What is the difference between "is" and "==" in python? [duplicate]

Question

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?

Answer 1

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.