'is' returns False for equal strings
`is` checks whether two names point to the same object; `==` checks whether the values are equal. Use == for strings and numbers.
What it means
Two strings with identical letters can be two different objects, for example one built by .lower() and one written as a literal. `is` then returns False even though == is True. It seems to work in quick tests because Python reuses some small strings and integers, which makes the bug intermittent. Python 3.8+ warns: SyntaxWarning: "is" with a literal.
Common causes
1. Comparing strings with is
lower() returns a new string object.
Breaks
if role.lower() is "admin":Works
if role.lower() == "admin":2. Comparing numbers with is
Small ints are cached; large ones are not.
Breaks
if count is 1000:Works
if count == 1000:How to find it in your own code
Use == for every value comparison. Keep is for identity checks with singletons: x is None, x is True, x is False.
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