Lambda recursion over list in Python

Question

I have a list which is a list of list (of list ...)

[ [], [ [] ] ]

Can I make a lambda function that returns the sum of the length of the lists at the bottom of this tree?

The one up here will return 0 because contains only empty lists. but this one:

[ ['foo'], [ [ [], [] ] ] ]

will return 3 which is the length of 'foo'. This one:

[ 'aa', [], [ ['ab'], [] ] ]

will return 4.

Thanks!

Why a lambda specifically? (I think the answer to the question as stated is "no" because Python lambdas don't have names, but it's a trivial exercise in recursion with a regular def.) — zwol
– zwol, Commented Mar 6, 2014 at 19:49
What have you tried so far? Perhaps we might be able to spot the error in logic. — wheaties
– wheaties, Commented Mar 6, 2014 at 19:49
that's not returning the length of the lists, but the sum of the length of non-empty elements... is that what you want? — isedev
– isedev, Commented Mar 6, 2014 at 19:49
See Can a lambda function call itself recursively in Python? — Janne Karila
– Janne Karila, Commented Mar 6, 2014 at 19:53
@JoranBeasley But it is lambda calculus at its finest... Looks impressive, is illegible and completely uncalled for. — Hyperboreus
– Hyperboreus, Commented Mar 6, 2014 at 20:04

Joran Beasley · Accepted Answer · 2014-03-06 21:36:56Z

1

recur = lambda l:len(l) if not isinstance(l,(tuple,list)) else sum(map(recur,l))

I think would work

or even cooler (self refering lambda :))

def myself (*args, **kw):
    caller_frame = currentframe(1)
    code = caller_frame.f_code
    return  FunctionType(code, caller_frame.f_globals)(*args,**kw)

print (lambda l:len(l) if not isinstance(l,(tuple,list)) else sum(map(myself,l)))(some_list)

or hyperboreus solution

lambda a:(lambda f, a: f(f, a))(lambda f, a:len(a) if not isinstance(a,(tuple,list)) else sum(f(f,e) for e in a), a)

which is whats known as a Y-Combinator ... which are nightmares to tell whats going on but they somehow work :P

edited Mar 6, 2014 at 21:36

answered Mar 6, 2014 at 19:52

Joran Beasley

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2 Comments

Hyperboreus Over a year ago

Just to keep the code golf going: Same thing with recursive, anonymous lamda and no stack inspection: lambda a:(lambda f, a: f(f, a))(lambda f, a:len(a) if not isinstance(a,(tuple,list)) else sum(f(f,e) for e in a), a). For Python3 at least.

Matteo Over a year ago

Thank you very much! I thought hardly before and it this is ingenious for me.. But how can I make that anonymous so I can call it in a while: while lambda: .......... : ?? You can read in my mind ;)

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