[Python-Dev] functools.compose to chain functions together

[Brett Cannon]

It would be best to discuss this on comp.lang.python or python-ideas to get
general support for the idea before trying to bring this to python-dev in
hopes of changing people's minds.

On Fri, Aug 14, 2009 at 11:39, Jason R. Coombs <jaraco at jaraco.com> wrote:

>  I’d like to express additional interest in python patch 1660179,
> discussed here:
>
>
>
> http://mail.python.org/pipermail/patches/2007-February/021687.html
>
>
>
> On several occasions, I’ve had the desire for something like this.  I’ve
> made due with lambda functions, but as was mentioned, the lambda is clumsy
> and harder to read than functools.compose would be.
>
>
>
> A potentially common use-case is when a library has a multi-decorator use
> case in which they want to compose a meta decorator out of one or more
> individual decorators.
>
>
>
> Consider the hypothetical library.
>
>
>
> # we have three decorators we use commonly
>
> def dec_register_function_for_x(func):
>
>                 # do something with func
>
>                 return func
>
>
>
> def dec_alter_docstring(func):
>
>                 # do something to func.__doc__
>
>                 return func
>
>
>
> def inject_some_data(data):
>
>                 def dec_inject_data(func):
>
>                                 func.data = data # this may not be legal,
> but assume it does something useful
>
>                                 return func
>
>                 return dec_inject_data
>
>
>
> # we could use these decorators explicitly throughout our project
>
> @dec_register_function_for_x
>
> @dec_alter_docstring
>
> @dec_inject_some_data(‘foo data 1’)
>
> def our_func_1(params):
>
>                 pass
>
>
>
> @dec_register_function_for_x
>
> @dec_alter_docstring
>
> @dec_inject_some_data(‘foo data 2’)
>
> def our_func_2(params):
>
>                 pass
>
>
>
> For two functions, that’s not too onerous, but if it’s used throughout the
> application, it would be nice to abstract the collection of decorators.  One
> could do this with lambdas.
>
>
>
> def meta_decorator(data):
>
>                 return lambda func:
> dec_register_function_for_x(dec_alter_docstring(dec_inject_some_data(data)(func)))
>
>
>
> But to me, a compose function is much easier to read and much more
> consistent with the decorator usage syntax itself.
>
>
>
> def meta_decorator(data):
>
> return compose(dec_register_function_for_x, dec_alter_docstring,
> dec_inject_some_data(data))
>
>
>
> The latter implementation seems much more readable and elegant.  One
> doesn’t even need to know the decorator signature to effectively compose
> meta_decorators.
>
>
>
> I’ve heard it said that Python is not a functional language, but if that
> were really the case, then functools would not exist. In addition to the
> example described above, I’ve had multiple occasions where having a general
> purpose function composition function would have simplified the
> implementation by providing a basic functional construct. While Python isn’t
> primarily a functional language, it does have some functional constructs,
> and this is one of the features that makes Python so versatile; one can
> program functionally, procedurally, or in an object-oriented way, all within
> the same language.
>
>
>
> I admit, I may be a bit biased; my first formal programming course was
> taught in Scheme.
>
>
>
> Nevertheless, I believe functools is the ideal location for a very basic
> and general capability such as composition.
>
>
>
> I realize this patch was rejected, but I’d like to propose reviving the
> patch and incorporating it into functools.
>
>
>
> Regards,
>
> Jason
>
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