__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

Create your module file that has your classes. Import the module. Run getattr on the module you just imported. You can do a dynamic import using __import__ and pull the module from sys.modules.

Here's your module some_module.py:

class Foo(object):
    pass

class Bar(object):
    pass

And in another module:

import some_module

Foo = getattr(some_module, 'Foo')

Doing this dynamically:

import sys

__import__('some_module')
mod = sys.modules['some_module']
Foo = getattr(mod, 'Foo')

Answer 2

This is a hack, but you can wrap the module with a class:

class Wrapper(object):
  def __init__(self, wrapped):
    self.wrapped = wrapped
  def __getattr__(self, name):
    # Perform custom logic here
    try:
      return getattr(self.wrapped, name)
    except AttributeError:
      return 'default' # Some sensible default

sys.modules[__name__] = Wrapper(sys.modules[__name__])

Answer 3

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.

---

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.

Note that if you use this functionality 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.

Answer 4

We don't usually do it that way.

What we do is this.

class A(object):
....

# The implicit global instance
a= A()

def salutation( *arg, **kw ):
    a.salutation( *arg, **kw )

Why? So that the implicit global instance is visible.

For examples, look at the random module, which creates an implicit global instance to slightly simplify the use cases where you want a "simple" random number generator.

Answer 5

This is hackish, but...

import types

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

    def farewell(self, greeting, accusative):
         print greeting, accusative

def AddGlobalAttribute(classname, methodname):
    print "Adding " + classname + "." + methodname + "()"
    def genericFunction(*args):
        return globals()[classname]().__getattribute__(methodname)(*args)
    globals()[methodname] = genericFunction

# set up the global namespace

x = 0   # X and Y are here to add them implicitly to globals, so
y = 0   # globals does not change as we iterate over it.

toAdd = []

def isCallableMethod(classname, methodname):
    someclass = globals()[classname]()
    something = someclass.__getattribute__(methodname)
    return callable(something)


for x in globals():
    print "Looking at", x
    if isinstance(globals()[x], (types.ClassType, type)):
        print "Found Class:", x
        for y in dir(globals()[x]):
            if y.find("__") == -1: # hack to ignore default methods
                if isCallableMethod(x,y):
                    if y not in globals(): # don't override existing global names
                        toAdd.append((x,y))


for x in toAdd:
    AddGlobalAttribute(*x)


if __name__ == "__main__":
    salutation("world")
    farewell("goodbye", "world")

This works by iterating over the all the objects in the global namespace. If the item is a class, it iterates over the class attributes. If the attribute is callable it adds it to the global namespace as a function.

It ignore all attributes which contain "__".

I wouldn't use this in production code, but it should get you started.

Answer 6

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.