How do I unload (reload) a Python module?

Solution 1:

You can reload a module when it has already been imported by using importlib.reload():

from importlib import reload  # Python 3.4+
import foo

while True:
    # Do some things.
    if is_changed(foo):
        foo = reload(foo)

In Python 2, reload was a builtin. In Python 3, it was moved to the imp module. In 3.4, imp was deprecated in favor of importlib. When targeting 3 or later, either reference the appropriate module when calling reload or import it.

I think that this is what you want. Web servers like Django's development server use this so that you can see the effects of your code changes without restarting the server process itself.

To quote from the docs:

Python module’s code is recompiled and the module-level code re-executed, defining a new set of objects which are bound to names in the module’s dictionary by reusing the loader which originally loaded the module. The init function of extension modules is not called a second time.

As with all other objects in Python the old objects are only reclaimed after their reference counts drop to zero.

The names in the module namespace are updated to point to any new or changed objects.

Other references to the old objects (such as names external to the module) are not rebound to refer to the new objects and must be updated in each namespace where they occur if that is desired.

As you noted in your question, you'll have to reconstruct Foo objects if the Foo class resides in the foo module.

Solution 2:

In Python 3.0–3.3 you would use: imp.reload(module)

The BDFL has answered this question.

However, imp was deprecated in 3.4, in favour of importlib (thanks @Stefan!).

I think, therefore, you’d now use importlib.reload(module), although I’m not sure.

Solution 3:

It can be especially difficult to delete a module if it is not pure Python.

Here is some information from: How do I really delete an imported module?

You can use sys.getrefcount() to find out the actual number of references.

>>> import sys, empty, os
>>> sys.getrefcount(sys)
9
>>> sys.getrefcount(os)
6
>>> sys.getrefcount(empty)
3

Numbers greater than 3 indicate that it will be hard to get rid of the module. The homegrown "empty" (containing nothing) module should be garbage collected after

>>> del sys.modules["empty"]
>>> del empty

as the third reference is an artifact of the getrefcount() function.

Solution 4:

reload(module), but only if it's completely stand-alone. If anything else has a reference to the module (or any object belonging to the module), then you'll get subtle and curious errors caused by the old code hanging around longer than you expected, and things like isinstance not working across different versions of the same code.

If you have one-way dependencies, you must also reload all modules that depend on the reloaded module to get rid of all the references to the old code. And then reload modules that depend on the reloaded modules, recursively.

If you have circular dependencies, which is very common for example when you are dealing with reloading a package, you must unload all the modules in the group in one go. You can't do this with reload() because it will re-import each module before its dependencies have been refreshed, allowing old references to creep into new modules.

The only way to do it in this case is to hack sys.modules, which is kind of unsupported. You'd have to go through and delete each sys.modules entry you wanted to be reloaded on next import, and also delete entries whose values are None to deal with an implementation issue to do with caching failed relative imports. It's not terribly nice but as long as you have a fully self-contained set of dependencies that doesn't leave references outside its codebase, it's workable.

It's probably best to restart the server. :-)

Solution 5:

For Python 2 use built-in function reload:

reload(module)

For Python 2 and Python 3.2—3.3 use reload from module imp:

import imp
imp.reload(module)

For Python ≥3.4, imp is deprecated in favor of importlib, so use this:

import importlib
importlib.reload(module)

or:

from importlib import reload
reload(module)

TL;DR:

Python ≥ 3.4: importlib.reload(module)
Python 3.2 — 3.3: imp.reload(module)
Python 2: reload(module)