Everything is an object: identity, type and value

In Python, every value is an object: numbers, strings, lists, functions, even classes and modules. And every object has three things:

• an identity, which never changes while the object exists. id(x) shows it as a number (in CPython, the version running here, it's the object's address in memory);
• a type, which also never changes: type(x);
• a value, which may or may not be able to change, depending on the type.

A name like x isn't any of these. It's a reference that leads to an object.

Two equal lists, or one list?

== asks "do these have the same value?". is asks "are these the same object?", which is the same as comparing their ids. Run this and compare the numbers:

mine = ["pears", "rice"]
yours = ["pears", "rice"]
ours = mine

print(mine == yours, mine is yours)  # True False
print(mine == ours, mine is ours)    # True True
print(id(mine), id(yours), id(ours))

Two of the three ids match. yours was built separately, so it's a second object that happens to hold the same items. ours = mine copied nothing at all: it made a second name for the one list that already existed.

Changing an object, or getting a new one

Watch the id when the value changes:

basket = ["pears"]
print(id(basket))
basket.append("rice")
print(id(basket))  # the same: the list itself changed

count = 10
print(id(count))
count = count + 1
print(id(count))   # different: count now names another object

A list is mutable: append changes the object in place, so its id stays. An int is immutable: there's no way to change the object 10, so count + 1 makes a new object, 11, and the name moves to it. Strings, floats, tuples, True and None are immutable too.

Types are objects too

print(type(42), type("hi"), type(print))
print(type(int))           # <class 'type'>: int is an object whose type is type
say = print                # a function is an object: give it another name
say("Hello from say")

When to use is

Use == to compare values, almost always. Use is for None: there is only ever one None object, so if result is None: is exact and fast.

Don't use is to compare numbers or strings. Whether two equal values share one object is up to Python, not you:

a = int("256")
b = int("256")
print(a is b)   # True: CPython keeps one object for each small int
c = int("1000")
d = int("1000")
print(c is d)   # False: two objects, equal values

Python even warns about it: x is 1000 gets a SyntaxWarning suggesting ==.

More in the Python docs: objects, values and types, id() and identity comparisons.