import string
print string.ascii_lowercase # abcdefghijklmnopqrstuvwxyz
print type(string.ascii_lowercase) # <type 'str'>
print string.ascii_lowercase is str # False
Shouldn't it be True?
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Which version of Python?Hamish Grubijan– Hamish Grubijan2010-01-17 15:03:54 +00:00Commented Jan 17, 2010 at 15:03
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4 Answers
The is operator compares the identity of two objects. This is what I believe it does behind the scenes:
id(string.ascii_lowercase) == id(str)
Actual strings are always going to have a different identity than the type str, so this will always be False.
Here is the most Pythonic way to test whether something is a string:
isinstance(string.ascii_lowercase, basestring)
This will match both str and unicode strings.
1 Comment
eozzy
Thanks for the id() example, got it now! :)
use:
>>> isinstance('dfab', str)
True
is intended for identity testing.
1 Comment
artlung
+1 docs.python.org/library/… "Return true if the object argument is an instance of the classinfo argument, or of a (direct or indirect) subclass thereof."
string.ascii_lowercase is str should not be True.
type(string.ascii_lowercase) is str is True.
The is keyword checks object identity, not type.
You may have seen code like foo is None often and thought that None is a type. None is actually a singleton object.
4 Comments
eozzy
Oh, why not? any technical reason that I must know?
Skilldrick
because
string.ascii_lowercase is a string, whereas str is a type.
James Brooks
is test's whether they are the same object. i.e. they are both pointers to an object, are those objects the same.Skilldrick
See what happens when you do
type(str)Don't you want type(string.ascii_lowercase) is str ?
