Best way to check if an item is present in a list of lists?

I have example list like this:

example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['', 'gg']]

Now, I check if it has empty string like this:

has_empty = False;
for list1 in example_list:
    for val1 in list1:
        if val1 == '':
            has_empty = True

print(has_empty)

This works OK as it prints True, but looking for more pythonik method?

Answers:

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Method 1

You can use itertools.chain.from_iterable:

>>> from itertools import chain
>>> example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['', 'gg']]
>>> '' in chain.from_iterable(example_list)
True

Just in case if the inner lists are bigger(more than 100 items) then using any with a generator will be faster than the above example because then the speed penalty of using a Python for-loop is compensated by the fast in-operation:

>>> any('' in x for x in example_list)
True

Timing comparisons:

>>> example_list = [['aaa']*1000, ['fff', 'gg']*1000, ['gg']*1000]*10000 + [['']*1000]
>>> %timeit '' in chain.from_iterable(example_list)
1 loops, best of 3: 706 ms per loop
>>> %timeit any('' in x for x in example_list)
1 loops, best of 3: 417 ms per loop

# With smaller inner lists for-loop makes `any()` version little slow

>>> example_list = [['aaa'], ['fff', 'gg'], ['gg', 'kk']]*10000 + [['']]
>>> %timeit '' in chain.from_iterable(example_list)
100 loops, best of 3: 2 ms per loop
>>> %timeit any('' in x for x in example_list)
100 loops, best of 3: 2.65 ms per loop

Method 2

You can do use combination of any, map and chain:

In [19]: example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['', 'gg']]

In [20]: import operator, itertools

In [21]: any(map(operator.not_, itertools.chain(*example_list)))
Out[21]: True

In [22]: example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['not empty', 'gg']]

In [23]: any(map(operator.not_, itertools.chain(*example_list)))
Out[23]: False

Method 3

Just convert the whole list to string and check for the presence of empty strings, i.e. '' or "" in it.

>>> example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['', 'gg']]
>>> any(empty in str(example_list) for empty in ("''", '""'))
True
>>> example_list = [['aaa'], ['fff', 'gg'], ['ff'], [ 'gg', ''']]
>>> any(empty in str(example_list) for empty in ("''", '""'))
False

Note that this won’t work with lists which have empty string as part of the string itself – example 'hello "" world'

Another approach could be to flatten the dict and check for presence of empty strings in it

>>> example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['gg', 'hello "" world']]
>>> '' in [item for sublist in example_list for item in sublist]
False
>>> example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['', 'gg', 'hello "" world']]
>>> '' in [item for sublist in example_list for item in sublist]
True

Method 4

using map and lambda:

>>> def check_empty(l):
...     k=map(lambda x: '' in x and True,l)
...     for x in k:
...         if x==True:
...             return True
...     return False
... 
>>> check_empty(example_list)
False
>>> example_list
[['aaa'], ['fff', 'gg'], ['ff'], ['gg', 'hello "" world']]
>>> example_list = [['aaa'], ['fff', 'gg',''], ['ff'], ['gg', 'hello "" world']]
>>> check_empty(example_list)
True

Method 5

example_list = [['aaa'], ['fff', 'gg'], ['ff'], ['', 'gg']]

has_empty = False

for list in example_list:

    if '' in list:
        has_empty = True
print(has_empty)

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0