Python 3.8+astterm

ast.NamedExpr

expressions

Node for the walrus operator (x := value): target is a Name with Store, value is the expression.

Syntax

ast.NamedExpr(target, value)

It is an expression, not a statement: (x := 1) at top level parses as an Expr wrapping a NamedExpr, while x := 1 without parentheses is a SyntaxError already in ast.parse. The target can only be a plain name: unlike Assign it never holds an attribute, a subscript or a tuple. Search for assignments in NamedExpr too, or names bound in if (n := len(a)) > 10 slip through.

Examples

import ast
stmt = ast.parse('if (n := len(a)) > 10: pass').body[0]
w = stmt.test.left
print(type(w).__name__, w.target.id, type(w.target.ctx).__name__)  # → NamedExpr n Store

import ast
try:
    ast.parse('x := 1')
except SyntaxError as e:
    print(type(e).__name__)  # → SyntaxError

See also

Common errors

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