Using Variables for Class Names in Python?

Question

I want to know how to use variables for objects and function names in Python. In PHP, you can do this:

$className = "MyClass";

$newObject = new $className();

How do you do this sort of thing in Python? Or, am I totally not appreciating some fundamental difference with Python, and if so, what is it?

Asked By: Sam McAfee

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Source

Answer 1

In Python,

className = MyClass
newObject = className()

The first line makes the variable className refer to the same thing as MyClass. Then the next line calls the MyClass constructor through the className variable.

As a concrete example:

>>> className = list
>>> newObject = className()
>>> newObject
[]

(In Python, list is the constructor for the list class.)

The difference is that in PHP, you represent the name of the class you want to refer to as a string, while in Python you can reference the same class directly. If you must use a string (for example if the name of the class is created dynamically), then you will need to use other techniques.

Answered By: Greg Hewgill

Answer 2

If you have this:

class MyClass:
    def __init__(self):
        print "MyClass"

Then you usually do this:

>>> x = MyClass()
MyClass

But you could also do this, which is what I think you’re asking:

>>> a = "MyClass"
>>> y = eval(a)()
MyClass

But, be very careful about where you get the string that you use “eval()” on — if it’s come from the user, you’re essentially creating an enormous security hole.

Update: Using type() as shown in coleifer’s answer is far superior to this solution.

Answered By: TimB

Answer 3

Assuming that some_module has a class named “class_name”:

import some_module
klass = getattr(some_module, "class_name")
some_object = klass()

I should note that you should be careful here: turning strings into code can be dangerous if the string came from the user, so you should keep security in mind in this situation. 🙂

One other method (assuming that we still are using “class_name”):

class_lookup = { 'class_name' : class_name }
some_object = class_lookup['class_name']()  #call the object once we've pulled it out of the dict

The latter method is probably the most secure way of doing this, so it’s probably what you should use if at all possible.

Answered By: Jason Baker

Answer 4

If you need to create a dynamic class in Python (i.e. one whose name is a variable) you can use type() which takes 3 params:
name, bases, attrs

>>> class_name = 'MyClass'
>>> klass = type(class_name, (object,), {'msg': 'foobarbaz'})

<class '__main__.MyClass'>

>>> inst = klass()
>>> inst.msg
foobarbaz

Note however, that this does not ‘instantiate’ the object (i.e. does not call constructors etc. It creates a new(!) class with the same name.

Answered By: coleifer

Answer 5

I use:

newObject = globals()[className]()

Answered By: Yanone

Answer 6

I prefer using dictionary to store the class to string mapping.

>>> class AB:
...   def __init__(self, tt):
...     print(tt, "from class AB")
... 
>>> class BC:
...   def __init__(self, tt):
...     print(tt, "from class BC")
... 
>>> x = { "ab": AB, "bc": BC}
>>> x
{'ab': <class '__main__.AB'>, 'bc': <class '__main__.BC'>}
>>> 
>>> x['ab']('hello')
hello from class AB
<__main__.AB object at 0x10dd14b20>
>>> x['bc']('hello')
hello from class BC
<__main__.BC object at 0x10eb33dc0>

Answered By: Akash Ranjan

Using Variables for Class Names in Python?

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