Python: Check if an object is a list of strings
To test if all the items in a list are strings, use the all
built-in and a generator:
if all(isinstance(s, str) for s in lis):
Note though that, if your list is empty, this will still return True
since that is technically a list of 0 strings. However, since you want to consider []
as being False
, you will need to do this:
if lis and all(isinstance(s, str) for s in lis):
So, your function should be something like this:
def checktype(obj):
# This if statement makes sure input is a list that is not empty
if obj and isinstance(obj, list):
return all(isinstance(s, str) for s in obj)
else:
return False
This function will only return True
if its input is a list that is not empty and that is composed entirely of strings. Anything else (such as []
, ['a', 1]
, ('a', 'b')
, etc) will make it return False
.
Also, using all
in this way has an added bonus in that it stops checking on the first item it finds that returns False
(is not a string). This allows you to work with very large lists quite efficiently.
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
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:
- Is is non-empty?
- Is it a list?
- 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)))