In python, I have to instantiate certain class, knowing its name in a string, but this class 'lives' in a dynamically imported module. An example follows:

loader-class script:

import sys

class loader:

  def __init__(self, module_name, class_name): # both args are strings

    try:

      __import__(module_name)

      modul = sys.modules[module_name]

      instance = modul.class_name() # obviously this doesn't works, here is my main problem!

    except ImportError:

       # manage import error

some-dynamically-loaded-module script:

class myName:

  # etc...

I use this arrangement to make any dynamically-loaded-module to be used by the loader-class following certain predefined behaviours in the dyn-loaded-modules...

Any ideas are appreciated.

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)

    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

If you want this sentence from foo.bar import foo2 to be loaded dynamically, you should do this

foo = __import__("foo")

bar = getattr(foo,"bar")

foo2 = getattr(bar,"foo2")



instance = foo2()