how to dynamically create an instance of a class in python?

user_400275 0
Viewed 131986

I have list of class names and want to create their instances dynamically. for example:

names=[
'foo.baa.a',
'foo.daa.c',
'foo.AA',
 ....
]

def save(cName, argument):
 aa = create_instance(cName) # how to do it?
 aa.save(argument)

save(random_from(names), arg)

How to dynamically create that instances in Python? thanks!

python
7 Answers

This worked for me:

from importlib import import_module

class_str: str = 'A.B.YourClass'
try:
    module_path, class_name = class_str.rsplit('.', 1)
    module = import_module(module_path)
    return getattr(module, class_name)
except (ImportError, AttributeError) as e:
    raise ImportError(class_str)
user_32989640

My problem was that I wanted to pass arguments into __init__ with the arguments being specified in a string on the command line. For example, the equivalent of

import a.b.ClassB as ClassB
instance = ClassB.ClassB('World')

The string on the command line is "a.b.ClassB.ClassB('World')"

With the following class in module a.b.ClassB

class ClassB():

    def __init__(self, name:str):
        self._name = name

    def hello(self):
        print("Hello " + self._name + "!")

we can create this class with the following 

import importlib

def create_instance(class_str:str):
    """
    Create a class instance from a full path to a class constructor
    :param class_str: module name plus '.' plus class name and optional parens with arguments for the class's
        __init__() method. For example, "a.b.ClassB.ClassB('World')"
    :return: an instance of the class specified.
    """
    try:
        if "(" in class_str:
            full_class_name, args = class_name = class_str.rsplit('(', 1)
            args = '(' + args
        else:
            full_class_name = class_str
            args = ()
        # Get the class object
        module_path, _, class_name = full_class_name.rpartition('.')
        mod = importlib.import_module(module_path)
        klazz = getattr(mod, class_name)
        # Alias the the class so its constructor can be called, see the following link.
        # See https://www.programiz.com/python-programming/methods/built-in/eval
        alias = class_name + "Alias"
        instance = eval(alias + args, { alias: klazz})
        return instance
    except (ImportError, AttributeError) as e:
        raise ImportError(class_str)

if __name__ == "__main__":
    instance = create_instance("a.b.ClassB.ClassB('World')")
    instance.hello()
user_7879710
user_118882020
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