How Can I Reload Python Modules Without Restarting the Server?

Reloading Python Modules for Dynamic Server Upgrades

In scenarios where long-running Python servers need updates to their services without the overhead of server restarts, the question arises: how can this be achieved? Specifically, how can a module be unloaded and subsequently reloaded to reflect the changes made to it?

Solution: Using Importlib.reload()

For Python 3.4 and above, the importlib.reload() function provides the solution for reloading already imported modules. It takes the initially imported module as its argument and returns the reloaded module, which incorporates any changes since the initial import.

Implementation

To illustrate the process, consider a situation where the foo.py module has been modified. Here's how to reload it:

from importlib import reload
import foo

while True:
    # Perform any necessary tasks
    if is_changed(foo):
        foo = reload(foo)

This approach allows you to seamlessly update your service without restarting the server, similar to how Django's development server works for code changes.

Technicalities

It's important to note that reloading a module involves:

• Recompiling the module's code and executing the module-level code.
• Updating the module's dictionary to point to any new or altered objects.
• Preserving old objects until their reference counts reach zero.
• Ignoring any existing references to old objects outside the module namespace.

Consequently, if you have instances of the Foo class defined in the foo module, you will need to re-instantiate them after reloading the module.

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