Fix Python – getattr on a module

Question 1

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

Question 2

A while ago, Guido declared that all special method lookups on
new-style classes bypass __getattr__ and __getattribute__. Dunder methods had previously worked on modules – you could, for example, use a module as a context manager simply by defining __enter__ and __exit__, before those tricks broke.

Recently some historical features have made a comeback, the module __getattr__ among them, and so the existing hack (a module replacing itself with a class in sys.modules at import time) should be no longer necessary.

In Python 3.7+, you just use the one obvious way. To customize attribute access on a module, define a __getattr__ function at the module level which should accept one argument (name of attribute), and return the computed value or raise an AttributeError:

# my_module.py

def __getattr__(name: str) -> Any:
    ...

This will also allow hooks into “from” imports, i.e. you can return dynamically generated objects for statements such as from my_module import whatever.

On a related note, along with the module getattr you may also define a __dir__ function at module level to respond to dir(my_module). See PEP 562 for details.

Question

Example

Answer