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In CPython, integer objects from -5 through 256 are reused, but that is an implementation detail—not a Python-wide guarantee. Use == to compare integer values; use is only when you need to know whether two references point to the exact same object.
What CPython’s small-integer cache does
CPython keeps an array of integer objects for every integer from -5 through 256, inclusive. When CPython creates an integer in that range, it returns a reference to the existing object. The range is documented in the Python 3.14.8 C API documentation.
This is a CPython implementation detail, not a rule of the Python language. Other Python implementations need not use the same cache, and the range is not a permanent promise even for CPython. The Python language reference notes that literal identity behavior and its boundary can change: Python 3.14.8 language reference.
What `is` and `==` actually compare
| Expression | Question it answers | Example use |
|---|---|---|
x is y |
Are x and y references to the same object? |
Check whether a value is None or a particular sentinel object. |
x == y |
Do x and y compare as equal in value? |
Check whether two integers have the same numeric value. |
Identity is about object sameness; equality is about values. For built-in integers, use equality when the question is whether the numbers are equal. Python’s language reference distinguishes identity comparisons from value comparisons.
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Why integer identity checks can seem inconsistent
When a runtime reuses one immutable integer object, two references can be identical and equal at the same time. That overlap does not make identity a reliable test for numeric equality.
x = 7
y = 7
print(x == y) # True: same numeric value
print(x is y) # May be True in CPython; do not depend on it
x = int("1000")
y = int("1000")
print(x == y) # True: equal integer values
The second example asks the useful question—whether the values are equal—without depending on whether the two references identify the same object. Do not infer that integers outside CPython’s documented cache range must be distinct, or that every integer inside it must produce a particular result for every expression. The cache range describes object reuse in CPython; it does not define the result of every identity comparison.
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Literals may share objects, too
The language reference says that separate evaluations of same-valued literals may yield the same object or distinct objects with equal values. It illustrates CPython cases in which 7 is 7 is true while two occurrences of 123456789 are not identical. These examples demonstrate possible behavior, not a portable rule. Compilation and implementation decisions can affect literal reuse.
CPython’s warning for an integer literal
Python 3.14.8 documentation says CPython emits a SyntaxWarning for comparisons such as x is 7, and suggests == instead. Treat this as documented CPython behavior, not a promise about every Python implementation or version. See the language reference.
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When `is` is the right operator
Use identity when the exact object matters, rather than when only its value matters. The Python Programming FAQ recommends identity for singleton checks and explains why equality is generally preferred in other circumstances.
value is Nonechecks whethervalueis theNonesingleton.- A private sentinel can distinguish “not supplied” from ordinary values:
_MISSING = object(), followed byif value is _MISSING:. - Identity can verify that another name or a container entry refers to the exact object you expect.
For instance, the sentinel check is useful when None itself is a valid argument value:
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_MISSING = object()
def get_setting(value=_MISSING):
if value is _MISSING:
return "no value supplied"
return value
Why `id()` does not solve the comparison problem
id(obj) reports an object’s identity, not a permanent numeric label for its value. The FAQ documents that an object’s ID is unique during its lifetime; in CPython, it is the object’s memory address, which may be reused after the object is deleted. Use == for integer-value comparisons, not comparisons of id() results.
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