[Python-ideas] @contextlib.contextmanager(with_context=True) - passing a context to contextmanager

July 9, 2015

      contextlib.contextmanager in Python 3 added the ability to define functions
which work both as decorators and context managers, which is great and one
of the features I appreciate the most about Python 3 (seriously).
I am facing a use case where I find contextlib.contextmanager has some
limitations though. I have a decorator which looks like this:

def wrap_exceptions(fun):
    """Decorator which translates bare OSError and IOError exceptions
    into NoSuchProcess and AccessDenied.
    """
    @functools.wraps(fun)
    def wrapper(self, *args, **kwargs):
        # here "self" is the Process class instance
        try:
            return fun(self, *args, **kwargs)
        except EnvironmentError as err:
            if err.errno in (errno.ENOENT, errno.ESRCH):
                raise NoSuchProcess(self.pid, self._name)
            if err.errno in (errno.EPERM, errno.EACCES):
                raise AccessDenied(self.pid, self._name)
            raise
    return wrapper

...and I use it like this:

class Process:

    @wrap_exceptions
    def exe():
        ...

The two key things about this decorator are:
- it's designed to be used with class methods
- it has a reference to the method's class instance (self)

I would like to push this a bit further and make wrap_exceptions() work
also a contextmanager, which is what contextlib.contextmanager should allow
me to do in an easy way. As contextlib.contextmanager stands right it won't
allow my use case though as there's no way to pass a reference of the class
instance (Process) to wrap_exceptions. So here is my proposal: what if we
add a new "with_context" argument to contextlib.contextmanager?
The resulting code would look like this:

@contextlib.contextmanager(with_context=True)
def wrap_exceptions(ctx):
    # ctx is the Process class instance
    try:
        yield
    except EnvironmentError as err:
        pid = ctx.pid
        name = ctx.name
        if err.errno in (errno.ENOENT, errno.ESRCH):
            raise NoSuchProcess(pid, name)
        if err.errno in (errno.EPERM, errno.EACCES):
            raise AccessDenied(pid, name)
        raise

class Process:

    @wrap_exceptions()
    def exe():
        ...

class Process:

    def exe(self):
        with wrap_exceptions(self):
            ...

It must be noted that:

- when with_context=True and wrap_exceptions is used as a decorator it can
only be used to decorate class methods and not regular functions
- when used as a decorator "self" is automatically passed as the first
argument for wrap_exceptions

I'm not sure if this is actually possible as I haven't gone through
contextlib.contextmanager in details (it's quite magical).

Thoughts?

-- 
Giampaolo - http://grodola.blogspot.com

[Python-ideas] @contextlib.contextmanager(with_context=True) - passing a context to contextmanager

Giampaolo Rodola'