python-indent-aware-edits

Python indent-aware edits: compileall won't catch a dropped with / try scope

When to use

You are making a multi-line edit inside a block-introducing statement (with, try, for, if, def, class) using an editor tool that does literal search-and-replace (e.g. replace_in_file, multi_replace, sed -i with a multi-line pattern). The edit rewrites or replaces a nested suite — not just a single token.

If the edit is purely mechanical in-line rename (e.g. iteritems(d) → d.items(), basestring → str) done by sed that never touches the leading whitespace of any line, this skill does not apply.

Problem

Python's grammar treats any indentation greater than the enclosing block as a valid new suite. That means an accidentally over-indented line is still syntactically legal — compileall, ast.parse, python -c 'import mymodule' all pass — but the semantic scope has silently changed.

Concretely: an old_string that captured only the body lines of a with block (not the with statement itself) can be replaced with a new_string whose first line is indented to the with level. The body lifts out of with, the file f closes, and the line below it (still indented deeper) becomes the body of the outer statement instead. The module still imports. The bug only fires when the specific function is called.

Real symptom caught during a blade-build py2-shim cleanup:

# before (correct)
def dump(self, output_file_name):
    with open(output_file_name, 'w') as f:
        print('# header', file=f)
        for name, value in sorted(iteritems(self.configs)):
            self._dump_section(name, value, f)

A replace_in_file meant to rewrite only the for line used old_string starting after the with, and the tool re-emitted the for with the method-body indent (8 spaces) instead of the with-body indent (12 spaces):

# after (broken but syntactically valid)
def dump(self, output_file_name):
    with open(output_file_name, 'w') as f:
        print('# header', file=f)
    for name, value in sorted(self.configs.items()):   # ← escaped the `with`
            self._dump_section(name, value, f)         # ← over-indented, still legal