placeholder uses operator overloading to create partially bound functions on-the-fly. When used in a binary expression, it will return a callable object with the other argument bound. It's useful for replacing
lambda in functional programming, and resembles Scala's placeholders.
from placeholder import _ # single underscore _.age < 18 # lambda obj: obj.age < 18 _[key] ** 2 # lambda obj: obj[key] ** 2
_ has special meaning in other contexts, such as the previous output in interactive shells. Assign to a different name as needed. Kotlin uses
it, but in Python
it is a common short name for an iterator.
_ is a singleton of an
F class, and
F expressions can also be used with functions.
from placeholder import F -F(len) # lambda obj: -len(obj)
All applicable double underscore methods are supported.
Every effort is made to optimize the placeholder instance. It's 20-40x faster than similar libraries on PyPI.
Placeholders are also iterable, allowing direct access to the underlying functions.
(func,) = _.age # operator.attrgetter('age')
Performance should generally be comparable to inlined expressions, and faster than lambda. Below are some example benchmarks.
min(data, key=operator.itemgetter(-1)) # 1x min(data, key=_[-1]) # 1.3x min(data, key=lambda x: x[-1]) # 1.6x
% pip install placeholder
100% branch coverage.
% pytest [--cov]