Operator overloading for fast anonymous functions.

functional, lambda, scala, underscore, partial
pip install placeholder==1.5


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A 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

Note _ 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]