Instantiate a Python class from a name

Question

So i have a set of classes and a string with one of the class names. How do I instantiate a class based on that string?

class foo:
  def __init__(self, left, right):
     self.left = left
     self.right = right

str = "foo"
x = Init(str, A, B)

I want x to be an instantiation of class foo.

Duplicate. http://stackoverflow.com/questions/734970/python-reference-to-a-class-from-a-string, http://stackoverflow.com/questions/1176136/convert-string-to-python-class-object, http://stackoverflow.com/questions/553784/can-you-use-a-string-to-instantiate-a-class-in-python, etc. — S.Lott, Feb 09 '10 at 11:17

score 31 · Answer 1 · edited Jan 14 '22 at 14:21

31

In your case you can use something like:

get_class = lambda x: globals()[x]
c = get_class("foo")

And it's even easier to get the class from the module:

import somemodule
getattr(somemodule, "SomeClass")

edited Jan 14 '22 at 14:21

SkunkSpinner

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answered May 11 '11 at 06:13

Radagast

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score 20 · Accepted Answer · answered Feb 09 '10 at 02:34

If you know the namespace involved, you can use it directly -- for example, if all classes are in module zap, the dictionary vars(zap) is that namespace; if they're all in the current module, globals() is probably the handiest way to get that dictionary.

If the classes are not all in the same namespace, then building an "artificial" namespace (a dedicated dict with class names as keys and class objects as values), as @Ignacio suggests, is probably the simplest approach.

score 8 · Answer 3 · answered Feb 09 '10 at 02:22

8

classdict = {'foo': foo}

x = classdict['foo'](A, B)

answered Feb 09 '10 at 02:22

Ignacio Vazquez-Abrams

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How can I do it using reflection? – klynch Feb 09 '10 at 02:33
1

You *could* use `getattr()`, but that could expose you to security problems. Using a class decorator will let you enumerate classes automatically, but those only exist in newer versions of Python. – Ignacio Vazquez-Abrams Feb 09 '10 at 02:36
How would you use the decorator? Would it append to a list of instantiable classes - and I would have to iterate through this list to pick the correct class to instantiate? – Utkarsh Sinha Jan 21 '17 at 07:36
@UtkarshSinha: ... No, it would add an entry to `classdict`. – Ignacio Vazquez-Abrams Jan 21 '17 at 10:28

Mischa Arefiev · Answer 4 · 2011-12-28T12:27:39.880

classname = "Foo"
foo = vars()[classname](Bar, 0, 4)

Or perhaps

def mkinst(cls, *args, **kwargs):
    try:
        return globals()[cls](*args, **kwargs)
    except:
        raise NameError("Class %s is not defined" % cls)

x = mkinst("Foo", bar, 0, 4, disc="bust")
y = mkinst("Bar", foo, batman="robin")

Miscellaneous notes on the snippet:

*args and **kwargs are special parameters in Python, they mean «an array of non-keyword args» and «a dict of keyword args» accordingly.

PEP-8 (official Python style guide) recommends using cls for class variables.

vars() returns a dict of variables defined in the local scope.

globals() returns a dict of variables currently present in the environment outside of local scope.

score 2 · Answer 5 · edited Dec 28 '11 at 11:15

2

try this

cls = __import__('cls_name')

and this - http://effbot.org/zone/import-confusion.htm maybe helpful

edited Dec 28 '11 at 11:15

Tom van der Woerdt

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answered Mar 10 '11 at 00:11

roma

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score 0 · Answer 6 · answered Mar 16 '15 at 12:03

0

You might consider usage of metaclass as well:

Cls = type('foo', (), foo.__dict__)
x = Cls(A, B)

Yet it creates another similar class.

answered Mar 16 '15 at 12:03

Aliaksei Kuzmin

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Instantiate a Python class from a name

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