Modeling a soccer team with substitutions
You naming convention does not follow PEP 8. Try to follow it as much as you can as it helps having same consistent guidelines all over Python code.



I do not quite understand how the on field thingy is supposed to work so I'll skip on that part.



Try to avoid global variables. Try to think how things would be affected if you were trying to handle multiple teams. Most probably things would go wrong.

I guess your different lists should be part of the team class. Also, there is not much point for having a Player class as a player on its own is nothing but a name. As far as I understand soccer, the number is for a given player in a given team. Thus, the team should be some kind of containers of pairs name/number.

Also, it seems like your are doing the same things for different teams of players that could be handled the same way. Instead of write code, it's easier to use smart data structures and make them work for you. In your case, it seems like a dictionnary from position to list of (player name + player number) is good idea. Default composition is to have empty lists and to fill them later on.

Here's what my code looks like :

class Team(object):
    def __init__(self, name):
        self.name = name
        self.composition = {
            'forward' : [],
            'midfield': [],
            'defense': [],
            'goalie': []
        }

    def add_player(self, name, number, position):
        if position not in self.composition:
            print 'Only %s accepted' % ', '.join(self.composition.keys())
        else:
            self.composition[position].append((name,number))
            print 'Added %s to the team!' % name

bcn = Team('Barcelona')
bcn.add_player('Messi', 10, 'forward')
bcn.add_player('Neymar', 11, 'forward')
bcn.add_player('Song', 9, 'midfield')
bcn.add_player('Valdes', 1, 'goalie')
bcn.add_player('Pique', 25, 'defense')
bcn.add_player('Pedro', 8, 'forward')
bcn.add_player('Josay', 10, 'invalid')

# It could be a good idea to create a method to do this :-)    
for pos, players in bcn.composition.iteritems():
    print pos
    for p in players:
        print p