What is the best (idiomatic) way to check the type of a Python variable? [duplicate]

Solution 1:

What happens if somebody passes a unicode string to your function? Or a class derived from dict? Or a class implementing a dict-like interface? Following code covers first two cases. If you are using Python 2.6 you might want to use collections.Mapping instead of dict as per the ABC PEP.

def value_list(x):
    if isinstance(x, dict):
        return list(set(x.values()))
    elif isinstance(x, basestring):
        return [x]
    else:
        return None

Solution 2:

type(dict()) says "make a new dict, and then find out what its type is". It's quicker to say just dict. But if you want to just check type, a more idiomatic way is isinstance(x, dict).

Note, that isinstance also includes subclasses (thanks Dustin):

class D(dict):
    pass

d = D()
print("type(d) is dict", type(d) is dict)  # -> False
print("isinstance (d, dict)", isinstance(d, dict))  # -> True

Solution 3:

built-in types in Python have built in names:

>>> s = "hallo"
>>> type(s) is str
True
>>> s = {}
>>> type(s) is dict
True

btw note the is operator. However, type checking (if you want to call it that) is usually done by wrapping a type-specific test in a try-except clause, as it's not so much the type of the variable that's important, but whether you can do a certain something with it or not.

Solution 4:

isinstance is preferrable over type because it also evaluates as True when you compare an object instance with it's superclass, which basically means you won't ever have to special-case your old code for using it with dict or str subclasses.

For example:

 >>> class a_dict(dict):
 ...     pass
 ... 
 >>> type(a_dict()) == type(dict())
 False
 >>> isinstance(a_dict(), dict)
 True
 >>> 

Of course, there might be situations where you wouldn't want this behavior, but those are –hopefully– a lot less common than situations where you do want it.