none-coalesce operator¶

a ?? b evaluates to a when a is not None, otherwise to b:

name = user.display_name ?? "anonymous"

transpiles to:

name = __by_t_0__ if (__by_t_0__ := user.display_name) is not None else "anonymous"

a compound left operand is bound to a temp by the walrus so it is evaluated exactly once. a bare name needs no temp and is repeated directly — a ?? b is a if a is not None else b

python refuses the walrus in some places: a comprehension's iterable, a comprehension in a class body, an annotation, and a generic definition's type parameters, bases and type value. there the temp is a comprehension variable instead, which still evaluates the left operand once and the right one only when it is needed:

names = [n for n in user.nicknames ?? []]

transpiles to:

names = [n for n in ([__by_t_0__ for __by_t_0__ in (user.nicknames,) if __by_t_0__ is not None] or [[]])[0]]

semantics¶

?? tests is not None (identity) — not falsiness. an empty string, zero, or an empty list is not coalesced. only None triggers the fallback

precedence¶

?? has the same precedence as or and groups left to right with it. it binds looser than and, not, comparisons and arithmetic, and tighter than a conditional expression:

a ?? b or c   # (a ?? b) or c
a or b ?? c   # (a or b) ?? c
a ?? b and c  # a ?? (b and c)

parenthesize when mixing it with boolean operators

interaction with ?.¶

see optional chaining. ?? and ?. compose; complex left operands are evaluated at most once even when chained