Welcome to python-aspectlib’s documentation!¶

aspectlib is an aspect-oriented programming, monkey-patch and decorators library. It is useful when changing behavior in existing code is desired.

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.

Installation¶

pip install aspectlib

Or, if you live in the stone age:

easy_install aspectlib

For you convenience there is a python-aspectlib meta-package that will just install aspectlib, in case you run pip install python-aspectlib by mistake.

Requirements¶

OS:	Any
Runtime:	Python 2.6, 2.7, 3.3, 3.4 or PyPy

Python 3.2, 3.1 and 3.0 are NOT supported (some objects are too crippled).

Rationale¶

There are perfectly sane use cases for monkey-patching (aka weaving):

Instrumenting existing code for debugging, profiling and other measurements.
Testing less flexible code. In some situations it’s infeasible to use dependency injection to make your code more testable.

Then in those situations:

You would need to handle yourself all different kids of patching (patching a module is different than patching a class, a function or a method for that matter). aspectlib will handle all this gross patching mumbo-jumbo for you, consistently, over many Python versions.
Writing the actual wrappers is repetitive, boring and error-prone. You can’t reuse wrappers but you can reuse function decorators.

Frequently asked questions¶

Why is it called weave and not patch ?¶

Because it does more things that just patching. Depending on the target object it will patch and/or create one or more subclasses and objects.

Why doesn’t aspectlib implement AOP like in framework X and Y ?¶

Some frameworks don’t resort to monkey patching but instead force the user to use ridiculous amounts of abstractions and wrapping in order to make weaving possible. Notable example: spring-python.

For all intents and purposes I think it’s wrong to have such high amount of boilerplate in Python.

Also, aspectlib is targeting a different stage of development: the maintenance stage - where the code is already written and needs additional behavior, in a hurry :)

Where code is written from scratch and AOP is desired there are better choices than both aspectlib and spring-python.

Why was aspectlib written ?¶

aspectlib was initially written because I was tired of littering other people’s code with prints and logging statements just to fix one bug or understand how something works. aspectlib.debug.log is aspectlib‘s crown jewel. Of course, aspectlib has other applications, see the Rationale.

Examples¶

Retry decorator¶

TODO: Make a more configurable retry decorator and add it in aspectlib.contrib.

class Client(object):
    def __init__(self, address):
        self.address = address
        self.connect()
    def connect(self):
        # establish connection
    def action(self, data):
        # do some stuff

def retry(retries=(1, 5, 15, 30, 60), retry_on=(IOError, OSError), prepare=None):
    assert len(retries)

    @aspectlib.Aspect
    def retry_aspect(*args, **kwargs):
        durations = retries
        while True:
            try:
                yield aspectlib.Proceed
                break
            except retry_on as exc:
                if durations:
                    logging.warn(exc)
                    time.sleep(durations[0])
                    durations = durations[1:]
                    if prepare:
                        prepare(*args, **kwargs)
                else:
                    raise

    return retry_aspect

Now patch the Client class to have the retry functionality on all its methods:

aspectlib.weave(Client, retry())

or with different retry options (reconnect before retry):

aspectlib.weave(Client, retry(prepare=lambda self, *_: self.connect())

or just for one method:

aspectlib.weave(Client.action, retry())

You can see here the advantage of having reusable retry functionality. Also, the retry handling is decoupled from the Client class.

Debugging¶

... those damn sockets:

>>> import aspectlib, socket, sys
>>> with aspectlib.weave(
...     socket.socket,
...     aspectlib.debug.log(
...         print_to=sys.stdout,
...         stacktrace=None,
...     ),
...     lazy=True,
... ):
...     s = socket.socket()
...     s.connect(('google.com', 80))
...     s.send(b'GET / HTTP/1.0\r\n\r\n')
...     s.recv(8)
...     s.close()
...
{socket...}.connect(('google.com', 80))
{socket...}.connect => None
{socket...}.send(...'GET / HTTP/1.0\r\n\r\n')
{socket...}.send => 18
18
{socket...}.recv(8)
{socket...}.recv => ...HTTP/1.0...
...'HTTP/1.0'
...

The output looks a bit funky because it is written to be run by doctest.

Testing¶

Mock behavior for tests:

class MyTestCase(unittest.TestCase):

    def test_stuff(self):

        @aspectlib.Aspect
        def mock_stuff(self, value):
            if value == 'special':
                yield aspectlib.Return('mocked-result')
            else:
                yield aspectlib.Proceed

        with aspectlib.weave(foo.Bar.stuff, mock_stuff):
            obj = foo.Bar()
            self.assertEqual(obj.stuff('special'), 'mocked-result')

Reference¶

aspectlib¶

`aspectlib.ALL_METHODS`	Compiled regular expression objects
`aspectlib.NORMAL_METHODS`	Compiled regular expression objects
`aspectlib.weave`	Send a message to a recipient
`aspectlib.Rollback`	When called, rollbacks all the patches and changes the `weave()` has done.
`aspectlib.Aspect`	Container for the advice yielding generator.
`aspectlib.Proceed`	Instructs the Aspect Calls to call the decorated function. Can be used multiple times.
`aspectlib.Return`	Instructs the Aspect to return a value.

class aspectlib.Aspect(advise_function)[source]¶: Container for the advice yielding generator. Can be used as a decorator on other function to change behavior according to the advices yielded from the generator.

class aspectlib.Proceed(*args, **kwargs)[source]¶

Instructs the Aspect Calls to call the decorated function. Can be used multiple times.

If not used as an instance then the default args and kwargs are used.

class aspectlib.Return(value)[source]¶: Instructs the Aspect to return a value.

class aspectlib.Rollback(rollback=None)[source]¶

When called, rollbacks all the patches and changes the weave() has done.

__enter__()[source]¶: Returns self.

__exit__(*_)[source]¶: Performs the rollback.

rollback(*_)¶: Alias of __exit__.

__call__(*_)¶: Alias of __exit__.

aspectlib.ALL_METHODS Weave all magic methods. Can be used as the value for methods argument in weave.¶: Compiled regular expression objects

aspectlib.NORMAL_METHODS Only weave non-magic methods. Can be used as the value for methods argument in weave.¶: Compiled regular expression objects

aspectlib.weave(target, aspect[, subclasses=True, methods=NORMAL_METHODS, lazy=False, aliases=True])[source]¶

Send a message to a recipient

Raises TypeError:
Parameters:	target (`aspectlib.Aspect`, function decorator or list of) – The object to weave. aspects – The aspects to apply to the object. subclasses (bool) – If `True`, subclasses of target are weaved. Only available for classes aliases (bool) – If `True`, aliases of target are replaced. lazy (bool) – If `True` only patch target’s `__init__`, the rest of the methods are patched after `__init__` is called. Only available for classes methods (list or regex or string) – Methods from target to patch. Only available for classes
Returns:	`aspectlib.Rollback` instance
	If target is a unacceptable object, or the specified options are not available for that type of object.

Changed in version 0.4.0: Replaced only_methods, skip_methods, skip_magicmethods options with methods. Renamed on_init option to lazy. Added aliases option. Replaced skip_subclasses option with subclasses.

aspectlib.debug¶

`aspectlib.debug.log`	Decorates func to have logging.
`aspectlib.debug.format_stack`	Returns a one-line string with the current callstack.
`aspectlib.debug.frame_iterator`	Yields frames till there are no more.
`aspectlib.debug.strip_non_ascii`	Convert to string (using str) and replace non-ascii characters with a dot (`.`).

aspectlib.debug.format_stack(skip=0, length=6, _sep='/')[source]¶: Returns a one-line string with the current callstack.

aspectlib.debug.frame_iterator(frame)[source]¶: Yields frames till there are no more.

aspectlib.debug.strip_non_ascii(val)[source]¶: Convert to string (using str) and replace non-ascii characters with a dot (.).

aspectlib.debug.log(func=None, stacktrace=10, stacktrace_align=60, attributes=(), module=True, call=True, call_args=True, call_args_repr=<built-in function repr>, result=True, exception=True, exception_repr=<built-in function repr>, result_repr=<function strip_non_ascii at 0x2dbf848>, use_logging='CRITICAL', print_to=None)[source]¶

Decorates func to have logging.

Parameters:

func (function) – Function to decorate. If missing log returns a partial which you can use as a decorator.
stacktrace (int) – Number of frames to show.
stacktrace_align (int) – Column to align the framelist to.
attributes (list) – List of instance attributes to show, in case the function is a instance method.
module (bool) – Show the module.
call (bool) – If True, then show calls. If False only show the call details on exceptions (if exception is enabled) (default: True)
call_args (bool) – If True, then show call arguments. (default: True)
call_args_repr (bool) – Function to convert one argument to a string. (default: repr)
result (bool) – If True, then show result. (default: True)
exception (bool) – If True, then show exceptions. (default: True)
exception_repr (function) – Function to convert an exception to a string. (default: repr)
result_repr (function) – Function to convert the result object to a string. (default: strip_non_ascii - like str but nonascii characters are replaced with dots.)
use_logging (string) – Emit log messages with the given loglevel. (default: "CRITICAL")
print_to (fileobject) – File object to write to, in case you don’t want to use logging module. (default: None - printing is disabled)

Returns:

A decorator or a wrapper.

Example:

>>> @log(print_to=sys.stdout)
... def a(weird=False):
...     if weird:
...         raise RuntimeError('BOOM!')
>>> a()
a()                                                           <<< ...
a => None
>>> try:
...     a(weird=True)
... except Exception:
...     pass # naughty code !
a(weird=True)                                                 <<< ...
a ~ raised RuntimeError('BOOM!',)

You can conveniently use this to logs just errors, or just results, example:

>>> import aspectlib
>>> with aspectlib.weave(int, log(call=False, result=False, print_to=sys.stdout)):
...     try:
...         int('invalid')
...     except Exception:
...         pass # naughty code !
int('invalid')                                                <<< ...
int ~ raised ValueError("invalid literal for int() with base 10: 'invalid'",)

This makes debugging naughty code easier.

Changed in version 0.5.0: Renamed arguments to call_args. Renamed arguments_repr to call_args_repr. Added call option. Fixed issue with logging from old-style methods (object name was a generic “instance”).

aspectlib.test¶

`aspectlib.test.record`	Factory or decorator (depending if func is initially given).
`aspectlib.test.mock`	Factory for a decorator that makes the function return a given return_value.

This module is designed to be a lightweight, orthogonal and easy to learn replacement for the popular mock framework.

Example usage, suppose you want to test this class:

>>> class ProductionClass(object):
...     def method(self):
...         return 'stuff'
>>> real = ProductionClass()

With aspectlib.test:

>>> from aspectlib import weave, test
>>> patch = weave(real.method, [test.mock(3), test.record(call=True)])
>>> real.method(3, 4, 5, key='value')
3
>>> assert real.method.calls == [(real, (3, 4, 5), {'key': 'value'})]

As a bonus, you have an easy way to rollback all the mess:

>>> patch.rollback()
>>> real.method()
'stuff'

With mock:

>>> from mock import Mock
>>> real = ProductionClass()
>>> real.method = Mock(return_value=3)
>>> real.method(3, 4, 5, key='value')
3
>>> real.method.assert_called_with(3, 4, 5, key='value')

aspectlib.test.mock(return_value, call=False)[source]¶

Factory for a decorator that makes the function return a given return_value.

Parameters:	return_value – Value to return from the wrapper. call (bool) – If `True`, call the decorated function. (default: `False`)
Returns:	A decorator.

aspectlib.test.record(func=None, call=False, history=None)[source]¶

Factory or decorator (depending if func is initially given).

Parameters:	history (list) – An object where the Call objects are appended. If not given a new list object will be created. call (bool) – If `True` the func will be called. (default: `False`)
Returns:	A wrapper that has a calls property.

The decorator returns a wrapper that records all calls made to func. The history is available as a call property. If access to the function is too hard then you need to specify the history manually.

Example:

>>> @record
... def a():
...     pass
>>> a(1, 2, 3, b='c')
>>> a.calls
[Call(self=None, args=(1, 2, 3), kwargs={'b': 'c'})]

Or, with your own history list:

>>> calls = []
>>> @record(history=calls)
... def a():
...     pass
>>> a(1, 2, 3, b='c')
>>> a.calls
[Call(self=None, args=(1, 2, 3), kwargs={'b': 'c'})]
>>> calls is a.calls
True

Development¶

Development is happening on Github.

TODO & Ideas¶

Validation¶

class BaseProcessor(object):
    def process_foo(self, data):
        # do some work

    def process_bar(self, data):
        # do some work

class ValidationConcern(aspectlib.Concern):
    @aspectlib.Aspect
    def process_foo(self, data):
        # validate data
        if is_valid_foo(data):
          yield aspectlib.Proceed
        else:
          raise ValidationError()

    @aspectlib.Aspect
    def process_bar(self, data):
        # validate data
        if is_valid_bar(data):
          yield aspectlib.Proceed
        else:
          raise ValidationError()

aspectlib.weave(BaseProcesor, ValidationConcern)

class MyProcessor(BaseProcessor):
    def process_foo(self, data):
        # do some work

    def process_bar(self, data):
        # do some work

# MyProcessor automatically inherits BaseProcesor's ValidationConcern

Changelog¶

Version 0.6.1¶

Fix checks inside aspectlib.debug.log that would inadvertently call __bool__/__nonzero.

Version 0.6.0¶

Don’t include __getattribute__ in ALL_METHODS - it’s too dangerous dangerous dangerous dangerous dangerous dangerous ... ;)
Do a more reliable check for old-style classes in debug.log
When weaving a class don’t weave attributes that are callable but are not actually routines (functions, methods etc)

Version 0.5.0¶

Changed aspectlib.debug.log:
- Renamed arguments to call_args.
- Renamed arguments_repr to call_args_repr.
- Added call option.
- Fixed issue with logging from old-style methods (object name was a generic “instance”).
Fixed issues with weaving some types of builtin methods.
Allow to apply multiple aspects at the same time.
Validate string targets before weaving. aspectlib.weave('mod.invalid name', aspect) now gives a clear error (invalid name is not a valid identifier)
Various documentation improvements and examples.

Version 0.4.1¶

Remove junk from 0.4.0’s source distribution.

Version 0.4.0¶

Changed aspectlib.weave:
- Replaced only_methods, skip_methods, skip_magicmethods options with methods.
- Renamed on_init option to lazy.
- Added aliases option.
- Replaced skip_subclasses option with subclasses.
Fixed weaving methods from a string target.

Welcome to python-aspectlib’s documentation!¶

Introduction¶

The aspect¶

The weaver¶

Installation¶

Requirements¶

Rationale¶

Frequently asked questions¶

Why is it called weave and not patch ?¶

Why doesn’t aspectlib implement AOP like in framework X and Y ?¶

Why was aspectlib written ?¶

Examples¶

Retry decorator¶

Debugging¶

Testing¶

Reference¶

aspectlib¶

aspectlib.debug¶

aspectlib.test¶

Development¶

TODO & Ideas¶

Validation¶

Changelog¶

Version 0.6.1¶

Version 0.6.0¶

Version 0.5.0¶

Version 0.4.1¶

Version 0.4.0¶

Indices and tables¶