Introduction¶
aspectlib provides two core tools to do AOP: Aspects and a weaver.
The aspect¶
An aspect can be created by decorating a generator with aspectlib.Aspect. The generator yields advices - simple behavior changing instructions.
The aspect is simple function decorator. Decorating a function with an aspect will change the function’s behavior according to the advices yielded by the generator.
Example:
@aspectlib.Aspect
def strip_return_value():
result = yield aspectlib.Proceed
yield aspectlib.Return(result.strip())
@strip_return_value
def read(name):
return open(name).read()
You can use these advices:
- Proceed or None - Calls the wrapped function with the default arguments. The yield returns the function’s return value or raises an exception. Can be used multiple times (will call the function multiple times).
- Proceed (*args, **kwargs) - Same as above but with different arguments.
- Return - Makes the wrapper return None instead. If aspectlib.Proceed was never used then the wrapped function is not called. After this the generator is closed.
- Return (value) - Same as above but returns the given value instead of None.
- raise exception - Makes the wrapper raise an exception.
The weaver¶
Patches classes and functions with the given aspect. When used with a class it will patch all the methods. In AOP parlance these patched functions and methods are referred to as cut-points.
Returns a Rollback object that can be used a context manager. It will undo all the changes at the end of the context.
Example:
@aspectlib.Aspect
def mock_open():
yield aspectlib.Return(StringIO("mystuff"))
with aspectlib.weave(open, mock_open):
assert open("/doesnt/exist.txt").read() == "mystuff"
You can use aspectlib.weave() on: classes, instances, builtin functions, module level functions, methods, classmethods, staticmethods, instance methods etc.