How to check if all of the following items are in a list?

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I found, that there is related question, about how to find if at least one item exists in a list:
How to check if one of the following items is in a list?

But what is the best and pythonic way to find whether all items exists in a list?

Searching through the docs I found this solution:

>>> l = ['a', 'b', 'c']
>>> set(['a', 'b']) <= set(l)
True
>>> set(['a', 'x']) <= set(l)
False

Other solution would be this:

>>> l = ['a', 'b', 'c']
>>> all(x in l for x in ['a', 'b'])
True
>>> all(x in l for x in ['a', 'x'])
False

But here you must do more typing.

Is there any other solutions?

Operators like <= in Python are generally not overriden to mean something significantly different than “less than or equal to”. It’s unusual for the standard library does this–it smells like legacy API to me.

Use the equivalent and more clearly-named method, set.issubset. Note that you don’t need to convert the argument to a set; it’ll do that for you if needed.

set(['a', 'b']).issubset(['a', 'b', 'c'])

I would probably use set in the following manner :

set(l).issuperset(set(['a','b'])) 

or the other way round :

set(['a','b']).issubset(set(l)) 

I find it a bit more readable, but it may be over-kill. Sets are particularly useful to compute union/intersection/differences between collections, but it may not be the best option in this situation …

I like these two because they seem the most logical, the latter being shorter and probably fastest (shown here using set literal syntax which has been backported to Python 2.7):

all(x in {'a', 'b', 'c'} for x in ['a', 'b'])
#   or
{'a', 'b'}.issubset({'a', 'b', 'c'})

What if your lists contain duplicates like this:

v1 = ['s', 'h', 'e', 'e', 'p']
v2 = ['s', 's', 'h']

Sets do not contain duplicates. So, the following line returns True.

set(v2).issubset(v1)

To count for duplicates, you can use the code:

v1 = sorted(v1)
v2 = sorted(v2)


def is_subseq(v2, v1):
    """Check whether v2 is a subsequence of v1."""
    it = iter(v1)
    return all(c in it for c in v2) 

So, the following line returns False.

is_subseq(v2, v1)

This was what I was searching for online. However unfortunately did not find online, but while experimenting on python interpreter.

>>> case  = "caseCamel"
>>> label = "Case Camel"
>>> list  = ["apple", "banana"]
>>>
>>> (case or label) in list
False
>>> list = ["apple", "caseCamel"]
>>> (case or label) in list
True
>>> (case and label) in list
False
>>> list = ["case", "caseCamel", "Case Camel"]
>>> (case and label) in list
True
>>>

and if you have a looong list of variables held in a sublist variable

>>>
>>> list  = ["case", "caseCamel", "Case Camel"]
>>> label = "Case Camel"
>>> case  = "caseCamel"
>>>
>>> sublist = ["unique banana", "very unique banana"]
>>>
>>> # example for if any (at least one) item contained in superset (or statement)
...
>>> next((True for item in sublist if next((True for x in list if x == item), False)), False)
False
>>>
>>> sublist[0] = label
>>>
>>> next((True for item in sublist if next((True for x in list if x == item), False)), False)
True
>>>
>>> # example for whether a subset (all items) contained in superset (and statement)
...
>>> # a bit of demorgan's law
...
>>> next((False for item in sublist if item not in list), True)
False
>>>
>>> sublist[1] = case
>>>
>>> next((False for item in sublist if item not in list), True)
True
>>>
>>> next((True for item in sublist if next((True for x in list if x == item), False)), False)
True
>>>
>>>

An example of how to do this using a lambda expression would be:

issublist = lambda x, y: 0 in [_ in x for _ in y]

Not OP’s case, but – for anyone who wants to assert intersection in dicts and ended up here due to poor googling (e.g. me) – you need to work with dict.items:

>>> a = {'key': 'value'}
>>> b = {'key': 'value', 'extra_key': 'extra_value'}
>>> all(item in a.items() for item in b.items())
True
>>> all(item in b.items() for item in a.items())
False

That’s because dict.items returns tuples of key/value pairs, and much like any object in Python, they’re interchangeably comparable

Another solution would be:

l = ['a', 'b', 'c']
potential_subset1 = ['a', 'b']
potential_subset2 = ['a', 'x']
print(False not in [i in l for i in potential_subset1]) # True
print(False not in [i in l for i in potential_subset2]) # False

What makes my solution great is that you can write one-liners by putting the lists inline.


The answers/resolutions are collected from stackoverflow, are licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0 .

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