How to set a class inheritance at instance creation?

G

glomde

Hi I wonder if you can set what subclass a class should
have at instance creation.

The problem is that I have something like:

class CoreLang():
def AssignVar(self, var, value):
pass

class Lang1(CoreLang):
def AssignVar(self, var, value):
return var, "=", value

class Lang2(CoreLang):
def AssignVar(self, var, value):
return var, "<=", value

class WriteStruct():
def Generate(self, vars):
for var in vars:
print self.AssignVar()

The problem is that I want WriteStruct to sometimes be a subclass of
Lang1 and sometimes
of Lang2.
In the above example I could but the Generate Method in CoreLang. But
in my real
example I also want to able to subclass WriteStruct to be able to easy
customize WriteStruct.
Which I wouldnt be able to do if it was a method in CoreLang.

So in code I would like to write something like:

WriteStruct(Lang1).Generate(vars)

Even better would be that if I in the Lang1 class could
just do WriteStruct().Generate(vars) and Lang1 class would
magically make WriteStruct a subclass of itself.


Cheers,

/T
 
R

Ramashish Baranwal

Hi I wonder if you can set what subclass a class should
have at instance creation.

The problem is that I have something like:

class CoreLang():
def AssignVar(self, var, value):
pass

class Lang1(CoreLang):
def AssignVar(self, var, value):
return var, "=", value

class Lang2(CoreLang):
def AssignVar(self, var, value):
return var, "<=", value

class WriteStruct():
def Generate(self, vars):
for var in vars:
print self.AssignVar()

The problem is that I want WriteStruct to sometimes be a subclass of
Lang1 and sometimes
of Lang2.
In the above example I could but the Generate Method in CoreLang. But
in my real
example I also want to able to subclass WriteStruct to be able to easy
customize WriteStruct.
Which I wouldnt be able to do if it was a method in CoreLang.

So in code I would like to write something like:

WriteStruct(Lang1).Generate(vars)
Click to expand...

class WriteStruct:
def __init__(self, SubClass):
self._sub = SubClass()
def Generate(self, vars):
for var in vars:
print self._sub.AssignVar()

This does what you want but isn't inheritance.
Even better would be that if I in the Lang1 class could
just do WriteStruct().Generate(vars) and Lang1 class would
magically make WriteStruct a subclass of itself.
Click to expand...

I don't think I understood what you want here.

Ram
 
G

glomde

Why not just have Lang1 and Lang2 inherit from WriteStruct as well?
Click to expand...

This wont work I think since if add antoher Class:

class WriteStruct():

def func1(self);
print "Hello2"

def Generate(self):
self.func1()

class WriteStruct2(WriteStruct):

def func1(self);
print "Hello"

def Generate(self):
self.func1()

Den if Lang1, inherit both WriteStruct and WriteStruct2 I will
get name clashes.

In my real code I have very big Generate Method in WriteStruct that
calls submethods.
and thes submethods should be overriden by the subclasses. So I cant
change the name
on the submethods.
 
G

glomde

class WriteStruct:
def __init__(self, SubClass):
self._sub = SubClass()
def Generate(self, vars):
for var in vars:
print self._sub.AssignVar()

This does what you want but isn't inheritance.
Click to expand...

This would work I think. Thanks.
I don't think I understood what you want here.
Click to expand...
I just dont want to pass the class in the instancecreation.
Somhehow in my:

class Lang1(CoreLang):

def Function(self):
WriteStruct().Generate()

Then somehow this WriteStruct should magically know that has been
instantiated in Lang1.
But this is not really needed. Just wondered if it was possible.
 
R

Ramashish Baranwal

Hi I wonder if you can set what subclass a class should
have at instance creation.

The problem is that I have something like:

class CoreLang():
def AssignVar(self, var, value):
pass

class Lang1(CoreLang):
def AssignVar(self, var, value):
return var, "=", value

class Lang2(CoreLang):
def AssignVar(self, var, value):
return var, "<=", value

class WriteStruct():
def Generate(self, vars):
for var in vars:
print self.AssignVar()

The problem is that I want WriteStruct to sometimes be a subclass of
Lang1 and sometimes
of Lang2.
In the above example I could but the Generate Method in CoreLang. But
in my real
example I also want to able to subclass WriteStruct to be able to easy
customize WriteStruct.
Which I wouldnt be able to do if it was a method in CoreLang.

So in code I would like to write something like:

WriteStruct(Lang1).Generate(vars)
Click to expand...

class WriteStruct:
def __init__(self, SubClass):
self._sub = SubClass()
def Generate(self, vars):
for var in vars:
print self._sub.AssignVar()
Even better would be that if I in the Lang1 class could
just do WriteStruct().Generate(vars) and Lang1 class would
magically make WriteStruct a subclass of itself.
Click to expand...

I don't think I understood what you want here.

Ram
 
S

Stargaming

glomde said:
Hi I wonder if you can set what subclass a class should
have at instance creation.

The problem is that I have something like:

class CoreLang():
def AssignVar(self, var, value):
pass

class Lang1(CoreLang):
def AssignVar(self, var, value):
return var, "=", value

class Lang2(CoreLang):
def AssignVar(self, var, value):
return var, "<=", value

class WriteStruct():
def Generate(self, vars):
for var in vars:
print self.AssignVar()

The problem is that I want WriteStruct to sometimes be a subclass of
Lang1 and sometimes
of Lang2.
In the above example I could but the Generate Method in CoreLang. But
in my real
example I also want to able to subclass WriteStruct to be able to easy
customize WriteStruct.
Which I wouldnt be able to do if it was a method in CoreLang.

So in code I would like to write something like:

WriteStruct(Lang1).Generate(vars)

Even better would be that if I in the Lang1 class could
just do WriteStruct().Generate(vars) and Lang1 class would
magically make WriteStruct a subclass of itself.


Cheers,

/T
Click to expand...

If you really need to inherit at runtime, you could utilize `type()`.
.... print self, blah
....<__main__.MyCls object at 0x009E86D0> 4

Changing ``object`` (the tuple contains all bases) will change the
parent. But, better stick to previous solutions. :)

Stargaming
 
S

Steve Holden

glomde said:
Hi I wonder if you can set what subclass a class should
have at instance creation.

The problem is that I have something like:

class CoreLang():
def AssignVar(self, var, value):
pass

class Lang1(CoreLang):
def AssignVar(self, var, value):
return var, "=", value

class Lang2(CoreLang):
def AssignVar(self, var, value):
return var, "<=", value

class WriteStruct():
def Generate(self, vars):
for var in vars:
print self.AssignVar()

The problem is that I want WriteStruct to sometimes be a subclass of
Lang1 and sometimes
of Lang2.
In the above example I could but the Generate Method in CoreLang. But
in my real
example I also want to able to subclass WriteStruct to be able to easy
customize WriteStruct.
Which I wouldnt be able to do if it was a method in CoreLang.

So in code I would like to write something like:

WriteStruct(Lang1).Generate(vars)

Even better would be that if I in the Lang1 class could
just do WriteStruct().Generate(vars) and Lang1 class would
magically make WriteStruct a subclass of itself.
Click to expand...
You should rethink your program design.

It seems that when you create a WriteStruct you should really be passing
its __init__() method the class that you want it to be a "subclass" of,
creating an instance of that class, and then using generic delegation to
that subclass (using a modified __getattr__()) to handle methods that
aren't found in the WriteStruct.

I can see there are circumstances in which this might not work, but I
believe your current ugly intentions reveal a design smell that you
really need to get rid of if you want a clean program.

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
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Get on the web: Blog, lens and tag your way to fame!!
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tagged items: del.icio.us/steve.holden/python
All these services currently offer free registration!
-------------- Thank You for Reading ----------------
 
S

Steve Holden

glomde said:
Hi I wonder if you can set what subclass a class should
have at instance creation.

The problem is that I have something like:

class CoreLang():
def AssignVar(self, var, value):
pass

class Lang1(CoreLang):
def AssignVar(self, var, value):
return var, "=", value

class Lang2(CoreLang):
def AssignVar(self, var, value):
return var, "<=", value

class WriteStruct():
def Generate(self, vars):
for var in vars:
print self.AssignVar()

The problem is that I want WriteStruct to sometimes be a subclass of
Lang1 and sometimes
of Lang2.
In the above example I could but the Generate Method in CoreLang. But
in my real
example I also want to able to subclass WriteStruct to be able to easy
customize WriteStruct.
Which I wouldnt be able to do if it was a method in CoreLang.

So in code I would like to write something like:

WriteStruct(Lang1).Generate(vars)

Even better would be that if I in the Lang1 class could
just do WriteStruct().Generate(vars) and Lang1 class would
magically make WriteStruct a subclass of itself.
Click to expand...
You should rethink your program design.

It seems that when you create a WriteStruct you should really be passing
its __init__() method the class that you want it to be a "subclass" of,
creating an instance of that class, and then using generic delegation to
that subclass (using a modified __getattr__()) to handle methods that
aren't found in the WriteStruct.

I can see there are circumstances in which this might not work, but I
believe your current ugly intentions reveal a design smell that you
really need to get rid of if you want a clean program.

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
------------------ Asciimercial ---------------------
Get on the web: Blog, lens and tag your way to fame!!
holdenweb.blogspot.com squidoo.com/pythonology
tagged items: del.icio.us/steve.holden/python
All these services currently offer free registration!
-------------- Thank You for Reading ----------------
 
G

glomde

You should rethink your program design.

It seems that when you create a WriteStruct you should really be passing
its __init__() method the class that you want it to be a "subclass" of,
creating an instance of that class, and then using generic delegation to
that subclass (using a modified __getattr__()) to handle methods that
aren't found in the WriteStruct.
Click to expand...

This is what I am going to do. For some reason I got stuck to think
I needed to solve it with inheritance.

Didnt think of the possibility to modify getattr to make
the delegation be much nicer. Thanks for the tip
 

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