How to check if an object is a list or tuple (but not string)?

user_55246 0
Viewed 566443

This is what I normally do in order to ascertain that the input is a list/tuple - but not a str. Because many times I stumbled upon bugs where a function passes a str object by mistake, and the target function does for x in lst assuming that lst is actually a list or tuple.

assert isinstance(lst, (list, tuple))

My question is: is there a better way of achieving this?

assert list python types
19 Answers

In "duck typing" manner, how about

try:
    lst = lst + []
except TypeError:
    #it's not a list

or

try:
    lst = lst + ()
except TypeError:
    #it's not a tuple

respectively. This avoids the isinstance / hasattr introspection stuff.

You could also check vice versa:

try:
    lst = lst + ''
except TypeError:
    #it's not (base)string

All variants do not actually change the content of the variable, but imply a reassignment. I'm unsure whether this might be undesirable under some circumstances.

Interestingly, with the "in place" assignment += no TypeError would be raised in any case if lst is a list (not a tuple). That's why the assignment is done this way. Maybe someone can shed light on why that is.

user_100997560

Another version of duck-typing to help distinguish string-like objects from other sequence-like objects.

The string representation of string-like objects is the string itself, so you can check if you get an equal object back from the str constructor:

# If a string was passed, convert it to a single-element sequence
if var == str(var):
    my_list = [var]

# All other iterables
else: 
    my_list = list(var)

This should work for all objects compatible with str and for all kinds of iterable objects.

user_11588110

Python 3 has this:

from typing import List

def isit(value):
    return isinstance(value, List)

isit([1, 2, 3])  # True
isit("test")  # False
isit({"Hello": "Mars"})  # False
isit((1, 2))  # False

So to check for both Lists and Tuples, it would be:

from typing import List, Tuple

def isit(value):
    return isinstance(value, List) or isinstance(value, Tuple)
user_14884450 
assert (type(lst) == list) | (type(lst) == tuple), "Not a valid lst type, cannot be string"
user_98419070

in python >3.6 

import collections
isinstance(set(),collections.abc.Container)
True
isinstance([],collections.abc.Container)
True
isinstance({},collections.abc.Container)
True
isinstance((),collections.abc.Container)
True
isinstance(str,collections.abc.Container)
False
user_130454260
Related