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Python / Core Python Fundamentals Interview Questions

What is None in Python, and when should you use 'is' versus '=='?

None is Python's null value — a singleton object of type NoneType. It represents the absence of a value: default function returns, uninitialised optional variables, missing dict values. There is exactly one None object in any Python process.

The key distinction for comparisons:

  • == tests equality: do the two objects have the same value? It calls __eq__ and can be overridden.
  • is tests identity: are the two names pointing to the exact same object in memory (same id())?
# Correct: test for None with 'is'
result = some_function()
if result is None:
    print('No result returned')

# Why not ==?
class Weird:
    def __eq__(self, other):
        return True   # lies — claims equality with everything

w = Weird()
print(w == None)   # True  — because __eq__ lies
print(w is None)   # False — identity check cannot be faked

PEP 8 says explicitly: use is and is not when comparing against None or the boolean singletons True/False. The is check is also marginally faster because it does not invoke any dunder method — it is a direct pointer comparison.

A related trap: CPython caches small integers (typically -5 to 256) and short strings, so a = 256; b = 256; a is b is True. But a = 1000; b = 1000; a is b may be False. Never rely on is for value comparisons — only use it for singletons like None.

Why should you use 'is None' instead of '== None'?
CPython caches small integers up to approximately which value?

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