Convert a string to a Class object in Python

# Table of Contents

1. Convert a string to a Class object in Python
2. Convert a string to a Class object using eval()
3. Convert a string to a Class object using globals()
4. Convert a string to a Class object using importlib.import_module()

# Convert a string to a Class object in Python

To convert a string to a class object:

1. Use the sys.modules dictionary to get the current module.
2. The class will be accessible in the module's attributes.
3. Pass the string to the getattr() function to get the class object.

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import sys


class Employee():
    pass


def get_class(class_name):
    return getattr(sys.modules[__name__], class_name)


cls = get_class('Employee')
print(cls)  # 👉️ <class '__main__.Employee'>

Sys.modules is a dictionary that maps module names to modules that have already been loaded.

The __name__ global variable stores the module's name.

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import sys

# 👇️ <module '__main__' from '/home/borislav/Desktop/bobbyhadz_python/main.py'>
print(sys.modules[__name__])

Once you have the module object, use the getattr() method to convert the string to a class object.

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import sys


class Employee():
    pass


def get_class(class_name):
    return getattr(sys.modules[__name__], class_name)


cls = get_class('Employee')
print(cls)  # 👉️ <class '__main__.Employee'>

The getattr() function returns the value of the provided attribute of the object.

The function takes the following parameters:

If a class with the specified name doesn't exist, you'd get an AttributeError.

You can pass a third argument to the getattr() method if you need to specify a default value.

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import sys


def get_class(class_name):
    # 👇️ returns None if class with given name doesn't exist
    return getattr(sys.modules[__name__], class_name, None)


cls = get_class('ABC123')
print(cls)  # 👉️ None

# Convert a string to a Class object using eval()

Alternatively, you can use the eval() function.

The eval() function will take the string and will evaluate it as a Python expression returning the class.

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class Employee():
    pass

def get_class(class_name):
    return eval(class_name)


print(get_class('Employee'))  # 👉️ <class '__main__.Employee'>

The eval() function takes an expression, parses it and evaluates it as a Python expression using the globals and locals dictionaries as the global and local namespace.

The eval() function basically tries to run the given expression, so it should never be used with untrusted code.

If you get linting errors when using eval(), use the globals() dictionary instead.

# Convert a string to a Class object using globals()

This is a two-step process:

1. Use the globals() function to get a dictionary containing the current scope's global variables.
2. Use the string as a dictionary key to get access to the class object.

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# ✅ if the class is in the same module

class Employee():
    pass

def get_class(class_name):
    return globals()[class_name]


print(get_class('Employee'))  # 👉️ <class '__main__.Employee'>

The globals function returns a dictionary that implements the current module namespace.

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class Employee():
    pass

# {'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x7f851caa1de0>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, '__file__': '/home/borislav/Desktop/bobbyhadz_python/main.py', '__cached__': None, 'Employee': <class '__main__.Employee'>}
print(globals())

The last step is to use the string to access the dictionary's key.

There is a key with the class's name that points to the class object.

If a class with the given name doesn't exist in the module, you'd get a KeyError exception.

The previous 3 approaches only work if the class is defined in the same module.

If the class is defined in a different module, use the importlib module.

# Convert a string to a Class object using importlib.import_module()

To convert a string to a class object when the class is defined in a different module:

1. Use the importlib.import_module() method to import the module.
2. The class will be accessible in the module's attributes.
3. Pass the string to the getattr() function to get the class object.

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import importlib


def get_class(module_name, class_name):
    module = importlib.import_module(module_name)
    return getattr(module, class_name)


print(get_class('another', 'Employee'))  # 👉️ <class 'another.Employee'>

The example assumes that there is a module named another.py located in the same directory that contains an Employee class.

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class Employee():
    pass

The importlib.import_module method takes the name of a module and imports it.

The name argument can be absolute or relative, e.g. pkg.module or ..module.

If you use a relative package name, e.g. ..module, you have to pass a second argument to the import_module() method, e.g. import_module('..module', pkg.subpkg') imports pkg.module.

Once we have the module imported, we can use the getattr() function to get the class object from the given string.

The getattr() function takes an optional third argument that serves as a default value for when the attribute doesn't exist on the object.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Creating class instances from a Dictionary in Python
• How to Create an incremental ID in a Class in Python
• Purpose of 'return self' from a class method in Python
• Call a class method from another Class in Python
• How to access Parent class Attributes in Python
• Flake8: f-string is missing placeholders [Solved]