Check if object is list of lists of strings?

Question

What is the elegant way to check if object is list of lists of strings, without nested loops? Probably here must be conventional ways to construct structured iterations.

UPD

Something like this:

l = [['a', 'b', 'c'], ['d', 1], 3, ['e', 2, 'f']]

def recurse(iterable, levels):
    results = []
    try:
        fn = levels[0]
    except IndexError:
        return
    for e in iterable:
        results.append(fn(e))
        try:
            results.extend(recurse(e, levels[1:]))
        except TypeError:
            pass
    return results

instance_of = lambda t: lambda e: isinstance(e, t)
print(recurse(l, [instance_of(list), instance_of(basestring)]))

UPD #2

Ive made some kind of homebrew functional programming, now it checks for list of lists of lists of strings:

from collections import Iterable
from itertools import imap, chain


def compose(f, g):
    return lambda *a, **kw: f(g(*a, **kw))

def concat(iterable):
    return chain.from_iterable(iterable)

def mk_iter(o):
    if isinstance(o, Iterable):
        return o
    else:
        return [o]

def put_in(f, g):
    """To support spirit of the Olympics :)"""
    return lambda e: concat(
        [mk_iter(f(e)),
         concat(imap(compose(mk_iter, g), mk_iter(e)))]
    )


ckr = lambda t: lambda e: isinstance(e, t)

l = [[['a', 'b'], ['c']], [['d'], ['1']], [1]]

fns = [ckr(list), ckr(list),ckr(list), ckr(str)]
fns.reverse()

print(list(reduce(lambda x, y: put_in(y, x), fns)(l)))

The task's nature is to test everything in every element of a list. That's a nested loop on the logical level. Each attempt to do it otherwise (using chain or similar) will only lead to hiding that fact. — Alfe
– Alfe, Commented Feb 12, 2014 at 10:02

thefourtheye · Accepted Answer · 2014-02-12 09:49:11Z

2

lol = [["a", "b"], ["c"], ["d", "e"], [1]]

from itertools import chain
print isinstance(lol, list) and all(isinstance(items, list) \
        and all(isinstance(item, str) for item in items) for items in lol)

edited Feb 12, 2014 at 9:49

answered Feb 12, 2014 at 9:41

thefourtheye

241k53 gold badges466 silver badges505 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Steve Jessop Over a year ago

and all(isinstance(item, list) for item in list_of_lists) and isinstance(list_of_list, list) ;-)

DhruvPathak · Accepted Answer · 2014-02-12 09:51:02Z

1

>>> lls = [ ["he","li"],["be","b"],["c","n","o"],["f","ne","na"] ]
>>> isinstance(lls,list) and all([ all(isinstance(y,str) for y in x) and isinstance(x,list) for x in lls])
True
>>> not_lls = [ ["he","li"],["be",1]]
>>> isinstance(lls,list) and all([ all(isinstance(y,str) for y in x) and isinstance(x,list) for x in not_lls])
False
>>> not_also_lls = [ ["he","li"],{}]
>>> isinstance(lls,list) and all([ all(isinstance(y,str) for y in x) and isinstance(x,list) for x in not_also_lls])
False

edited Feb 12, 2014 at 9:51

answered Feb 12, 2014 at 9:45

DhruvPathak

43.4k17 gold badges125 silver badges180 bronze badges

3 Comments

DhruvPathak Over a year ago

@thefourtheye nice catch, added the check for main list also

thefourtheye Over a year ago

he he he... Now compare this answer with my answer... :)

Alfe Over a year ago

I'd prefer not to build useless lists, so just strip the []. And swap the check to avoid running into an error when testing [3]: isinstance(lls, list) and all(isinstance(x, list) and all(isinstance(y, str) for y in x) for x in lls)

georg · Accepted Answer · 2014-02-12 10:21:13Z

In a more generic way:

def validate(x, types):
    if not isinstance(x, types[0]):
        raise ValueError('expected %s got %s for %r' % (types[0], type(x), x))
    if len(types) > 1:
        for y in x:
            validate(y, types[1:])

Usage:

try:
    validate(
        [['a', 'b', 'c'], ['d', 1], 3, ['e', 2, 'f']],
        [list, list, str])
except ValueError as e:
    print e  # expected <type 'str'> got <type 'int'> for 1
try:
    validate(
        [['a', 'b', 'c'], ['d', 'X'], 3, ['e', 2, 'f']],
        [list, list, str])
except ValueError as e:
    print e # expected <type 'list'> got <type 'int'> for 3

try:
    validate(
        [['a', 'b', 'c'], ['d', 'X'], ['3'], ['e', '2', 'f']],
        [list, list, str])
except ValueError as e:
    print e
else:
    print 'ok' # ok

Alfe · Accepted Answer · 2014-02-12 10:15:24Z

0

Plain and straight forward:

def isListOfStrings(los):
  return isinstance(los, list) and all(isinstance(e, str) for e in los)

def isListOfListOfStrings(lolos):
  return isinstance(lolos, list) and all(isListOfStrings(los) for los in lolos)

print isListOfListOfStrings([["foo"]])

I can't recommend a recursion here because the test for the outer does not really resemble the test for the inner. Recursions are more useful if the depth of the recursion isn't fixed (as in this case) and the task is similar on each level (not the case here).

edited Feb 12, 2014 at 10:15

answered Feb 12, 2014 at 10:07

Alfe

60.2k21 gold badges117 silver badges172 bronze badges

1 Comment

thefourtheye Over a year ago

Try isListOfListOfStrings(1)

Collectives™ on Stack Overflow

Check if object is list of lists of strings?

UPD

UPD #2

4 Answers 4

1 Comment

3 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

UPD

UPD #2

4 Answers 4

1 Comment

3 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related