Dynamic instantiation from string name of a class in dynamically imported module?

Question

You can use getattr

getattr(module, class_name)

to access the class. More complete code:

module = __import__(module_name)
class_ = getattr(module, class_name)
instance = class_()

As mentioned below, we may use importlib

import importlib
module = importlib.import_module(module_name)
class_ = getattr(module, class_name)
instance = class_()

tl;dr

Import the root module with importlib.import_module and load the class by its name using getattr function:

# Standard import
import importlib
# Load "module.submodule.MyClass"
MyClass = getattr(importlib.import_module("module.submodule"), "MyClass")
# Instantiate the class (pass arguments to the constructor, if needed)
instance = MyClass()

explanations

You probably don't want to use __import__ to dynamically import a module by name, as it does not allow you to import submodules:

>>> mod = __import__("os.path")
>>> mod.join
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'join'

Here is what the python doc says about __import__:

Note: This is an advanced function that is not needed in everyday Python programming, unlike importlib.import_module().

Instead, use the standard importlib module to dynamically import a module by name. With getattr you can then instantiate a class by its name:

import importlib
my_module = importlib.import_module("module.submodule")
MyClass = getattr(my_module, "MyClass")
instance = MyClass()

You could also write:

import importlib
module_name, class_name = "module.submodule.MyClass".rsplit(".", 1)
MyClass = getattr(importlib.import_module(module_name), class_name)
instance = MyClass()

This code is valid in python ≥ 2.7 (including python 3).

Use getattr to get an attribute from a name in a string. In other words, get the instance as

instance = getattr(modul, class_name)()

Copy-paste snippet:

import importlib
def str_to_class(module_name, class_name):
    """Return a class instance from a string reference"""
    try:
        module_ = importlib.import_module(module_name)
        try:
            class_ = getattr(module_, class_name)()
        except AttributeError:
            logging.error('Class does not exist')
    except ImportError:
        logging.error('Module does not exist')
    return class_ or None

One can simply use the pydoc.locate function.

from pydoc import locate
my_class = locate("module.submodule.myclass")
instance = my_class()