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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 count as empty. Choose the check that matches what “missing” means in your code.

Check whether a value is specifically None

None is Python’s singleton for representing the absence of a value in many situations. Test for it with the identity operator:

if value is None:
    handle_missing()

This check matches only None. Values such as 0, False, '', [], and {} pass through the else path if you add one. That makes is None the right choice when those values can be valid inputs. Python documents None as a single object and the identity operator as a test of whether two references point to the same object.

Check whether a value is false or empty

Use a truth-value test when all false-valued inputs should take the same path:

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if not value:
    handle_empty_or_false_value()

For built-in types, this catches None, False, numeric zero, and empty strings and containers. The Python truth-value rules also apply to custom objects: unless a class defines __bool__() to return False or __len__() to return zero, its instances are true.

Choose based on what counts as missing

Check What it matches Use it when
value is None Only None None alone means missing or unset
not value None, False, zero, and false-valued empty or custom objects Every false-valued value should follow the same path

This is a question of meaning, not performance: use the condition that matches your input contract.

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Check for an empty string or blank text

To match only an empty string, compare with "":

if value == "":
    handle_empty_string()

That does not treat None or zero as an empty string. A string containing spaces is not empty either. If whitespace-only text should count as blank, strip it first:

if value is not None and not value.strip():
    handle_blank_text()

This 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

The expression value or fallback uses truthiness. It returns the fallback not just for None, but also for values such as 0, False, and "". Python’s Boolean operation rules specify that or returns one of its operands.

If only None should trigger the fallback, write the condition explicitly:

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result = fallback if value is None else value

Avoid these common mistakes

  • Using if not value to mean only “not provided.” It also catches false and empty values such as zero, False, and an empty string.
  • Writing value == None. Prefer value is None to express that you are checking for the None singleton, rather than comparing for equality.
  • Assuming custom objects use built-in emptiness rules. Their truth value can be affected by their __bool__() or __len__() implementation.
  • Expecting whitespace to be an empty string. Whitespace makes a string nonempty; use strip() when your rule is that whitespace-only text counts as blank.

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