Add converter and formatter parameters to csv.reader and csv.writer

Feature or enhancement

Proposal:

The conversion between Python values and CSV fields is hard-coded in both directions. The reader converts unquoted fields with float(), and only in the QUOTE_NONNUMERIC and QUOTE_STRINGS modes. The writer converts every non-string value with str().

This is the common cause of several open issues:

• gh-74232 -- bool is written unquoted as True, which cannot be read back.
• gh-98485 -- the same for complex; Fraction and IntEnum are affected too, and Decimal silently round-trips through float.
• gh-110852 -- there is no way to write floats with a fixed precision, because preformatted strings are quoted in the QUOTE_NONNUMERIC mode.
• gh-85002 -- there is no way to reject values which are neither strings nor numbers.

I propose two parameters, mirroring parse_float in json:

• csv.reader(f, converter=None) -- called as converter(index, field) instead of float().
• csv.writer(f, formatter=None) -- called as formatter(index, value) instead of str(). It must return a string.

index is the 0-based position of the field in the record. Both default to None, which keeps the current behavior. The hooks only replace the existing calls -- what is not passed to float() or str() now is not passed to them either. Quoting is still decided by the original value.

The index goes first, like in enumerate(). This also makes a wrong one-argument callable fail at once: converter=int raises TypeError on the first field instead of taking the index as the base.

The index makes the hooks per-column, which is what the dtype and converters parameters of pandas.read_csv() are used for:

>>> types = [str, int, Decimal, Fraction]
>>> list(csv.reader(['spam,42,1.10,1/2'], quoting=csv.QUOTE_NONNUMERIC,
...                 converter=lambda i, field: types[i](field)))
[['spam', 42, Decimal('1.10'), Fraction(1, 2)]]

>>> def money(index, value):
...     return format(value, '.2f') if index == 2 else str(value)
>>> csv.writer(sys.stdout, formatter=money).writerow(['a', 1, 0.0, 3.14159])
a,1,0.00,3.14159

gh-85002 no longer needs a parameter of its own -- a strict writer is a formatter which refuses everything except numbers.

I have a working prototype (about 90 lines in Modules/_csv.c).

Open question: should these be parameters of the reader and the writer, or attributes of the dialect? A dialect is a portable description of the file syntax -- it is registered under a global name, sniffed, and copied -- so keeping callables out of it seems better.

Linked PRs

• gh-155099

Start with Modules/_csv.c and the reader/writer entry points described in the proposal; review the existing float() and str() conversion paths and the prototype mentioned there. Compare the parameter and dialect alternatives, then use the linked gh-155099 work to determine the agreed API and completion criteria.