Do something every time a module is imported

Question

Is there a way to do something (like print "funkymodule imported" for example) every time a module is imported from any other module? Not only the first time it's imported to the runtime or reloaded?

• NullUserException: Why would you want to import a module that's already been imported?
• millimoose: This probably isn't possible. PEP-396 describes such a mechanism, but was withdrawn.
• Bakuriu: The only way I can think of is to replace sys.modules with a custom mapping, reimplementing __getitem__, but the import machinery probably calls directly PyDict_GetItem from the C side so even this shouldn't work.
• Ned Batchelder: You can do this much more easily with grep...
• mgibsonbr: Once I ran into a problem of cyclic dependencies - module A was importing module B, that in turn was importing A again before A was ready. That was pretty hard to debug, and I ended up using something like this. In my case at least, using grep wouldn't be enough - since I was not only interested in how many times the import happened, but in their order as well (and I didn't know which modules were problematic). Thus I think it's an interesting question, although not something I'd do in production code.

Answer 1

One possibility would be to monkey patch __import__:

>>> old_import = __import__
>>> def my_import(module,*args,**kwargs):
... print module, 'loaded'
... return old_import(module,*args,**kwargs)
...
>>> __builtins__.__import__ = my_import
>>> import datetime
datetime loaded
>>> import datetime
datetime loaded
>>> import django
django loaded

It worked fine on command line (using Python 2.7.3 on Windows XP), but I dunno if would work in other environments.

To access the module object (instead of just the module name - so you can do something useful with it) just intercept the return value instead of the argument:

>>> def my_import(*args,**kwargs):
... ret = old_import(*args,**kwargs)
... print ret
... return ret
...
>>> __builtins__.__import__ = my_import
>>> import datetime
<module 'datetime' (built-in)>
>>> import django
<module 'django' from 'C:\Python27\lib\site-packages\django\__init__.pyc'>

Update: Just confirmed it works if used inside a python file too - though in this case, the correct way of assigning it is __builtins__['__import__'] = my_import.

• andyortlieb: I want to know every time module A gets imported from any other module. Not how many times module B is importing module A.
• mgibsonbr: This code will be called every time a module is imported (using import module), no matter who is importing it. Even imports inside imports will work (though for the inner modules, only the first time will show it - since the module body is not run again).
• andyortlieb: It's a cool solution. I was hoping to find some way to avoid monkeypatching import, especially since there's only exactly one specific module that I care about tracking in this sense. But it could definitely get me what I need.
• martineau: You could probably just do a return old_import(*args,**kwargs) at the end after whatever extra stuff you wanted done (unless you wanted to do something afterwards, of course).