Python: how to automatically create an instance in another class

Question

In writing a Python (2.5) program, I tried to create a class and, in its __init__ function, automatically create an instance of another class with its name as an argument to the __init__ function, something like this:

    class Class1:
        def __init__(self,attribute):
            self.attribute1=attribute

    class Class2:
        def __init__(self,instanceName):
            #any of Class2's attributes
            exec instanceName + '=Class1('attribute1')'
            # this should produce an instance of Class1 whose name is instanceName

But when I make an instance of Class2, instance=Class2('instance2'), and try to get attribute1 of instance2 (which should have been created from Class2's __init__ function) I get an error message:

    Traceback (most recent call last):
      File "<pyshell#29>", line 1, in <module>
        print instance2.attribute1
    NameError: name 'instance2' is not defined

I don't know what the problem is, since name='instance3' and exec name+'=Class1('attribute1') does work, though this is probably because I don't have much experience with Python. How would I be able to do something like this automatically when an instance is created?

What exactly are you trying to do? I'm pretty sure there's a way to do what you want that doesn't involve this structure. — Waleed Khan
– Waleed Khan, Commented Nov 27, 2013 at 1:25
Seems like an XY problem - why do you have the requirement to assign the instance to a specific name? And right now you're trying to create it as a local variable, which makes little sense as it's only accessible in the local scope. What's your use case? — l4mpi
– l4mpi, Commented Nov 27, 2013 at 1:30
This still does not work when I add exec 'global '+instancename, which should make it accessible as a global variable. — m.baxter
– m.baxter, Commented Nov 27, 2013 at 1:50
My purpose in this is to be able to create an instance of Class1 from one of Class2 while also being able to create an instance without using Class2, and this is the only way I know of to do this. — m.baxter
– m.baxter, Commented Nov 27, 2013 at 1:52

inspectorG4dget · Accepted Answer · 2013-11-27 01:21:59Z

1

I have to run, so hopefully, someone else can fix any mistakes in this post:

class Class1:
  def __init__(self, attribute):
    self.attribute1 = attribute

class Class2:
  def __init__(self, instanceName):
    setattr(self, instanceName, Class1(...))  # replace ... with whatever parameters you want

answered Nov 27, 2013 at 1:21

inspectorG4dget

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2 Comments

m.baxter Over a year ago

This, setattr(self,instanceName,class1('attribute text')) does seem to work, making instanceName a global instance of class1.

m.baxter Over a year ago

Though this worked in the example I gave, I got the same error message NameError: name 'instance2' is not defined when I incorporated this into the more complex program I was originally working on.

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