Possible Duplicate:
Python ‘==’ vs ‘is’ comparing strings, ‘is’ fails sometimes, why?
Is
a == b
the same as
a is b
?
If not, what is the difference?
Edit: Why does
a = 1
a is 1
return True, but
a = 100.5
a is 100.5
return False?
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1dupe stackoverflow.com/questions/1504717/… among many othersSilentGhost– SilentGhost2009-10-28 00:00:44 +00:00Commented Oct 28, 2009 at 0:00
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Oops, sorry. That one didn't show up in the list when I asked this--feel free to close this.Matthew– Matthew2009-10-28 00:09:21 +00:00Commented Oct 28, 2009 at 0:09
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2 Answers
No, these aren't the same. is is a check for object identity - ie, checking if a and b are exactly the same object. Example:
a = 100.5
a is 100.5 # => False
a == 100.5 # => True
a = [1,2,3]
b = [1,2,3]
a == b # => True
a is b # => False
a = b
a == b # => True
a is b # => True, because if we change a, b changes too.
So: use == if you mean the objects should represent the same thing (most common usage) and is if you mean the objects should be in identical pieces of memory (you'd know if you needed the latter).
Also, you can overload == via the __eq__ operator, but you can't overload is.
7 Comments
Pavel Minaev
Java doesn't have
is, so it's hard to say how this is the exact opposite of it.
hasen
in java,
== tests memory addresshasen
good answer, but doesn't answer the part about
a is 1 returning true.
Peter
yep, that was an edit after the fact. this is interpreter-dependent, and reflects the fact that small integers are cached for speed. thus things equal to
1 will point to the same address for performance reasons. you can't rely on this, though.
Jeremy S
@hasen j I'm not sure about the underlying engine, but if
a = [1,2,3] and b = [1,2,3], then wouldn't a == b evaluate to true regardless of memory address? |
As already very clearly explained above.
is : used for identity testing (identical 'objects')
== : used for equality testing (~~ identical value)
Also keep in mind that Python uses string interning (as an optimisation) so you can get the following strange side-effects:
>>> a = "test"
>>> b = "test"
>>> a is b
True
>>> "test_string" is "test" + "_" + "string"
True
>>> a = 5; b = 6; c = 5; d = a
>>> d is a
True # --> expected
>>> b is a
False # --> expected
>>> c is a
True # --> unexpected
answered Oct 27, 2009 at 23:44
3 Comments
hasen
For numbers, it only works for integers that take on byte (e.g.
<= 256)hasen
And for strings, it doesn't "detect" similar strings magically. Consider:
>>> a = "test" >>> b = "test" >>> a is b True >>> a = "test_string" >>> c = b + "_string" >>> a == c True >>> a is c False Peter
this is due to the length of the strings / integers, and is interpreter dependent: don't rely on this behaviour.