When should you use is instead of == in Python?

Distinguishes object identity from value equality and prevents fragile code based on interning or implementation accidents.

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TL;DR

is asks whether two references identify the exact same object. == asks whether the objects are equal by their value semantics. I use is for singletons and intentional sentinels, especially value is None. I use == for strings, numbers, collections, and domain values. Two separate lists can compare equal while left is right is false because they contain equal values but are different objects.

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Correct use of identity for singletons and sentinels, value equality for domain values, and awareness of custom equality behavior.

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Identity and equality answer different questions even when they happen to produce the same result for a particular value.
is asks whether two references identify the exact same object. == asks whether the objects are equal by their value semantics. I use is for singletons and intentional sentinels, especially value is None. I use == for strings, numbers, collections, and domain values. Two separate lists can compare equal while left is right is false because they contain equal values but are different objects.
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Details

Using is to compare strings or numbers by value.
Assuming equal objects must have the same identity.
Depending on small-integer or string interning.
Using == None when identity expresses the intent directly.

When should you use is instead of == in Python?

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