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Typed lambdas that are short and readable

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Write short and fully-typed lambdas where you need them.


  • Allows to write lambdas as _
  • Fully typed with annotations and checked with mypy, PEP561 compatible
  • Has a bunch of helpers for better composition
  • Easy to start: has lots of docs, tests, and tutorials


pip install lambdas

You also need to configure mypy correctly and install our plugin:

# In setup.cfg or mypy.ini:
plugins =

We recommend to use the same mypy settings we use.


Imagine that you need to sort an array of dictionaries like so:

>>> scores = [
...     {'name': 'Nikita', 'score': 2},
...     {'name': 'Oleg', 'score': 1},
...     {'name': 'Pavel', 'score': 4},
... ]

>>> print(sorted(scores, key=lambda item: item['score']))
[{'name': 'Oleg', 'score': 1}, {'name': 'Nikita', 'score': 2}, {'name': 'Pavel', 'score': 4}]

And it works perfectly fine. Except, that you have to do a lot of typing for such a simple operation.

That's where lambdas helper steps in:

>>> from lambdas import _

>>> scores = [
...     {'name': 'Nikita', 'score': 2},
...     {'name': 'Oleg', 'score': 1},
...     {'name': 'Pavel', 'score': 4},
... ]

>>> print(sorted(scores, key=_['score']))
[{'name': 'Oleg', 'score': 1}, {'name': 'Nikita', 'score': 2}, {'name': 'Pavel', 'score': 4}]

It might really save you a lot of effort, when you use a lot of lambda functions. Like when using returns library.

We can easily create math expressions:

>>> from lambdas import _

>>> math_expression = _ * 2 + 1
>>> print(math_expression(10))
>>> complex_math_expression = 50 / (_ ** 2) * 2
>>> print(complex_math_expression(5))

Work in progress:

  • _.method() is not supported yet for the same reason
  • TypedDicts are not tested with __getitem__
  • __getitem__ does not work with list and tuples (collections), only dicts (mappings)

For now you will have to use regular lamdbas in these cases.

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