of 'derived_class'

One solution is to ensure you bind the word 'derived_class' to the
correct class:

class final_class_1(derived_class):
def __init__(self, derived_class=derived_class):
derived_class.__init__ ( self )

This should work (untested).

It's worth noting that one can avoid this problem in Python 3 by using
super():
.... def __init__(self): print('init A')
.... .... def __init__(self):
.... super().__init__()
....