CodingwithDom
Subscribe

What is the difference between == and is in Python?

Python 2 min read Full lesson: Python Basics — Lists, Dictionaries and Functions
Short answer

== compares values; is compares identity — whether two names point at the exact same object in memory. Use is only with None, True and False.

python
a = [1, 2, 3]
b = [1, 2, 3]
c = a

a == b    # True  — same contents
a is b    # False — two separate list objects
a is c    # True  — c is another name for the same list

#Why this bites

Small integers and short strings are cached by CPython, so is sometimes appears to work:

python
x = 256
y = 256
x is y      # True — small ints are interned

x = 257
y = 257
x is y      # False on most builds

Code that uses is for numbers will pass your tests with small values and fail in production with large ones. This is why linters flag it.

#Where is is correct

python
if value is None:
    ...
if flag is True:      # rarely needed; usually just `if flag:`

None is a singleton — there is exactly one None in a running program — so identity is the right question. It is also faster and cannot be fooled by a class that overrides __eq__.

#Never write == None

if value == None works but is non-idiomatic, and it breaks if the object defines a weird __eq__. is None is the correct form, every time.

Related answers