CODEWITHSUNDEEP
python
Nyambaa
Nyambaa

Reputation: 41757

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

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?

Upvotes: 77

Views: 162800

Answers (7)

David Wood
David Wood

Reputation: 434

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()

Upvotes: 3

hb2pencil
hb2pencil

Reputation: 892

You can use the python builtin eval() statement to instantiate your classes. Like this:

aa = eval(cName)()

Notice!

using eval is dangerous and is a key for lots of security risks based on code injections.

Upvotes: 10

Nic
Nic

Reputation: 11

Best Answer I found: Better Way is to make a dictionary: objects ={} Names =[object1,object2, object3]

For objname in Names: objects[objname]=classname()

Found in: https://www.thecodingforums.com/threads/create-object-name-from-string-value.712461/

Upvotes: 0

Tobias Ernst
Tobias Ernst

Reputation: 4634

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)

Upvotes: 24

elewinso
elewinso

Reputation: 2521

Assuming you have already imported the relevant classes using something like

from [app].models import *

all you will need to do is

klass = globals()["class_name"]
instance = klass()

Upvotes: 104

S.Lott
S.Lott

Reputation: 391818

You can often avoid the string processing part of this entirely.

import foo.baa 
import foo.AA
import foo

classes = [ foo.baa.a, foo.daa.c, foo.AA ]

def save(theClass, argument):
   aa = theClass()
   aa.save(argument)

save(random.choice(classes), arg)

Note that we don't use a string representation of the name of the class.

In Python, you can just use the class itself.

Upvotes: 9

Scott Lance
Scott Lance

Reputation: 2249

This is often referred to as reflection or sometimes introspection. Check out a similar questions that have an answer for what you are trying to do:

Does Python Have An Equivalent to Java Class forname

Can You Use a String to Instantiate a Class in Python

Upvotes: 28

Related Questions