What is the best way to check if a variable is a list?

Question

I found this 3 ways to check it, but I don’t know which of them is the best:

x = ['Bla', 'Bla', 'Bla', 'etc']

if isinstance(a, list): print('Perfect!')
if type(a) is list:     print('Incredible!')
if type(a) == type([]): print('Awesome!')

Which of these is better?

Also, Can I use these ways to check whether an x is a string, tuple, dictionary, int, float, etc? If this is possible, in the first two methods do I have to convert a list to a tuple, string, dictionary, int, float, etc (no?), but in the third? I have to use (), {}, ”, and what more for int and float?

Asked By: Ender Look

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Answer 1

These all express different things, so really it depends on exactly what you wish to achieve:

isinstance(x, list) check if the type of x is either list or has list as a parent class (lets ignore ABCs for simplicity etc);
type(x) is list checks if the type of x is precisely list;
type(x) == list checks for equality of types, which is not the same as being identical types as the metaclass could conceivably override __eq__

So in order they express the following:

isinstance(x, list): is x like a list
type(x) is list: is x precisely a list and not a sub class
type(x) == list: is x a list, or some other type using metaclass magic to masquerade as a list.

Answered By: donkopotamus

Answer 2

Do you need to know if it’s a list, or just if it’s iterable (if you can use it in a for loop, for example)? Generally the “Pythonic way” is to just go ahead and do it in a try-except, because many things can be iterable: strings, lists, sets, deques, custom types, etc. (All it takes is an __iter__ or __getitem__ method)

If you REALLY need to know what type it is, isinstance() is typically the way to go since it will also cover subclasses.

As far as using type() == something is concerned, int, float, list, etc are all types: type(1) == int is True.

My typical approach, where I might have a string, a list (or tuple, etc.) of strings, or an int or other object which can be converted to a string, would be this (for Python 2 – Py3 no longer has basestring so you’ll need to check for str and/or bytes), assuming foo is your variable:

if isinstance(foo, basestring):
    foo = (foo,) # turn it into an iterable (tuple)
    # or, doStuff(foo) followed by a return or break
try:
    for x in foo:
        doStuff(str(x)) # do something with each element
except TypeError: # TypeError: 'some' object is not iterable
    doStuff(str(foo))

Answered By: DimeCadmium

Answer 3

Usually we prefer, isinstance(a, list) because it allows a to be either a list or list subclass.

For better speed, an exact check can to an identity test, type(a) is list. This is a bit faster than using ==.

That said, the norm in Python is to avoid type checks altogether and instead do “duck typing”. You call list methods on a and if they succeed, then we deem a to be sufficiently list like.

Answered By: Raymond Hettinger

What is the best way to check if a variable is a list?

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