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loader

In Python, a loader is responsible for loading a module or package into a Python runtime environment. Loaders are a crucial part of Python’s import system, as they define how to fetch and execute the code of a module or package.

When you import a module, a finder first locates it and returns a module spec. Python then uses the loader named by that spec to load the module’s code. The loader then executes that code in the module’s namespace, populating it so the module is ready for use. Loaders can vary based on the source of the module, such as file systems, zip files, or even network locations.

Example

Here’s an example of how to create a custom loader:

Language: Python Filename: loader.py 
import importlib.util
import importlib.abc
import sys

class CustomLoader(importlib.abc.Loader):
    def create_module(self, spec):
        return None

    def exec_module(self, module):
        print(f"Loading module: {module.__name__}")
        module.__dict__["greet"] = lambda: "Hello from the custom loader!"

spec = importlib.util.spec_from_loader("module", CustomLoader())
module = importlib.util.module_from_spec(spec)
spec.loader.exec_module(module)

# Usage
print(module.greet())  # Output: Hello from the custom loader!

This example demonstrates how to create a custom loader that modifies the module’s namespace to include a new function called greet().

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For additional information on related topics, take a look at the following resources:

By Leodanis Pozo Ramos • Updated June 1, 2026
Python Glossary Share Feedback