Prevent Python from caching the imported modules

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

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

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.

For Python version 3.4 and above

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

Refer below documentation for details.