How do I unload (reload) a Python module?

The reload() function reloads a previously imported module without restarting Python. This is useful when you modify a module's source code and want to test changes in an interactive session.

Python 2 vs Python 3

In Python 2, reload() was a built-in function. In Python 3, it was moved to the importlib module.

Python 2 Syntax

import mymodule
# Edit mymodule.py and want to reload it
reload(mymodule)

Python 3 Syntax

import importlib
import mymodule

# Edit mymodule.py and want to reload it
importlib.reload(mymodule)

Example

Let's create a simple module and demonstrate reloading ?

# First, let's simulate a module with a simple function
import types
import sys

# Create a module dynamically
mymodule = types.ModuleType('mymodule')
mymodule.greet = lambda: print("Hello from original module!")
sys.modules['mymodule'] = mymodule

# Import and use the module
import mymodule
mymodule.greet()

Hello from original module!

Now let's modify and reload the module ?

import importlib

# Modify the module (simulating editing the source file)
mymodule.greet = lambda: print("Hello from reloaded module!")

# Reload the module
importlib.reload(mymodule)
mymodule.greet()

Hello from reloaded module!

How It Works

When you reload a module, Python:

• Recompiles the module's source code
• Re-executes the module-level code
• Creates new objects bound to names in the module's dictionary
• Updates the module namespace with new or changed objects

Important Notes

References to old objects: Existing references to the old module objects are not automatically updated. You must update them manually.

import importlib

# Create a reference to a function
old_function = mymodule.greet

# Reload the module
importlib.reload(mymodule)

# old_function still points to the original function
print("Old reference:")
old_function()

print("New reference:")
mymodule.greet()

Old reference:
Hello from reloaded module!
New reference:
Hello from reloaded module!

Extension modules: The initialization function of C extension modules is not called a second time during reload.

Common Use Cases

• Interactive development and testing
• Debugging module code without restarting Python
• Hot-swapping code in development environments

Conclusion

Use importlib.reload() in Python 3 to reload modified modules during development. Remember that existing references to old objects won't be automatically updated.