Non-Name Metavariables

Metavariables can only occur where ast.Name nodes can occur in the AST. For example, $foo.bar works because foo.bar is a valid expression where foo is a Name node.

Works:

$foo.attr
$foo + 3

Error

Doesn’t work:

obj.$foo
def $foo(): pass
import $foo as $bar

When debugging this error message, it can help to use ast.dump() with every $ removed to see if the AST produces a Name node there. For example, when debugging why obj.$foo or import $foo won’t work, we could print these ASTs:

>>> import ast
>>> print(ast.dump(ast.parse('obj.foo').body[0]))
Expr(value=Attribute(value=Name(id='obj', ctx=Load()), attr='foo', ctx=Load()))

>>> print(ast.dump(ast.parse('import foo').body[0]))
Import(names=[alias(name='foo', asname=None)])

We can see from this the place we wanted to use a metavariable is not a Name node (attr='foo', name='foo'). These places cannot be matched using metavariables in a pattern.

For more about metavariables and Python patterns, see Python Patterns and Templates.