python - __getattr__ on a module

Question

How can implement the equivalent of a __getattr__ on a class, on a module?

Example

When calling a function that does not exist in a module's statically defined attributes, I wish to create an instance of a class in that module, and invoke the method on it with the same name as failed in the attribute lookup on the module.

class A(object):
    def salutation(self, accusative):
        print "hello", accusative

# note this function is intentionally on the module, and not the class above
def __getattr__(mod, name):
    return getattr(A(), name)

if __name__ == "__main__":
    # i hope here to have my __getattr__ function above invoked, since
    # salutation does not exist in the current namespace
    salutation("world")

Which gives:

matt@stanley:~/Desktop$ python getattrmod.py 
Traceback (most recent call last):
  File "getattrmod.py", line 9, in <module>
    salutation("world")
NameError: name 'salutation' is not defined

Answer 1

There are two basic problems you are running into here:

1. __xxx__ methods are only looked up on the class
2. TypeError: can't set attributes of built-in/extension type 'module'

(1) means any solution would have to also keep track of which module was being examined, otherwise every module would then have the instance-substitution behavior; and (2) means that (1) isn't even possible... at least not directly.

Fortunately, sys.modules is not picky about what goes there so a wrapper will work, but only for module access (i.e. import somemodule; somemodule.salutation('world'); for same-module access you pretty much have to yank the methods from the substitution class and add them to globals() eiher with a custom method on the class (I like using .export()) or with a generic function (such as those already listed as answers). One thing to keep in mind: if the wrapper is creating a new instance each time, and the globals solution is not, you end up with subtly different behavior. Oh, and you don't get to use both at the same time -- it's one or the other.

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Update

From Guido van Rossum:

There is actually a hack that is occasionally used and recommended: a module can define a class with the desired functionality, and then at the end, replace itself in sys.modules with an instance of that class (or with the class, if you insist, but that's generally less useful). E.g.:

# module foo.py

import sys

class Foo:
    def funct1(self, <args>): <code>
    def funct2(self, <args>): <code>

sys.modules[__name__] = Foo()

This works because the import machinery is actively enabling this hack, and as its final step pulls the actual module out of sys.modules, after loading it. (This is no accident. The hack was proposed long ago and we decided we liked enough to support it in the import machinery.)

So the established way to accomplish what you want is to create a single class in your module, and as the last act of the module replace sys.modules[__name__] with an instance of your class -- and now you can play with __getattr__/__setattr__/__getattribute__ as needed.

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Note 1: If you use this functionality then anything else in the module, such as globals, other functions, etc., will be lost when the sys.modules assignment is made -- so make sure everything needed is inside the replacement class.

Note 2: To support from module import * you must have __all__ defined in the class; for example:

class Foo:
    def funct1(self, <args>): <code>
    def funct2(self, <args>): <code>
    __all__ = list(set(vars().keys()) - {'__module__', '__qualname__'})

Depending on your Python version, there may be other names to omit from __all__. The set() can be omitted if Python 2 compatibility is not needed.