aio-libs / multidict / 34026791742

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Use repr() to escape keys in MultiDict __repr__ output (#1342)

## What do these changes do?

Make `repr(MultiDict(...))` produce a parseable string when keys contain
quote characters. The pure-Python and C-extension code paths previously
wrapped keys in literal single quotes, so a key like `a'b` produced the
broken output `<MultiDict('a'b': 1)>`. They now format keys via Python's
`repr()`, which picks valid quoting automatically.

Before:

```python
>>> repr(MultiDict([("a'b", 1)]))
"<MultiDict('a'b': 1)>"        # not valid Python
```

After:

```python
>>> repr(MultiDict([("a'b", 1)]))
'<MultiDict("a\'b": 1)>'       # valid string literal
```

## Are there changes in behavior for the user?

Only for keys containing characters that need escaping in a Python
string literal (single quotes, control characters, …). Such repr output
was already broken, so any consumer was either tolerating bad output or
already wrapping keys themselves. Plain-ASCII keys produce
byte-identical
output, so the existing repr tests continue to pass unchanged.

## How

- **Pure-Python** (`multidict/_multidict_py.py`): four `f"'{key}'"` /
`f"'{key}': {v!r}"` patterns in `_ItemsView.__repr__`,
`_KeysView.__repr__`,
`MultiDict.__repr__`, and `MultiDictProxy.__repr__` switched to
`{key!r}`.
- **C extension** (`multidict/_multilib/hashtable.h`): the three-step
  *write `'`, write str, write `'`* sequence in `md_repr` is replaced by
  a single `PyUnicodeWriter_WriteRepr(writer, key)`. All views
  (`MultiDict`, proxies, keys/items views) delegate to this function, so
  one fix covers them all.

## Testing

- Three new tests in `tests/test_multidict.py::BaseMultiDictTest`
  (parametrized over `MultiDict`/`CIMultiDict` and both implementations)
  assert the repr of a multidict with a quoted key, plus its keys and
  items views, is the expected escaped form.
- Full local suite: `1682 passed` (was 1658 before, +24 from the new
  parametrized tests) on Python 3.12, C-ext + pure-Python.
- ... (continued)

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