Is it pythonic: naming lambdas

Question

I\'m beginning to appreciate the value of lambda expressions in python, particularly when it comes to functional programming, map, functions returning functions

Answer 1

This is not Pythonic and PEP8 discourages it:

Always use a def statement instead of an assignment statement that binds a lambda expression directly to an identifier.

Yes:

def f(x): return 2*x

No:

f = lambda x: 2*x

The first form means that the name of the resulting function object is specifically 'f' instead of the generic '<lambda>'. This is more useful for tracebacks and string representations in general. The use of the assignment statement eliminates the sole benefit a lambda expression can offer over an explicit def statement (i.e. that it can be embedded inside a larger expression)

A rule of thumb for this is to think on its definition: lambdas expressions are anonymous functions. If you name it, it isn't anonymous anymore. :)

Answer 2

These are functionally (no pun intended) identical.

indexer = lambda a0, a1, idx: a0[idx] + a1[idx]

def indexer(a0, a1, idx):
    return a0[idx] + a1[idx]

If you have an aesthetic preference for one or the other, then use it.

Answer 3

I've written a named lambda expression to do the indexing instead of writing a whole other function

Well, you are writing a whole other function. You're just writing it with a lambda expression.

Why not use def? You get nicer stack traces and more syntactical flexibility, and you don't lose anything. It's not like def can't occur inside another function:

def fcn_operating_on_arrays(array0, array1):
    def indexer(a0, a1, idx):
        return a0[idx] + a1[idx]
    ...