What is the difference between "is" and "==" in python?

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.

Answer 2

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