Prevent Python from caching the imported modules

Question

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??

Solution 1

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.

Solution 2

import checks to see if the module is in sys.modules, and if it is, it returns it. If you want import to load the module fresh from disk, you can delete the appropriate key in sys.modules first.

There is the reload builtin function which will, given a module object, reload it from disk and that will get placed in sys.modules. Edit -- actually, it will recompile the code from the file on the disk, and then re-evalute it in the existing module's __dict__. Something potentially very different than making a new module object.

Mike Graham is right though; getting reloading right if you have even a few live objects that reference the contents of the module you don't want anymore is hard. Existing objects will still reference the classes they were instantiated from is an obvious issue, but also all references created by means of from module import symbol will still point to whatever object from the old version of the module. Many subtly wrong things are possible.

Edit: I agree with the consensus that restarting the interpreter is by far the most reliable thing. But for debugging purposes, I guess you could try something like the following. I'm certain that there are corner cases for which this wouldn't work, but if you aren't doing anything too crazy (otherwise) with module loading in your package, it might be useful.

def reload_package(root_module):
    package_name = root_module.__name__

    # get a reference to each loaded module
    loaded_package_modules = dict([
        (key, value) for key, value in sys.modules.items() 
        if key.startswith(package_name) and isinstance(value, types.ModuleType)])

    # delete references to these loaded modules from sys.modules
    for key in loaded_package_modules:
        del sys.modules[key]

    # load each of the modules again; 
    # make old modules share state with new modules
    for key in loaded_package_modules:
        print 'loading %s' % key
        newmodule = __import__(key)
        oldmodule = loaded_package_modules[key]
        oldmodule.__dict__.clear()
        oldmodule.__dict__.update(newmodule.__dict__)

Which I very briefly tested like so:

import email, email.mime, email.mime.application
reload_package(email)

printing:

reloading email.iterators
reloading email.mime
reloading email.quoprimime
reloading email.encoders
reloading email.errors
reloading email
reloading email.charset
reloading email.mime.application
reloading email._parseaddr
reloading email.utils
reloading email.mime.base
reloading email.message
reloading email.mime.nonmultipart
reloading email.base64mime

Solution 3

With IPython comes the autoreload extension that automatically repeats an import before each function call. It works at least in simple cases, but don't rely too much on it: in my experience, an interpreter restart is still required from time to time, especially when code changes occur only on indirectly imported code.

Usage example from the linked page:

In [1]: %load_ext autoreload

In [2]: %autoreload 2

In [3]: from foo import some_function

In [4]: some_function()
Out[4]: 42

In [5]: # open foo.py in an editor and change some_function to return 43

In [6]: some_function()
Out[6]: 43

Solution 4

For Python version 3.4 and above

import importlib 
importlib.reload(<package_name>) 
from <package_name> import <method_name>

Refer below documentation for details.

Solution 5

There are some really good answers here already, but it is worth knowing about dreload, which is a function available in IPython which does as "deep reload". From the documentation:

The IPython.lib.deepreload module allows you to recursively reload a module: changes made to any of its dependencies will be reloaded without having to exit. To start using it, do:

http://ipython.org/ipython-doc/dev/interactive/reference.html#dreload

It is available as a "global" in IPython notebook (at least my version, which is running v2.0).

HTH

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Comments

[Olivier Verdier]

I tried those approaches, both by deleting the appropriate entry in sys.modules, and by using reload. It occasionally works, but sometimes create very, very subtle and strange bugs, where some variables suddenly become None for no reason at all.

[msw]

"Doctor, it hurts when I go like this", "Well, don't do that". I've always looked at reload as merely a convenience while debugging and think it a documentation bug that it is prescribed for actual live loading given how bad the semantic errors that it affords can be.

[dashesy]

I found that while %autoreload 2 does not work when there are deep hierarchies, this solution always works:stackoverflow.com/a/13096672/311567 which I hope will become the default in ipython

[Evgeni Sergeev]

Superstition! Hot reloading is achieved relatively elegantly even in such general cases as in PyUnit, but in specific cases setting it up doesn't take long and pays off very well. For example, in my scheme there is a lightweight main module containing a singleton that all my other instances hang off, and that only calls their methods, not functions. Reloading is by for each i: del sys.modules[mymodule_i], reload(..)-ing them, then swapping the __class__ of each instance from old to new.

[Mike Graham]

Debuggable, understandable code is plain and simple. Adding complexity in this way is not sensible or reasonable. I don't recommend doing anything remotely as slick as this ever.

[Vasif]

I believe the question is about restarting python interpreter. You seem to be using C# and python engine within it. ? Proabaly not what the OP is looking for.

[Geoffrey Anderson]

@user7175781 the syntax isn't working on my system no matter what I try. I guess PythonNet is different syntax and API than ipython. I was hoping to get your example to work but after many errors reported by the interpreter I finally realized you said PythonNet language.

[Daniel D.]

I found the del sys.modules[key] line of your answer useful for my scenario. I agree this doesn't prevent caching as the question asked, but these functions could help one to build a behaviour like expiring the python cache periodically.

[Cobertos]

Except perhaps if the target environment cannot be restarted so easily. Blender, for example, requires an application restart to clear the interpreter state which is horrible user experience for add-ons that get toggled on and off (the add-on UI allows this) if the add-on is larger than one module.

[Michael A.]

reload takes only parental module. You need recursively reload all of them, including module.version. more details: pprint(sys.modules)

[jtlz2]

imp is deprecated

[Luxalpa]

Depending on the project size, Houdini can easily take 5+ minutes to start. This is not a usable solution for every Python use case.