chrisgrimm / better_partial

Better partial function application in Python

Install with:

pip install better-partial

My library provides both a more ergonomic and expressive way of partially applying functions than functools.partial. We begin by decorating a function with our partial operator. Let's consider the following function:

from better_partial import partial, _
# Note that "_" can be imported under a different name if it clashes with your conventions

@partial
def f(a, b, c, d, e):
  return a, b, c, d, e

We can then evaluate f like a standard function:

f(1, 2, 3, 4, 5) # --> (1, 2, 3, 4, 5)

but under the hood, my decorator now enables partial applications of f. Suppose we wanted to produce a function with all of f's arguments fixed except for c. We can accomplish this via the placeholder _ as follows:

g = f(1, 2, _, 4, 5)
g(3) # --> (1, 2, 3, 4, 5)

Alternatively, we might want to produce a function in which only c is specified. We can accomplish this using ...:

g = f(..., c=3)
g(1, 2, 4, 5) # --> (1, 2, 3, 4, 5)

... must be specified as the last positional argument and indicates that all following positional arguments should be treated as placeholders. This means that we can specify a and d as follows:

g = f(1, ..., d=4)
g(2, 3, 5) # --> (1, 2, 3, 4, 5)

The functions returned by partial applications are themselves partially applicable:

g = f(..., e=5)
h = g(_, 2, 3, 4)
h(1) # --> (1, 2, 3, 4, 5)

This enables a diversity of ways to partially apply functions. Consider the following equivalent expressions for (1, 2, 3, 4, 5):

f(1, 2, 3, 4, 5)
f(_, 2, _, 4, _)(1, 3, 5)
f(1, _, 3, ...)(2, 4, 5)
f(..., a=1, e=5)(_, 3, _)(2, 4)
f(..., e=5)(..., d=4)(1, 2, 3)
f(1, ..., e=5)(2, ..., d=4)(3)
f(..., e=5)(..., d=4)(..., c=3)(..., b=2)(..., a=1)
f(_, _, _, _, _)(1, 2, 3, 4, 5)
f(...)(1, 2, 3, 4, 5)