Function polymorphism in Python

Question

I've been looking around for the best way of doing this but I haven't really found anything completely convincing.

I am writing a system where you have User objects and a collection that manages those Users. Each User has a name and I want to specify a function in the manager that can either take the name of the User or the User object itself.

class UserManager: 
  def remove_user(self,user_or_username):
    #If user_or_username is a string
    remote.remove(user_or_username)
    #If user_or_username is a User object
    remote.remove(user_or_username.name)

Is there any nifty way of doing this or is usage of isinstance the way to go?

Polymorphism really only helps if you don't know at the point of call whether you have a name or a user object. Mostly that isn't the case. You should resist the temptation to overload functions unless you really need to. Here you have a method that when given a string just chains to another method. That's a code smell: you should just call the other method directly; when you have a username call manager.remove(username) directly. — Duncan
– Duncan, Commented May 15, 2012 at 13:07
Your point is valid @Duncan but in this case the manager is an intermediate layer and its job is to make the proper calls for the user of my application. I wanted to explore this option to know how much flexibility I had at my disposal! — Parham
– Parham, Commented May 15, 2012 at 14:21

Community · Accepted Answer · 2017-05-23 10:34:15Z

5

A solution like mgilson's, but slightly different:

def remove_user(self,user_or_username):
    try:
        #If user_or_username is a User object
        username = user_or_username.name
    except AttributeError:   #Oops -- didn't works.  ask forgiveness ;-)
        #If user_or_username is a string
        username = user_or_username
    remote.remove(username)

Why? Because this way, AttributeErrors in remove() are not suppressed.

It might be irrelevant, but I prefer concentrating exception handling to those places I really inted to have them.

edited May 23, 2017 at 10:34

CommunityBot

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answered May 15, 2012 at 13:03

glglgl

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4 Comments

Karl Knechtel Over a year ago

For even better factoring, you might consider extracting a function for the try/except block. It might be useful elsewhere, too.

glglgl Over a year ago

@KarlKnechtel Well, a username = getattr(user_or_username, 'name', user_or_username) would as well be a way to go...

mgilson Over a year ago

and +1 for figuring out how to do this in a 1-liner (using getattr) -- Not that I generally perfer 1-liners ...

mgilson Over a year ago

@glglgl Agreed -- Though I tend to think that LBYL vs. EAFP depends on the application (and programmer preference/background in other languages, etc.) ...

Mp0int · Accepted Answer · 2012-05-15 13:02:35Z

3

using isinstance is a good approach... There is one more approach for this solution

if hasattr(user_or_username, 'name'):
    # this object has <name> attribute
    remote.remove(user_or_username.name)
else:
    remote.remove(user_or_username)

answered May 15, 2012 at 13:02

Mp0int

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Comments

mgilson · Accepted Answer · 2012-05-15 12:58:58Z

Sometimes python people like to say "it's better to ask forgiveness than permission"...

  def remove_user(self,user_or_username):
    try:
        #If user_or_username is a User object
        remote.remove(user_or_username.name)
    except AttributeError:   #Oops -- didn't works.  ask forgiveness ;-)
        #If user_or_username is a string
        remote.remove(user_or_username)

But I say it's just a matter of preference really. You could also use isinstance just as easily if you know you're only going to be getting strings, or User instances.

robert · Accepted Answer · 2012-05-15 13:00:55Z

2

I would use isinstance, but this also works:

def remove_user(self, user):
   if hasattr(user, "name"):
      self.remove(user.name)
   else:
      self.remove(user)

answered May 15, 2012 at 13:00

robert

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