tbh, even assigning a lambda to a variable seems unpythonic to me. if it needs a name, define it as a regular function. The difference between a lambda function and a regular function is that the latter has a __name__ attribute and an explicit return statement.

if you must add a docstring to a lambda, do it like this:

f = lambda x: x + 1
f.__doc__ = """adds 1 to input-arg"""

help(f) 
# outputs the following:
help(f)
Help on function <lambda> in module __main__:

<lambda> lambda x
    adds 1 to arg

This way, the documentation is actually available to the interpreter as a function docstring.

Quoting directly from pep-8

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

Lambda is more of a concept or programming technique then anything else.

Basically it's the idea that you get a function (a first-class object in python) returned as a result of another function instead of an object or primitive type. I know, it's confusing.

See this example from the python documentation:

def make_incrementor(n):
  return lambda x: x + n
f = make_incrementor(42)
f(0)
>>> 42
f(1)
>>> 43

So make_incrementor creates a function that uses n in it's results. You could have a function that would increment a parameter by 2 like so:

f2 = make_incrementor(2)
f2(3)
>>> 5

This is a very powerful idea in functional programming and functional programming languages like lisp & scheme.

Hope this helps.