+++ slug = "none" title = "None" section = "Basics" summary = "None represents expected absence, distinct from missing keys and errors." doc_path = "/library/constants.html#None" see_also = [ "values", "truthiness", "exceptions", "dicts", ] +++

None represents the absence of a value. It is the usual sentinel when a function has no result to return but the absence itself is meaningful.

Because None is a singleton, idiomatic Python checks it with is None or is not None. This avoids confusing identity with value equality.

Absence has several nearby shapes in Python. A function can return None, a dictionary lookup can supply a default for a missing key, and an invalid operation can raise an exception.

:::program

result = None
print(result is None)

def find_score(name):
    if name == "Ada":
        return 10
    return None

score = find_score("Grace")
print(score is None)

profile = {"name": "Ada"}
print(profile.get("timezone", "UTC"))

try:
    int("python")
except ValueError:
    print("invalid number")

:::

:::cell None is Python's value for “nothing here.” Check it with is None because it is a singleton identity value.

result = None
print(result is None)

True

:::

:::cell Functions often return None when absence is expected and callers can continue. The function name and surrounding code should make that possibility clear.

def find_score(name):
    if name == "Ada":
        return 10
    return None

score = find_score("Grace")
print(score is None)

True

:::

:::cell A missing dictionary key is another absence boundary. Use get() when the mapping can supply a default, and use exceptions for invalid operations that cannot produce a value.

profile = {"name": "Ada"}
print(profile.get("timezone", "UTC"))

try:
    int("python")
except ValueError:
    print("invalid number")

UTC
invalid number

:::

:::note

• Use is None rather than == None; None is a singleton identity value.
• Use None for expected absence that callers can test.
• Use dictionary defaults for missing mapping keys and exceptions for invalid operations. :::