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A library for attaching additional functionality around existing functions.

Project description

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A library for attaching additional functionality around existing functions, such as before or after execution, or given a raised exception or specific return value.

@attach(on_before=lambda: print('Initializing connection'),
        on_after=lambda: print('Connect successfully established'),
        on_error=lambda x: print('Erred establishing connection: %s' % x),
        on_return=lambda x: print('Returned connection: %s' % x))
def connection(conn_str, params):
    conn = db(conn_str, params)


The latest version of is available via pip:

pip install

Alternatively, you can download and install from source:

python install

Getting Started

The attach function contains the following signature:

@attach(on_before=None, on_after=None, on_error=None, on_return=None,
        override_error=False, override_return=False, before_has_kwargs=False)
def func(...)

It serves as both a function decorator, and a runnable wrapper and is configurable through it’s dynamic parameters. Most of which are function callbacks which allow the user to highly configure the additional behavior.

Before / After Execution

Either prior to the wrapped function being executed, or afterwards, another function can be called. The most simplistic use case for this is logging the beginning and ending of execution of a function.

@attach(on_before=lambda:'Execution began'), on_after=lambda:'Execution ended'))
def func():

If an exception is raised by the wrapped function (or the on_before function), the on_after function isn’t called.

More complex usage comes from digesting the parameters meant for the wrapped function and transforming them in some way. This is accomplished by simply returning an object from the on_before function and the values will be used instead of the ones passed in.

def sanitize(string):
    # Do some stuff
    return new_string

@attach(on_before=lambda x: sanitize(x))
def func(string):

If an iterable is returned, it is used as the args of the wrapped function. The before_with_kwargs argument can be set to True to specify that the return value be used as the kwargs of the wrapped function (which means it should be a dictionary. If an iterable is returned and this parameter is set, the last value is used as the kwargs, and the rest as the args.

def sanitize(string):
    # Do some stuff
    return new_string

@attach(on_before=lambda x: sanitize(x), {'use_ssl': True})
def func(string):

Error Handling

The on_error can be used to execute a function if an exception is raised. By default, the original exception is still raised after the on_error callback is called. This can be changed by setting override_error to True. This can be used to instead return a value or raise a different exception.

def on_error(e):
    print('Caught error: ' + str(e))
    if isinstance(e, TypeError):
        return -1

@attach(on_error=on_error, override_error=True)
def func():
    raise TypeError

# -1 is returned instead of raising TypeError

Return Value Handling

Like raised exception, return values can consumed by a on_return function in a similar manner. By default, the original return value is still returned after the on_return callback is called. This can be changed by setting override_return to True. A common use case for this is when interacting with functions that yield a return value that indicates a failed state (like -1 or None), while other values indicate a successful state (like 0 or an object). This behavior can be transformed into a simple bool True or False return value instead.

def on_return(val):
    if val in (-1, None):
        return False
    return True

@attach(on_return=on_return, override_return=True)
def func()
    return -1

# False is returned instead of -1

If an exception is raised by the wrapped function (or the on_before or on_after functions), the on_return function isn’t called.

Advanced Usage

Instead of using as a decorator, attach can be used as an instead for wrapping an arbitrary number of function calls. This can be achieved via the run method.

def func_a():

def func_b():

attacher = attach(on_before=..., on_after=..., on_error=..., on_return=...)

# Using same configured attach instance, args, kwargs), args, kwargs)

Besides using the provided run method, like any decorator functions can be locally wrapped, passed around, and executed.

def func():

attacher = attach(on_before=..., on_after=..., on_error=..., on_return=...)
attach_func = attacher(func)
attach_func(args, kwargs)

# Or as a one-off like so
attach(...)(func)(args, kwargs)

Each of the function parameters that can be passed into attach, can actually be configured to accepts different number of parameters depending on the function. They can each either accept 0 parameters, the parameters that would be typically passed in, or the wrapped function’s args and kwargs in addition to the parameters typically given.

Optionally passing in the args and kwargs allows for building more complex callback functions. Each of the possible function variations are shown below.

def on_before(): ...
def on_before(*args, **kwargs): ...

def on_after(): ...
def on_after(*args, **kwargs): ...

def on_error(): ...
def on_error(error): ...
def on_error(error, *args, **kwargs): ...

def on_return(): ...
def on_return(value): ...
def on_return(value, *args, **kwargs): ...


Contributions or suggestions are welcome! Feel free to open an issue if a bug is found or an enhancement is desired, or even a pull request.


All changes and versioning information can be found in the CHANGELOG.


Copyright (c) 2018 Jared Gillespie. See LICENSE for details.

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