Python: Check if an object is a list of strings

Question

How to check if an object is a list of strings? I could only check if an object is string as such:

def checktype(obj):
  if isinstance(obj,str):
    print "It's a string"

obj1 = ['foo','bar','bar','black','sheet']
obj2 = [1,2,3,4,5,'bar']
obj3 = 'bar'

for i in [obj1,obj2,obj3]:
  checktype(i)

Desired output:

It's a list of strings
It's not a list of strings or a single string
It's a single string

Solution 1

Something like this, I presume? You could do some checks to see if it's a single string.

>>> def checktype(obj):
        return bool(obj) and all(isinstance(elem, basestring) for elem in obj)

>>> obj1 = ['foo','bar','bar','black','sheet']
>>> obj2 = [1,2,3,4,5,'bar']
>>> obj3 = 'bar'
>>> for i in [obj1, obj2, obj3] :
        print checktype(i)


True
False
True

Why check for basestring instead of str?

You should check for basestring instead of str since it's a common class from which both the str and unicode types inherit from. Checking only the str leaves out the unicode types.

As per Steven Rumbalski's suggestions, if you need to specifically check for a list of strings, you could do.

>>> def is_list_of_strings(lst):
        return bool(lst) and not isinstance(lst, basestring) and all(isinstance(elem, basestring) for elem in lst)
        # You could break it down into `if-else` constructs to make it clearer to read.

>>> for i in [obj1, obj2, obj3] :
        print is_list_of_strings(i)


True
False
False

EDIT - As per abarnert's suggestion, you could also check for a list instead of not isinstance(lst, basestring), the code would get rewritten as.

>>> def is_list_of_strings(lst):
        return bool(lst) and isinstance(lst, list) and all(isinstance(elem, basestring) for elem in lst)
        # You could break it down into `if-else` constructs to make it clearer to read.

>>> for i in [obj1, obj2, obj3] :
        print is_list_of_strings(i)


True
False
False

Moving away from one liners, we could use.

>>> def is_list_of_strings(lst):
        if lst and isinstance(lst, list):
            return all(isinstance(elem, basestring) for elem in lst)
        else:
            return False

Solution 2

This answer is for Python 3. If for example the variable name is pins:

if not (pins and isinstance(pins, list) and all(isinstance(pin, str) for pin in pins)):
    raise TypeError('pins must be a list of one or more strings.')

It checks for three things:

1. Is is non-empty?
2. Is it a list?
3. Does it contain strings?

---

If you also need to check for uniqueness of the strings, include this fourth check:

and (len(pins) == len(set(pins)))

Solution 3

As a one liner:

assert all(map(lambda x: isinstance(x, str), my_list))

Solution 4

The answers I've read so far raise exepctions when given a non-list that isn't a string...and isn't iterable either. That question is addressed in:

In Python, how do I determine if an object is iterable?

Taking the duck-typing approach:

def categorize(x):
    result = "not a string or list of strings"
    if isinstance(x, basestring):
        return "It's a single string"
    try:
        if all(isinstance(y, basestring) for y in x):
            return "It's a list of strings"
    except TypeError:
        pass
    return "It's not a list of strings or a single string"

data = [ 5, "xyzzy", list("xyzzy"), ['1', '23', 456]]
for x in data:
    print x, categorize(x)

Output:

5 It's not a list of strings or a single string
xyzzy It's a single string
['x', 'y', 'z', 'z', 'y'] It's a list of strings
['1', '23', 456] It's not a list of strings or a single string

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