Olivier Verdier Olivier Verdier - 21 days ago 5
Python Question

Prevent Python from caching the imported modules

While developing a largeish project (split in several files and folders) in Python with IPython, I run into the trouble of cached imported modules.

The problem is that instructions

import module
only reads the module once, even if that module has changed! So each time I change something in my package, I have to quit and restart IPython. Painful.

Is there any way to properly force reloading some modules? Or, better, to somehow prevent Python from caching them?

I tried several approaches, but none works. In particular I run into really, really weird bugs, like some modules or variables mysteriously becoming equal to
None
...

The only sensible resource I found is Reloading Python modules, from pyunit, but I have not checked it. I would like something like that.

A good alternative would be for IPython to restart, or restart the Python interpreter somehow.

So, if you develop in Python, what solution have you found to this problem?

Edit

To make things clear: obviously, I understand that some old variables depending on the previous state of the module may stick around. That's fine by me. By why is that so difficult in Python to force reload a module without having all sort of strange errors happening?

More specifically, if I have my whole module in one file
module.py
then the following works fine:

import sys
try:
del sys.modules['module']
except AttributeError:
pass
import module

obj = module.my_class()


This piece of code works beautifully and I can develop without quitting IPython for months.

However, whenever my module is made of several submodules, hell breaks loose:

import os
for mod in ['module.submod1', 'module.submod2']:
try:
del sys.module[mod]
except AttributeError:
pass
# sometimes this works, sometimes not. WHY?


Why is that so different for Python whether I have my module in one big file or in several submodules? Why would that approach not work??

Answer

Quitting and restarting the interpreter is the best solution. Any sort of live reloading or no-caching strategy will not work seamlessly because objects from no-longer-existing modules can exist and because modules sometimes store state and because even if your use case really does allow hot reloading it's too complicated to think about to be worth it.