How to dynamically compose and access class attributes in Python? [duplicate]

Question

I have a Python class that have attributes named: date1, date2, date3, etc.

During runtime, I have a variable i, which is an integer.

What I want to do is to access the appropriate date attribute in run time based on the value of i.

For example,

if i == 1, I want to access myobject.date1

if i == 2, I want to access myobject.date2

And I want to do something similar for class instead of attribute.

For example, I have a bunch of classes: MyClass1, MyClass2, MyClass3, etc. And I have a variable k.

if k == 1, I want to instantiate a new instance of MyClass1

if k == 2, I want to instantiate a new instance of MyClass2

How can i do that?

EDIT

I'm hoping to avoid using a giant if-then-else statement to select the appropriate attribute/class.

Is there a way in Python to compose the class name on the fly using the value of a variable?

Answer 1

You can use getattr() to access a property when you don't know its name until runtime:

obj = myobject() i = 7 date7 = getattr(obj, 'date%d' % i) # same as obj.date7 

If you keep your numbered classes in a module called foo, you can use getattr() again to access them by number.

foo.py:   class Class1: pass   class Class2: pass   [ etc ]   bar.py:   import foo   i = 3   someClass = getattr(foo, "Class%d" % i) # Same as someClass = foo.Class3   obj = someClass() # someClass is a pointer to foo.Class3   # short version:   obj = getattr(foo, "Class%d" % i)() 

Having said all that, you really should avoid this sort of thing because you will never be able to find out where these numbered properties and classes are being used except by reading through your entire codebase. You are better off putting everything in a dictionary.

Answer 2

For the first case, you should be able to do:

getattr(myobject, 'date%s' % i) 

For the second case, you can do:

myobject = locals()['MyClass%s' % k]() 

However, the fact that you need to do this in the first place can be a sign that you're approaching the problem in a very non-Pythonic way.