Use value is None to check whether a variable is specifically None. Use if not value when any false-valued value—including None, False, zero, or an empty container—should take the same path. These checks answer different questions.
Choose the check that matches what “empty” means
| Check | What it matches | Use it when |
|---|---|---|
value is None |
Only the None singleton |
None alone means missing or unset |
not value |
None, False, numeric zero, empty built-in sequences and collections, and objects that define false truth behavior |
All false-valued inputs should be treated alike |
The distinction is about meaning, not speed. Python defines None as a single object commonly used to represent the absence of a value, so test for it by identity: value is None. Python Built-in Types documentation and the language reference on identity comparisons describe these rules.
Check only for None
if value is None:
handle_missing()
This leaves valid false-valued inputs alone. For example, if 0, False, '', [], or {} are meaningful values in your program, an explicit None check will not mistake them for missing data.
Prefer is None to == None. The former asks whether value is the same object as the None singleton; equality comparisons can follow an object’s custom equality behavior.
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Check for any false-valued value
if not value:
handle_empty_or_false_value()
In a condition, Python tests an object’s truth value. None, False, numeric zero, and empty built-in sequences and collections are false. By default, other objects are true unless their class defines __bool__() to return False or __len__() to return zero. See the official truth-value testing documentation.
That means if not value is broader than a check for an empty string or container. Use it only when the program should handle every false-valued case the same way.
Check for an empty string or blank text
Exactly an empty string
if value == "":
handle_empty_string()
This tests for the empty string specifically; it does not classify None or numeric zero as an empty string.
Whitespace-only text
if value is not None and not value.strip():
handle_blank_text()
A string containing spaces or other whitespace is nonempty, so it is truthy. Calling strip() makes a whitespace-only string count as blank. This expression assumes value is either None or a string; validate or convert other input types as your application requires.
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Use a fallback without discarding zero or False
This expression uses truthiness, so it selects fallback for any false-valued value:
result = value or fallbackIf only None should trigger the fallback, write the condition explicitly:
result = fallback if value is None else valuePython’s or operator returns one of its operands: it evaluates the left operand’s truth value and returns the fallback when that operand is false. The language reference on Boolean operations specifies this behavior.
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Common mistakes to avoid
- Treating “false” as synonymous with “not provided.”
if not valuealso catches zero,False, and empty values. Usevalue is Nonewhen those may be valid. - Using equality for a None check. Write
value is Noneto test the singleton directly. - Assuming custom objects use ordinary emptiness rules. A class can control its truth value through
__bool__()or__len__(). - Expecting whitespace to be an empty string. Whitespace makes a string nonempty; strip it explicitly if your rule treats blank text as empty.
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