Roy Smith wrote:
> Andy Leszczynski <leszczynscyATnospam.yahoo.com.nospam> writes:
>
(snip)
>>It's not a true statement. Nothing in the language enforces LSP. In
>>fact, there's not even a [way?] when a function/method is invoked to make
>>sure the type passed in is a subtype of the type you expect
>
>
> Well, that's not entirely true. I could write:
>
> def func (self, obj):
> assert (isinstance (obj, baseClass))
>
> It's sort of un-pythonic, but the language certainly lets you do it if you
> really want to.
It doesn't inforce LSP anyway.
class baseClass(object):
def f(self):
print "in %s f" % self
return 42
class derivedClass(baseClass):
f = "gotcha"
class somethingElse(object)
def f(self):
return 42
def func1(obj):
assert isinstance (obj, baseClass)
obj.f()
def func2(obj):
assert(callable(getAttr(obj, f))
obj.f()
b = baseClass()
d = derivedClass()
s = somethingElse()
func1(b) # ok
func1(d) # TypeError
func1(s) # AssertionError
func2(b) # ok
func2(d) # AssertionError
func2(s) # ok
Clearly, somethingElse is a subtype of baseClass, when derivedClass is
not. Don't confuse suclassing with subtyping. issinstance() let you
check for subclassing, not for subtyping. The only language-level
mechanism I know of that more or less inforce LSP is Eiffel's contracts.
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in '[EMAIL PROTECTED]'.split('@')])"
--
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