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dict.get() and missing vs None

dict.get(key, default) is convenient, but it does not tell you whether a key is missing or whether the key exists with a value of None. Those are often different situations.

What is happening?​

data = {"timeout": None}

print(data.get("timeout", 30))
print(data.get("retries", 3))

Output:

None
3

This is fine when None is a valid, meaningful value. The subtle bug appears when code treats .get() as a reliable way to distinguish "missing" from "present."

Why this surprises people​

Consider this:

data = {"timeout": None}

if data.get("timeout") is None:
    print("Use default timeout")

That condition is true whether:

  • "timeout" is missing
  • "timeout" exists and is explicitly None

If your program needs to treat those cases differently, .get() alone is not enough.

Prefer this instead​

Check membership when the distinction matters:

if "timeout" in data:
    timeout = data["timeout"]
else:
    timeout = 30

Another useful pattern is to use a sentinel object when None is a valid stored value.

Rules of thumb​

  • Use .get() when missing and default behavior can be treated the same way.
  • Do not rely on .get() to distinguish missing keys from stored None values.
  • Use in or a sentinel object when that distinction matters.
  • Be explicit when None is part of the data model.