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Module properties for Python

Project description


This package intends to offer a mechanism to add properties to your modules. The primary use-case of this functionality is to allow for the deferred execution of expensive-to-compute module/package level globals without needing to explicitly call a function.

Generally speaking, this module has two APIs that offer this functionality. You can use either of them or both of them, and everything will work more or less as you expect it to, as long as you follow the rules.

Note that this package supports the use of basically any descriptor at the module-level, not just properties.


The first method of using the package supports basically any descriptor being used as a decorator in the standard way. We show the use of property for the sake of brevity:

def module_property(mod):
    '''mod is the module that this property was defined in'''

import mprop; mprop.init()

Because remembering to put the import/init call at the bottom of a module can be annoying, we’ve got a special decorator that works just like the property object, but handles the mprop.init() call for you:

from mprop import mproperty

def module_property2(mod):
    '''I work exactly the same as the earlier module property, but you
    don't need to make a subsequent call to `mprop.init()`'''

Regardless of which method you use, if the name of your module is mod, you those that import the module can access the properties as normal:

from mprop import mproperty

def prop(mod):
    return "I was called!"

def fcn():
    # I can access the property via:
    print _pmodule.prop

import example

# the below should print "I was called!"
print example.prop
# this should also print "I was called!"

Referencing properties from within the module

After initialization, your code may want to reference the global properties. If you try to access the properties directly, you will get a NameError unless you locally aliased the value, the initialization has not completed, or unless someone else injected a value with that name into the globals.

In order to reference properties from within the module, any module that has been initialized with mprop.init() or has used the mproperty decorator will have in its global namespace an object called _pmodule, which is the “property-enhanced module” that offers property access.

If you find it necessary to require access to the original module object (which doesn’t support properties), you can access _module from the global namspace.

How it works

The short version: we replace the standard Python module instance in sys.modules during module import, which allows us to ensure that the module is replaced everywhere it is used. We perform some magic to ensure that everything available in the original module is available in the replacement module (the replacement module’s __dict__ is the module’s globals), and we post-process everything in the module’s global namespace to pull out descriptors as necessary.

When using the mproperty decorator, much the same goes on, but we add a special __getattribute__ method that handles descriptor extraction on first attribute access to the module.

Python magic that goes on

  • Using sys.getframe() to pull calling frame objects to get module globals and subsequently the module object itself
  • Replace sys.modules entries
  • Replace an instance __dict__
  • Assign descriptors during runtime by modifying the base class of the replacement module
  • Assign and delete class-level __getattribute__ methods

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