> I do remain a bit surprised that there seems to be no way to implement
> what I naively thought would be the obvious solution -- to remove an
> inherited method from the instance's dictionary.
Hi Brian,
If we're trying to do this, we probably don't want to "inherit" from a
parent. A subclass child is supposed to have, at the very least, the same
public methods as its parent. For example, if we have a:
######
class Account:
def __init__(self, amount):
self.amount = amount
def withdraw(self, x):
assert x > 0
self.amount = self.amount - x
def report(self):
print "You have ", self.amount, "dollars"
#######
And we'd like to reuse this, but for something that doesn't report itself,
we shouldn't use inheritance:
#######
class SecretAccount(Account):
def report(self):
pass
#######
This SecretAccount is now pretending to be an account that doesn't have a
usable report() method.
#######
>>> Account(5).report()
You have 5 dollars
>>> SecretAccount(5).report()
>>>
#######
But we can get into trouble again, if, later on, Account is expanded to
have a few more additional functions:
######
class Account: ## version 2
# [same as before]
def htmlReport(self):
print ("<html><body><p>You have %d dollars</p></body></html" %
self.amount)
######
And now, suddenly, SecretAccount again has a method that shows information
that it probably doesn't want out in the open.
The issue that that SecretAccount is really not trying to be an "Account":
it's just trying to reuse its functionality. Instead of doing things with
inheritance, we're probably looking for delegation:
#######
class SecretAccount:
def __init__(self, amount):
self.account = Account(amount)
def withdraw(self, x):
self.account.withdraw(x)
#######
And now we're more isolated from changes to the Account class.
There are some more Pythonic idioms to make delegation easier to code.
And the example above is hideously toyish. *grin*
But I hope the idea's a little clearer: inheritance ties the subclass down
to at least what the superclass has promised, and in many cases, that's
not what we want.
Best of wishes!
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