What's the canonical way to check for type in

Question

What is the best way to check whether a given object is of a given type? How about checking whether the object inherits from a given type?

Let's say I have an object o. How do I check whether it's a str?

Answer 1

Here is an example why duck typing is evil without knowing when it is dangerous. For instance: Here is the Python code (possibly omitting proper indenting), note that this situation is avoidable by taking care of isinstance and issubclassof functions to make sure that when you really need a duck, you don't get a bomb.

class Bomb:
    def __init__(self):
        ""

    def talk(self):
        self.explode()

    def explode(self):
        print "BOOM!, The bomb explodes."

class Duck:
    def __init__(self):
        ""
    def talk(self):
        print "I am a duck, I will not blow up if you ask me to talk."    

class Kid:
    kids_duck = None

    def __init__(self):
        print "Kid comes around a corner and asks you for money so he could buy a duck."

    def takeDuck(self, duck):
        self.kids_duck = duck
        print "The kid accepts the duck, and happily skips along"

    def doYourThing(self):
        print "The kid tries to get the duck to talk"
        self.kids_duck.talk()

myKid = Kid()
myBomb = Bomb()
myKid.takeDuck(myBomb)
myKid.doYourThing()

Answer 2

To check if o is an instance of str or any subclass of str, use isinstance (this would be the "canonical" way):

if isinstance(o, str):

To check if the type of o is exactly str (exclude subclasses):

if type(o) is str:

The following also works, and can be useful in some cases:

if issubclass(type(o), str):

See Built-in Functions in the Python Library Reference for relevant information.

One more note: in this case, if you're using python 2, you may actually want to use:

if isinstance(o, basestring):

because this will also catch Unicode strings (unicode is not a subclass of str; both str and unicode are subclasses of basestring). Note that basestring no longer exists in python 3, where there's a strict separation of strings (str) and binary data (bytes).

Alternatively, isinstance accepts a tuple of classes. This will return True if x is an instance of any subclass of any of (str, unicode):

if isinstance(o, (str, unicode)):

Answer 3

The most Pythonic way to check the type of an object is... not to check it.

Since Python encourages Duck Typing, you should just try...except to use the object's methods the way you want to use them. So if your function is looking for a writable file object, don't check that it's a subclass of file, just try to use its .write() method!

Of course, sometimes these nice abstractions break down and isinstance(obj, cls) is what you need. But use sparingly.