Support csv writer QUOTE_STRINGS and QUOTE_NOTNULL (#8109)

* Support csv quoting modes added in Python 3.12

Implement writer handling for QUOTE_STRINGS and QUOTE_NOTNULL so the exported constants no longer panic and follow CPython behavior for string, non-null, None, and quotechar validation cases.

Constraint: RustPython contribution policy requires CPython-compatible behavior through Rust-side changes and regression coverage outside Lib/.

Rejected: Marking CPython Lib tests or hiding the constants | would preserve a runtime panic for supported public constants.

Confidence: high

Scope-risk: narrow

Directive: Keep QUOTE_STRINGS and QUOTE_NOTNULL writer behavior aligned with CPython's csv module when extending dialect validation.

Tested: cargo check -p rustpython-stdlib; cargo run --release -- extra_tests/snippets/stdlib_csv.py; cargo run --release -- -m test test_csv; cargo clippy; pre-commit run --all-files via temporary venv/PATH shim; cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher -- --test-threads=1

Not-tested: Remote CI and PR creation were not run because remote git operations require explicit approval.

Assisted-by: Codex:gpt-5.5

* Update crates/stdlib/src/csv.rs

Co-authored-by: Shahar Naveh <50263213+ShaharNaveh@users.noreply.github.com>

* Update crates/stdlib/src/csv.rs

Co-authored-by: Shahar Naveh <50263213+ShaharNaveh@users.noreply.github.com>

* Keep csv writer follow-up lint clean

Apply maintainer-requested expression cleanups and rustfmt wrapping so the PR lint job can pass without changing behavior.

Constraint: GitHub Actions Lint failed on rustfmt output after the review suggestion.

Rejected: Changing csv writer semantics | the failing check was formatting-only and all behavior tests already passed.

Confidence: high

Scope-risk: narrow

Directive: Keep future review suggestions rustfmt-clean before pushing.

Tested: cargo fmt --check; cargo check -p rustpython-stdlib; cargo run --release -- extra_tests/snippets/stdlib_csv.py; cargo run --release -- -m test test_csv; cargo clippy; pre-commit run --all-files; cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher -- --test-threads=1

Not-tested: Remote CI has not rerun yet; this commit will trigger it after push.

Assisted-by: Codex:gpt-5.5

---------

Co-authored-by: Shahar Naveh <50263213+ShaharNaveh@users.noreply.github.com>

crates/stdlib/src/csv.rs

@@ -466,8 +466,7 @@ mod _csv {
                 QuoteStyle::All => Self::Always,
                 QuoteStyle::Nonnumeric => Self::NonNumeric,
                 QuoteStyle::None => Self::Never,
-                QuoteStyle::Strings => todo!(),
-                QuoteStyle::Notnull => todo!(),
+                QuoteStyle::Strings | QuoteStyle::Notnull => Self::Necessary,
             }
         }
     }
@@ -661,7 +660,10 @@ mod _csv {
                         |_| { vm.new_type_error(r#""quotechar" must be a 1-character string"#) }
                     )?)),
                     PyNone => {
-                        if let Some(QuoteStyle::All) = res.quoting {
+                        if res
+                            .quoting
+                            .is_some_and(|quoting| quoting != QuoteStyle::None)
+                        {
                             return Err(ArgumentError::Exception(
                                 vm.new_type_error("quotechar must be set if quoting enabled"),
                             ));
@@ -1114,6 +1116,55 @@ mod _csv {
         }
     }
 
+    fn write_quoted_field(
+        output: &mut Vec<u8>,
+        data: &[u8],
+        dialect: PyDialect,
+        vm: &VirtualMachine,
+    ) -> PyResult<()> {
+        let quotechar = dialect
+            .quotechar
+            .ok_or_else(|| vm.new_type_error("quotechar must be set if quoting enabled"))?;
+        output.push(quotechar);
+        for &byte in data {
+            if byte == quotechar {
+                if dialect.doublequote {
+                    output.push(quotechar);
+                    output.push(quotechar);
+                } else if let Some(escapechar) = dialect.escapechar {
+                    output.push(escapechar);
+                    output.push(byte);
+                } else {
+                    return Err(new_csv_error(vm, "need to escape, but no escapechar set"));
+                }
+            } else {
+                if dialect.escapechar == Some(byte) {
+                    output.push(byte);
+                }
+                output.push(byte);
+            }
+        }
+        output.push(quotechar);
+        Ok(())
+    }
+
+    fn field_needs_quotes(data: &[u8], dialect: PyDialect) -> bool {
+        data.iter().any(|&byte| {
+            byte == dialect.delimiter
+                || dialect.quotechar == Some(byte)
+                || matches!(byte, b'\r' | b'\n')
+                || matches!(dialect.lineterminator, Terminator::Any(t) if byte == t)
+        })
+    }
+
+    fn write_lineterminator(output: &mut Vec<u8>, terminator: Terminator) {
+        match terminator {
+            Terminator::CRLF => output.extend_from_slice(b"\r\n"),
+            Terminator::Any(byte) => output.push(byte),
+            _ => unreachable!(),
+        }
+    }
+
     #[pyclass(flags(DISALLOW_INSTANTIATION))]
     impl Writer {
         #[pygetset(name = "dialect")]
@@ -1121,8 +1172,65 @@ mod _csv {
             self.dialect
         }
 
+        fn writerow_quoted_strings(&self, row: PyObjectRef, vm: &VirtualMachine) -> PyResult {
+            let _state = self.state.lock();
+            let row: ArgIterable = ArgIterable::try_from_object(vm, row.clone()).map_err(|_e| {
+                new_csv_error(
+                    vm,
+                    format!("'{}' object is not iterable", row.class().name()),
+                )
+            })?;
+            let fields = row.iter(vm)?.collect::<PyResult<Vec<_>>>()?;
+            let single_field = fields.len() == 1;
+            let mut output = Vec::new();
+
+            for (index, field) in fields.into_iter().enumerate() {
+                if index > 0 {
+                    output.push(self.dialect.delimiter);
+                }
+
+                let stringified;
+                let (data, is_str, is_none): (&[u8], bool, bool) = match_class!(match field {
+                    ref s @ PyStr => (s.as_bytes(), true, false),
+                    crate::builtins::PyNone => (b"", false, true),
+                    ref obj => {
+                        stringified = obj.str(vm)?;
+                        (stringified.as_bytes(), false, false)
+                    }
+                });
+
+                let should_quote = match self.dialect.quoting {
+                    QuoteStyle::Strings => is_str || field_needs_quotes(data, self.dialect),
+                    QuoteStyle::Notnull => !is_none,
+                    _ => unreachable!(),
+                };
+                if should_quote {
+                    write_quoted_field(&mut output, data, self.dialect, vm)?;
+                } else if single_field && data.is_empty() {
+                    return Err(new_csv_error(
+                        vm,
+                        "single empty field record must be quoted",
+                    ));
+                } else {
+                    output.extend_from_slice(data);
+                }
+            }
+
+            write_lineterminator(&mut output, self.dialect.lineterminator);
+            let s = core::str::from_utf8(&output)
+                .map_err(|_| vm.new_unicode_decode_error("csv not utf8"))?;
+            self.write.call((s,), vm)
+        }
+
         #[pymethod]
         fn writerow(&self, row: PyObjectRef, vm: &VirtualMachine) -> PyResult {
+            if matches!(
+                self.dialect.quoting,
+                QuoteStyle::Strings | QuoteStyle::Notnull
+            ) {
+                return self.writerow_quoted_strings(row, vm);
+            }
+
             let mut state = self.state.lock();
             let WriteState { buffer, writer } = &mut *state;
 

extra_tests/snippets/stdlib_csv.py

@@ -1,4 +1,5 @@
 import csv
+import io
 
 from testutils import assert_raises
 
@@ -47,3 +48,27 @@ def test_delim():
 
 
 test_delim()
+
+
+def test_quote_strings_and_notnull_writer():
+    string_buf = io.StringIO()
+    csv.writer(string_buf, quoting=csv.QUOTE_STRINGS).writerow(["x", 1, None, ""])
+    assert string_buf.getvalue() == '"x",1,,""\r\n'
+
+    notnull_buf = io.StringIO()
+    csv.writer(notnull_buf, quoting=csv.QUOTE_NOTNULL).writerow(["x", 1, None, ""])
+    assert notnull_buf.getvalue() == '"x","1",,""\r\n'
+
+    for quoting in (csv.QUOTE_STRINGS, csv.QUOTE_NOTNULL):
+        buf = io.StringIO()
+        csv.writer(buf, quoting=quoting).writerow([None, None])
+        assert buf.getvalue() == ",\r\n"
+
+        with assert_raises(csv.Error):
+            csv.writer(io.StringIO(), quoting=quoting).writerow([None])
+
+        with assert_raises(TypeError):
+            csv.writer(io.StringIO(), quoting=quoting, quotechar=None)
+
+
+test_quote_strings_and_notnull_writer()