all Python 3astfunction

ast.fix_missing_locations()

helper functions

Fills in lineno and col_offset for every node of a tree that lacks them, copying them from the parent. A required step before compile() of a tree with new nodes.

Syntax

tree = ast.fix_missing_locations(tree)

Nodes built by hand or by a NodeTransformer have no positions, and compile() fails with TypeError: required field "lineno" missing. unparse and dump work fine meanwhile, so the error only appears at compile time. Call it on the whole transformed tree: tree = ast.fix_missing_locations(tree); it edits the tree in place and returns it. Positions already set are left alone.

Examples

import ast
tree = ast.parse('print(x)')
call = tree.body[0].value
call.args[0] = ast.BinOp(left=ast.Name(id='x', ctx=ast.Load()), op=ast.Mult(),
                         right=ast.Constant(value=2))
try:
    compile(tree, '<ast>', 'exec')
except TypeError as e:
    print(e)  # → required field "lineno" missing from expr
tree = ast.fix_missing_locations(tree)
exec(compile(tree, '<ast>', 'exec'), {'x': 21})  # → 42
print(call.args[0].lineno)  # → 1

See also

Common errors

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