[python] Is there a difference between "==" and "is"?

What's the difference between is and ==?

== and is are different comparison! As others already said:

• == compares the values of the objects.
• is compares the references of the objects.

In Python names refer to objects, for example in this case value1 and value2 refer to an int instance storing the value 1000:

value1 = 1000
value2 = value1

Because value2 refers to the same object is and == will give True:

>>> value1 == value2
True
>>> value1 is value2
True

In the following example the names value1 and value2 refer to different int instances, even if both store the same integer:

>>> value1 = 1000
>>> value2 = 1000

Because the same value (integer) is stored == will be True, that's why it's often called "value comparison". However is will return False because these are different objects:

>>> value1 == value2
True
>>> value1 is value2
False

When to use which?

Generally is is a much faster comparison. That's why CPython caches (or maybe reuses would be the better term) certain objects like small integers, some strings, etc. But this should be treated as implementation detail that could (even if unlikely) change at any point without warning.

You should only use is if you:

• want to check if two objects are really the same object (not just the same "value"). One example can be if you use a singleton object as constant.

• want to compare a value to a Python constant. The constants in Python are:

  • None
  • True¹
  • False¹
  • NotImplemented
  • Ellipsis
  • __debug__
  • classes (for example int is int or int is float)
  • there could be additional constants in built-in modules or 3rd party modules. For example np.ma.masked from the NumPy module)

In every other case you should use == to check for equality.

Can I customize the behavior?

There is some aspect to == that hasn't been mentioned already in the other answers: It's part of Pythons "Data model". That means its behavior can be customized using the __eq__ method. For example:

class MyClass(object):
    def __init__(self, val):
        self._value = val

    def __eq__(self, other):
        print('__eq__ method called')
        try:
            return self._value == other._value
        except AttributeError:
            raise TypeError('Cannot compare {0} to objects of type {1}'
                            .format(type(self), type(other)))

This is just an artificial example to illustrate that the method is really called:

>>> MyClass(10) == MyClass(10)
__eq__ method called
True

Note that by default (if no other implementation of __eq__ can be found in the class or the superclasses) __eq__ uses is:

class AClass(object):
    def __init__(self, value):
        self._value = value

>>> a = AClass(10)
>>> b = AClass(10)
>>> a == b
False
>>> a == a

So it's actually important to implement __eq__ if you want "more" than just reference-comparison for custom classes!

On the other hand you cannot customize is checks. It will always compare just if you have the same reference.

Will these comparisons always return a boolean?

Because __eq__ can be re-implemented or overridden, it's not limited to return True or False. It could return anything (but in most cases it should return a boolean!).

For example with NumPy arrays the == will return an array:

>>> import numpy as np
>>> np.arange(10) == 2
array([False, False,  True, False, False, False, False, False, False, False], dtype=bool)

But is checks will always return True or False!

¹ As Aaron Hall mentioned in the comments:

Generally you shouldn't do any is True or is False checks because one normally uses these "checks" in a context that implicitly converts the condition to a boolean (for example in an if statement). So doing the is True comparison and the implicit boolean cast is doing more work than just doing the boolean cast - and you limit yourself to booleans (which isn't considered pythonic).

Like PEP8 mentions:

Don't compare boolean values to True or False using ==.

Yes:   if greeting:
No:    if greeting == True:
Worse: if greeting is True: