On 1/2/06, Mike Meyer <[EMAIL PROTECTED]> wrote: > "John M. Gabriele" <[EMAIL PROTECTED]> writes: > > > Consider the following: > > > > #!/usr/bin/python > > > > #----------------------------------------------------------------- > > class Grand_parent( object ): > > > > def speak( self ): > > print 'Grand_parent.speak()' > > self.advise() > > > > def advise( self ): > > print 'Grand_parent.advise()' > > self.critique() > > > > def critique( self ): > > print 'Grand_parent.critique()' > > > > > > #----------------------------------------------------------------- > > class Parent( Grand_parent ): > > > > def speak( self ): > > print '\tParent.speak()' > > self.advise() > > > > def advise( self ): > > print '\tParent.advise()' > > self.critique() > > > > # ATM, the Parent is at a loss for words, and has no critique. > > > > > > #----------------------------------------------------------------- > > class Child( Parent ): > > > > def speak( self ): > > print '\t\tChild.speak()' > > self.advise() > > > > # Currently, the Child has no really useful advice to give. > > > > def critique( self ): > > print '\t\tChild.critique()' > > > > > > #----------------------------------------------------------------- > > print 'speak() calls advise(), then advise() calls critique().' > > print > > > > people = [ Grand_parent(), Parent(), Child() ] > > for person in people: > > person.speak() > > print > > > > > > > > ==================== > > The output is: > > > > speak() calls advise(), then advise() calls critique(). > > > > Grand_parent.speak() > > Grand_parent.advise() > > Grand_parent.critique() > > > > Parent.speak() > > Parent.advise() > > Grand_parent.critique() > > > > Child.speak() > > Parent.advise() > > Child.critique() > > > > > > What's going on here with that last "Child.critique()"? The > > Parent called self.critique(), and since it *had* no critique() > > method, it should've deferred to it's parent's critique() > > method, right? But instead, somehow Child.critique() got called. > > Why? > > Because that's the way Python searchs for object attributes. Nobody > made it explicit, so I will. This is simplified, ignoring various > complications: > > When looking up a value for self.foo, you first look for > attribute foo of self. You then check the class of self for > attribute foo. You then check the parent class of the last > class you checked for attribute foo. You repeat that last step > until there is no parent class. > > Like I said, that's simplified. But it's sufficent to explain what > you're seeing.
If your interested in the non-simplified complicated rules for exactly how methods are looked up (only different than the simple case when you get into multiple inheritence), see http://www.python.org/2.3/mro.html > > <mike > -- > Mike Meyer <[EMAIL PROTECTED]> > http://www.mired.org/home/mwm/ > Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information. > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
