Instantiate an object based on a variable name

[Steven D'Aprano]

On Thu, 31 Dec 2009 08:54:57 -0800, Wells wrote:

> Sorry, this is totally basic, but my Google-fu is failing:
> 
> I have a variable foo. I want to instantiate a class based on its value-
> how can I do this?

The right way to do it is like this:


>>> class C:
...     pass
...
>>> foo = C  # assign the class itself to the variable foo
>>> instance = foo()
>>> instance
<__main__.C instance at 0xb7ce60ec>


Many newbies don't think of that, because they're not used to classes 
being first-class objects like strings, floats, ints and similar. It's a 
very useful technique when you want to do something to a whole lot of 
different classes:

for theclass in [MyClass, C, Klass, int, str, SomethingElse]:
    process(theclass())


Here's an alternative, for those times you only have the name of the 
class (perhaps because you've read it from a config file):


>>> foo = "C"  # the name of the class
>>> actual_class = globals()[foo]
>>> instance = actual_class()
>>> instance
<__main__.C instance at 0xb7ce608c>


The obvious variation if the class belongs to another module is to use 
getattr(module, foo) instead of globals()[foo].


-- 
Steven