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Consider the following code:
import sys
class FirstClass:
def __init__(self, value): self.data = ""> def __add__(self, value): return FirstClass(self.data + value) def display(self): print self.data class
SecondClass(FirstClass):
def __add__(self, value): # Generalized version of SecondClass(self.data + value). sum = FirstClass(self.data) + value return SecondClass(sum.data) def display2(self): print self.data x = SecondClass(1)
# Must reimplement __add__ in SecondClass or else x after the assignment is # an instance of class FirstClass not SecondClass. This implies that all # subclasses of classes that overload operators such as + must reimplement # those methods to ensure that the class being returned is correct. x += 1 x.display2() Note the comments in the code and observe that
as is execution yields:
C:\Files\Python> problem.py
2 But if the reimplementation of __add__ in
SecondClass is removed (or commented out) then as claimed in the comment above the code does not work:
C:\Files\Python> problem.py
Traceback (most recent call last): File "C:\Files\Python\problem.py", line 25, in ? x.display2() AttributeError: FirstClass instance has no attribute 'display2' This is because x after the statement x += 1 has
been executed is an object of class FirstClass because the method implementing
addition in FirstClass explicitly returns a FirstClass object.
This seems to me to be a big problem because what
this means is that if class X overloads any operator, e.g. addition,
then all subclass of X (and all subclasses of those subclasses
recursively)
must reimplement all the operator methods
defined in X.
Is this a real problem with Python or just the way
I am coding it as I admit that I am new to Python?
If its my fault then what is the correct way to
implement SecondClass?
Thanks,
Edward
P.S. Why were slices ([low:high]) implemented
to return the low'th to high-1'th values of a sequence? For example I
would expect that [2:4] of "abcdef" would return "cde" not "cd" and this is the
case in other languages that do support slices of strings (HP BASIC) and arrays
(Perl). But I do accept that this a personal preference of mine so if
folks don't agree with me that is fine.
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