Class as an input in a function

Question

I have a file different_classes that contains three different classes. It is something like:

class first(object):
    def __init__(x, y, z):
    body of the first class

class second(first):
    def __init__(x, y, z, a=2, b=3):
    body of the second class

class third(object):
    def __init__(x, y, z):
    body of the third class

Now I have another file, say main.py where I want to be able to pass on the name of the class that needs to be called. For example, right now I do:

import different_classes
def create_blah():
    instance = different_classes.first()
    rest of the function body

when I want to use the first class in different_classes. If I want to use class second, I use different_classes.second().

Can I input the class name as an argument in the create_blah function. Something like:

def create_blah(class_type = "first", x=x1, y=y1, z=z1):
    instance = different_classes.class_type(x, y, z)

I know this may not be valid...but want to know if something similar can be done. Thanks!

Duncan · Accepted Answer · 2010-12-21 17:20:37Z

11

Rather than passing the name of the class, why not just pass the class itself:

def create_blah(class_type = different_classes.first, x=x1, y=y1, z=z1):
    instance = class_type(x, y, z)

Remember that a class is just an object like anything else in Python: you can assign them to variables and pass them around as arguments.

If you really do need to use the name, e.g. because you are reading it from a configuration file, then use getattr() to retrieve the actual class:

instance = getattr(different_classes, class_type)(x, y, z)

edited Dec 21, 2010 at 17:20

answered Dec 21, 2010 at 16:26

Duncan

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1 Comment

Curious2learn Over a year ago

Thanks. I can pass the class as you have suggested. Did not know that was possible.

robert · Accepted Answer · 2010-12-21 16:26:02Z

0

def create_blah(class_type = "first", x=x1, y=y1, z=z1):
  class_ = different_classes.__dict__.get(class_type, None)
  if isinstance(class_, type):
    instance = class_(x, y, z)

You could also pass around the class object: class_ = different_classes.first.

answered Dec 21, 2010 at 16:26

robert

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3 Comments

user395760 Over a year ago

Not explicitly incorrect, but the example code is an example of doing it wrong(tm) - therefore, aiming for a total score of 0, I downvoted.

robert Over a year ago

@delnan I answered the original poster's question. I didn't know if he was receiving his class name as a string or not. I do this stuff all the time to implement factory methods in Python.

Karl Knechtel Over a year ago

You might also instead use class_ = getattr(different_classes, class_type, None).

Falmarri · Accepted Answer · 2010-12-21 16:26:58Z

-1

Sort of. Thare are fancier ways, but I suggest this one.

def create_blah(class_type = "first", x=x1, y=y1, z=z1):
    if class_type == "first":
        instance=different_classes.first(x,y,z)
    ...

answered Dec 21, 2010 at 16:26

Falmarri

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1 Comment

Falmarri Over a year ago

Is there a reason for the -3? I said there are fancier ways. This is a perfectly legitimate way of doing this.

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