partiell

Partial argument application using ....

Description

An enhanced version of functools.partial() that allows partially applying positional arguments from both the left and right sides of the argument list.

We use ... (the Ellipsis object) as a placeholder in the argument list for the arguments that are not yet supplied. The object returned from partial() also supports being called with ... for further partial application.

Use partial as a function decorator (@partial) to make a function automatically support partial argument application with ....

Usage

As a drop-in replacement for functools.partial():

>>> from partiell import partial
>>> from operator import mul, truediv

>>> double = partial(mul, 2)  # same as functools.partial()
>>> double(5)
10

>>> halve = partial(truediv, ..., 2)  # functools.partial() cannot do this
>>> halve(3)
1.5

As a function decorator to enable smoother partial function application:

>>> @partial
... def f(x, y, z):
...    return x * 100 + y * 10 + z

f() can now be called with ... for partial function application:

>>> g = f(1, ...)  # Supply first argument only (x)
>>> g(2, 3)  # Supply the two remaining arguments (y, z)
123

Functions derived from f() automatically support ... themselves:

>>> h = g(2, ...)  # Supply g's first argument (y)
>>> h(3)  # Supply the final argument (z)
123

Using the ... placeholder also allows supplying arguments right-to-left:

>>> i = f(..., 3)  # Supply last argument only (z)
>>> i(1, 2)  # Supply the remaining arguments (x, y)
123

We can even supply arguments from both ends simultaneously:

>>> j = f(1, ..., 3)  # Supply first and last argument (x, z)
>>> j(2)  # Supply the remaining argument (y)
123

Discussion

Using ... as a placeholder for future function arguments allows for a "functional light" programming style that is somewhere between the verbosity of invoking partial() explicitly and the implicit currying provided by e.g. the @curry decorator in PyMonad.

The idea of using ... as a placeholder for function arguments and having it convert a function call into a partial function application is not new. AFAICS, it was first discussed on python-list in 2005, around the time partial() was first added to the Python standard library (PEP 309).

Installation

Run the following to install:

$ pip install partiell

Development

To work on partiell, clone this repo, and run the following (in a virtualenv) to get everything you need to develop and run tests:

$ pip install -e .[dev]

Alternatively, if you are using Nix, simply use the bundled shell.nix to get a development environment:

$ nix-shell

Use nox to run all tests (as defined in noxfile.py):

$ nox

Contributing

Main development happens at https://github.com/jherland/partiell/. Post issues and PRs there.