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

• python -m compileall exits 0.
• import config succeeds.
• 12-item import-level smoke test passes.
• The bug fires only when someone calls BladeConfig().dump(path) and gets ValueError: I/O operation on closed file — because f closed at the end of with, and the for is now outside it.

Solution

Three rules, in order of importance.

1. Anchor old_string on the block header, not inside the suite

When rewriting any line that lives inside a with / try / nested for / if block, include the block-introducing line in old_string. The tool then has an unambiguous indentation anchor and cannot accidentally shift the replacement out.

# BAD — old_string starts mid-suite, leading whitespace is ambiguous
old_string:
            for name, value in sorted(iteritems(self.configs)):
                self._dump_section(name, value, f)

# GOOD — include the `with` header so the indentation ladder is fixed
old_string:
    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)

2. After the edit, run the callee, not just the importer

compileall and import prove the module parses. They do not prove that a specific function still works. If you edited a function body, call that function at least once:

# Smoke the exact function you edited.
python3 - <<'PY'
import sys; sys.path.insert(0, 'src')
from blade.config import BladeConfig
import tempfile, os
out = os.path.join(tempfile.mkdtemp(), 'x')
BladeConfig().dump(out)
assert os.path.getsize(out) > 0
print('dump OK')
PY

3. Diff-audit indentation changes

Before committing, grep the diff for suspicious indent shifts on any line you didn't intend to re-indent:

# Any line where only leading whitespace changed between - and +?
git --no-pager diff -U0 -- path/to/file.py \
  | awk '/^-[^-]/{getline n; if (n ~ /^\+[^+]/ && \
         substr($0,2) !~ /^[[:space:]]/ "" substr(n,2)) print}'

Or simpler: just scan git diff output for any + line whose leading whitespace doesn't match the - line it replaces. Manual eyeballing of the hunks around with / try / nested for is usually enough.

Example

Full fix from the real incident above:

     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()):
-                self._dump_section(name, value, f)
+            for name, value in sorted(self.configs.items()):
+                self._dump_section(name, value, f)

Verification after the fix:

python3.10 -m compileall -q src/        # still passes (as before)
python3.12 -m compileall -q src/        # still passes (as before)
python3 -c "from blade.config import BladeConfig; \
            import tempfile, os; \
            out=os.path.join(tempfile.mkdtemp(),'x'); \
            BladeConfig().dump(out); \
            print(os.path.getsize(out))"
# -> prints 5572; function works end-to-end

Pitfalls

• "It compiles" is not verification. Over-indented suites, disconnected else clauses (attached to the wrong for/while), and return moved one level too shallow are all legal Python. Your CI and your import smoke cannot see them.
• Mixing mechanical sed with hand-written replace_in_file is the highest-risk combo. sed 's/iteritems(\([^)]*\))/\1.items()/g' preserves leading whitespace by construction; replace_in_file does not. After a mixed batch, run the end-to-end call for every file that was touched by the hand-written edits, not every file in the diff.
• ast.parse and py_compile agree with the compiler. They will not help. Tools that do help: pylint (W0101 unreachable, W0104 statement-with-no-effect sometimes catches dangling suite bodies), pyright --outputjson (flow-based, catches "variable possibly unbound after with"), or — most reliably — actually invoking the function.
• Diff-stat can hide it. A 1-file / +1 -1 diff that shifts a suite out of a with looks trivial in --stat. Review the diff, not the stats.

See also

• python-code-audit-sweep — parent skill; this one is the "editing hygiene" half of an audit PR.
• shell-heredoc-and-multiline-strings — when you need to pipe a verification script through the terminal tool without the shell eating it.
• agent-work-artifacts-layout — where to dump the temporary verification script.