How to apply a function to each sublist of a list in python?

Question

Lets say I have a list like this:

list_of_lists = [[\'how to apply\'],[\'a function\'],[\'to each list?\']]

And I have a function let\'s sa

Answer 1

Something that works on arbitrarily nested lists, e.g. [[1,2],[[5]],[7,[8,[9,11]]]] :

def apply_f(a,f):
 if isinstance(a,list):
     return map(lambda t:apply_f(t,f), a)
 else:
     return f(a)

here is an example of running this:

>>> ll=[[1,2],[[5]],[7,[8,[9,11]]]]
>>> apply_f(ll,lambda t:t**2)
[[1, 4], [[25]], [49, [64, [81, 121]]]]

---

and here is how to do the same only on selected levels:

def apply_f(a,f,depth,levels):
    if isinstance(a,list):
        return map(lambda t:apply_f(t,f,depth+1,levels), a)
    else:
        if depth in levels:
            return f(a)
        else:
            return a

getting e.g.

>>> apply_f(ll,lambda t:t**2, 0, [2,4])
[[1, 4], [[5]], [49, [8, [81, 121]]]]

there are some optimisations to do here, by avoiding passing f and levels around (making the recursive function an inner function in a wrapper, so that it can use f and levels from the outer scope), but that's minor. (Note that this is Python 2, for Python 3 you need to replace map with something else).

---

For a more generic input, the following would do the trick:

def apply_f(a,f):
    try: 
       return(f(a))
    except:
       return map(lambda t:apply_f(t,f), a)

So now also the following sort of works:

>> apply_f([(1,2),[[5]],[7,(8,[9,11])]],lambda t:t**2)
[[1, 4], [[25]], [49, [64, [81, 121]]]]

(things get slightly rewritten, as map() always produces lists...)