all Python 3astterm

ast.Is

operators

The marker for the comparison is in an ast tree: it sits in Compare.ops and has no fields.

Syntax

ast.Is()

Linters hunt for exactly this: 'x is 1' or 'x is "a"' is a Compare with Is and a Constant on the right — an identity test against a literal that Python 3.8+ flags with a SyntaxWarning. 'is not' is a single IsNot operator, not Is under Not. Compare marker nodes by type (isinstance), not with ==: ast nodes have no __eq__, so ast.Is() == ast.Is() is False.

Examples

import ast
src = 'if a is None: pass\nif b is 1: pass'
for node in ast.walk(ast.parse(src)):
    if isinstance(node, ast.Compare) and isinstance(node.ops[0], ast.Is):
        rhs = node.comparators[0]
        if isinstance(rhs, ast.Constant) and rhs.value is not None:
            print('literal is at line', node.lineno)  # → literal is at line 2

See also

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