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is vs ==

is and == answer different questions. == compares values, while is compares object identity. Using the wrong one can lead to confusing behavior.

What is happening?​

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

print(a == b)
print(a is b)

Output:

True
False

a and b contain the same values, but they are different objects.

Why this surprises people​

Many bugs come from using is when the real question is value equality:

name = "wolf"

if name is "wolf":
    print("match")

That may appear to work in some cases because of Python implementation details, but it is the wrong comparison.

When is is the right choice​

Use is when you want to know whether two references point to the same object.

The most common and important case is checking for None:

if value is None:
    ...

is is also correct for sentinel objects whose identity is meaningful.

Rules of thumb​

  • Use == for value comparisons.
  • Use is for None and sentinel objects.
  • Do not use is for strings, numbers, or lists when you mean equality.
  • Read is as "is the same object?" not "has the same value?"