[Tutor] How to make comparison work

Peter Otten __peter__ at web.de
Fri Apr 11 10:32:21 CEST 2014

Gregg Martinson wrote:

> I have been working through a fairly simple process to teach myself python
> and I am running into a problem with a comparison.  Can anyone tell me
> where I am going wrong?
> #!/usr/bin/env python
> class Team(object):
>     code = ""
>     opponents_debated=[]
>     wins=0
>     losses=0
>     competitors=[]

Defining the 'competitors' list here means that it is shared by all Team 
instances. As soon as any team A has debated with a team B B is added to 
this list. As any team immediately adds itself to the list no debate will 
ever take place.

Solution: instead of a class attribute make the list an instance attribute 
by moving the definition into the initialiser:

>     def __init__(self, code):
          self.competitors = []
>         self.code = code
>         self.competitors.append(code)
>         #self.school_teams.append(code)

Note that the difference between class and instance attributes exists for 
all attributes, but may not lead to an error when you rebind instead of 
mutating the attribute:

>>> class T:
...     wins = 0
...     def win(self):
...             self.wins = self.wins + 1
>>> a = T()
>>> b = T()
>>> a.win()
>>> a.win()
>>> b.win()
>>> a.wins
>>> b.wins
>>> T.wins

That is because the first time win() is called on an instance

self.wins = self.wins + 1

The instance attribute is not found and the right side falls back to look up 
self.wins in the class, i. e. the first time you are effectively running

self.wins = T.wins + 1

The left-hand side always denotes an assignment to the instance, so T.wins 
will always remain 0.

It is still good practice to define all attributes that are meant to be 
instance attributes in the initialiser:

class Team:
    def __init__(self, code):
        self.wins = 0
        self.losses = 0

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