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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

_ 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. See tests for more example usage.


Every effort is made to optimize the placeholder instance. It's 20-40x faster than similar libraries on PyPI.

However, there is slight overhead (in CPython) in making an object callable. Placeholders with single operators can access the func attribute directly for optimal performance.

_.age.func         # operator.attrgetter('age')
_[key].func        # operator.itemgetter(key)

However, placeholder performance should generally be comparable to inlined expressions, and faster than lambda. Below are some example benchmarks.

min(data, key=operator.itemgetter(-1))    # 22.7 ms
min(data, key=_[-1])                      # 25.9 ms
min(data, key=lambda x: x[-1])            # 27.2 ms


$ pip install placeholder


  • Python ~=2.7, >=3.4


100% branch coverage.

$ pytest [--cov]



  • Deprecated __ (double underscore)


  • Optimized composite functions
  • Renamed to _ (single underscore) for consistency


  • Unary operators
  • __call__ implements methodcaller
  • __getitem__ supports only single argument
  • Improved error handling
  • composer object deprecated in favor of optimized F expression