Modules/_csv.c (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Modules/_csv.c ↗

This annotation covers CSV writing and the DictReader/DictWriter wrappers. See modules_csv_detail for csv.reader, Dialect, field parsing, and csv.register_dialect.

Map

Lines	Symbol	Role
1-100	csv.writer	Write rows as CSV to a file-like object
101-220	Writer.writerow	Quote and join one row
221-340	csv.DictWriter	Write dicts as rows; validate fieldnames
341-500	Quoting modes	QUOTE_ALL, QUOTE_MINIMAL, QUOTE_NONNUMERIC, QUOTE_NONE

Reading

csv.writer

// CPython: Modules/_csv.c:920 Writer_new
static PyObject *
Writer_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    WriterObj *self = (WriterObj *)type->tp_alloc(type, 0);
    PyObject *output_file;
    PyObject *dialect = NULL;
    if (!_PyArg_ParseTupleAndKeywordsFast(args, kwds,
                                          "O|O", wr_kwlist,
                                          &output_file, &dialect)) {
        return NULL;
    }
    self->writeline = PyObject_GetAttr(output_file, _csvstate_global->write_str);
    ...
    return (PyObject *)self;
}

csv.writer(f) stores f.write in self->writeline. writerow builds the output string and calls writeline once per row. Any file-like object with a .write method works.

Writer.writerow

// CPython: Modules/_csv.c:980 Writer_writerow
static PyObject *
Writer_writerow(WriterObj *self, PyObject *seq)
{
    /* For each field: apply quoting rules, escape special chars,
       join with delimiter, append line terminator. */
    for (Py_ssize_t i = 0; i < seqlen; i++) {
        PyObject *field = PySequence_GetItem(seq, i);
        /* Convert to string, then quote if needed */
        if (csv_join_append(self, field, i == seqlen - 1) < 0) {
            return NULL;
        }
    }
    /* Write the completed line */
    PyObject *line = PyUnicode_Join(Py_None, self->rec);
    PyObject *result = PyObject_CallOneArg(self->writeline, line);
    ...
}

Quoting is determined by the dialect: QUOTE_MINIMAL only quotes fields containing the delimiter, quotechar, or line terminator. QUOTE_ALL always quotes. QUOTE_NONNUMERIC quotes non-numeric fields and converts all numeric fields to float on read.

DictWriter

# CPython: Lib/csv.py:145 DictWriter
class DictWriter:
    def __init__(self, f, fieldnames, restval='', extrasaction='raise', dialect='excel', **fmtparams):
        self.fieldnames = fieldnames
        self.restval = restval
        if extrasaction.lower() not in ('raise', 'ignore'):
            raise ValueError(...)
        self.extrasaction = extrasaction
        self.writer = writer(f, dialect, **fmtparams)

    def writerow(self, rowdict):
        if self.extrasaction == 'raise':
            wrong_fields = set(rowdict) - set(self.fieldnames)
            if wrong_fields:
                raise ValueError(f"dict contains fields not in fieldnames: {wrong_fields!r}")
        row = [rowdict.get(key, self.restval) for key in self.fieldnames]
        return self.writer.writerow(row)

DictWriter converts dicts to ordered lists using self.fieldnames as the key order. restval fills in missing keys. extrasaction='raise' (default) rejects dicts with extra keys.

gopy notes

csv.writer is module/csv.Writer in module/csv/module.go. writerow builds the CSV line as a Go string using strings.Builder, then calls the Python file's .write method via objects.CallMethod. DictWriter is implemented in pure Go following the same logic as the CPython Python-level wrapper.