Adam Hupp

Python λ Shorthand

In Python a lambda expression (anonymous function) is created with the lambda keyword:

map(lambda x: x+1, [1,2,3])

Some Scheme interpreters such as Dr. Scheme allow the λ symbol (U+03BB) as a shorthand for lambda. I started wondering what this would look like in Python. For example:

map(λ x: x+1, [1,2,3])

There are a few cases where the shorthand is beneficial. For example, compare:

[i for i in lst if i > 42]


filter(λ x: x > 42, lst)

While list comprehensions are the standard python way to accomplish this, I really prefer how the predicate comes first in the latter version. Most of the functions in the itertools module could work nicely with the shorthand form:

itertools.groupby(lst, λ x: x.somevalue)

For a slightly more gratuitously complex example, a recursive factorial with the Y combinator:

Y = (λ X:
         (λ p:
              X(λ arg: p(p)(arg)))
     ((λ p:
           X(λ arg: p(p)(arg)))))

F = (λ f: (λ x: (x*f(x-1) if x > 0 else 1)))

fact = Y(F)
fact(5) => 120

Update: This post used to contain an interpreter patch that defines λ as a synonym for lambda , but a much simpler solution is to do the transformation at display time in emacs. That setup is described here.