feat(python): csv: add schema argument

crates/polars-io/src/csv/read.rs

@@ -181,8 +181,8 @@ where
     /// in the csv parser and expects a complete Schema.
     ///
     /// It is recommended to use [with_dtypes](Self::with_dtypes) instead.
-    pub fn with_schema(mut self, schema: SchemaRef) -> Self {
-        self.schema = Some(schema);
+    pub fn with_schema(mut self, schema: Option<SchemaRef>) -> Self {
+        self.schema = schema;
         self
     }
 

crates/polars-io/src/csv/read_impl/batched_mmap.rs

@@ -308,7 +308,7 @@ pub fn to_batched_owned_mmap(
 ) -> OwnedBatchedCsvReaderMmap {
     // make sure that the schema is bound to the schema we have
     // we will keep ownership of the schema so that the lifetime remains bound to ourselves
-    let reader = reader.with_schema(schema.clone());
+    let reader = reader.with_schema(Some(schema.clone()));
     // extend the lifetime
     // the lifetime was bound to schema, which we own and will store on the heap
     let reader = unsafe {

crates/polars-io/src/csv/read_impl/batched_read.rs

@@ -405,7 +405,7 @@ pub fn to_batched_owned_read(
 ) -> OwnedBatchedCsvReader {
     // make sure that the schema is bound to the schema we have
     // we will keep ownership of the schema so that the lifetime remains bound to ourselves
-    let reader = reader.with_schema(schema.clone());
+    let reader = reader.with_schema(Some(schema.clone()));
     // extend the lifetime
     // the lifetime was bound to schema, which we own and will store on the heap
     let reader = unsafe {

crates/polars-lazy/src/frame/csv.rs

@@ -106,8 +106,8 @@ impl<'a> LazyCsvReader<'a> {
 
     /// Set the CSV file's schema
     #[must_use]
-    pub fn with_schema(mut self, schema: SchemaRef) -> Self {
-        self.schema = Some(schema);
+    pub fn with_schema(mut self, schema: Option<SchemaRef>) -> Self {
+        self.schema = schema;
         self
     }
 
@@ -261,7 +261,7 @@ impl<'a> LazyCsvReader<'a> {
             }
         }
 
-        Ok(self.with_schema(Arc::new(schema)))
+        Ok(self.with_schema(Some(Arc::new(schema))))
     }
 }
 

crates/polars-pipe/src/executors/sources/csv.rs

@@ -62,7 +62,7 @@ impl CsvSource {
         let reader = CsvReader::from_path(&path)
             .unwrap()
             .has_header(options.has_header)
-            .with_schema(self.schema.clone())
+            .with_schema(Some(self.schema.clone()))
             .with_delimiter(options.delimiter)
             .with_ignore_errors(options.ignore_errors)
             .with_skip_rows(options.skip_rows)

crates/polars/tests/it/io/csv.rs

@@ -387,7 +387,7 @@ fn test_empty_bytes_to_dataframe() {
     let result = CsvReader::new(file)
         .has_header(false)
         .with_columns(Some(schema.iter_names().map(|s| s.to_string()).collect()))
-        .with_schema(Arc::new(schema))
+        .with_schema(Some(Arc::new(schema)))
         .finish();
     assert!(result.is_ok())
 }
@@ -416,11 +416,11 @@ fn test_missing_value() {
     let file = Cursor::new(csv);
     let df = CsvReader::new(file)
         .has_header(true)
-        .with_schema(Arc::new(Schema::from_iter([
+        .with_schema(Some(Arc::new(Schema::from_iter([
             Field::new("foo", DataType::UInt32),
             Field::new("bar", DataType::UInt32),
             Field::new("ham", DataType::UInt32),
-        ])))
+        ]))))
         .finish()
         .unwrap();
     assert_eq!(df.column("ham").unwrap().len(), 3)

py-polars/polars/dataframe/frame.py

@@ -662,6 +662,7 @@ def _read_csv(
         quote_char: str | None = r'"',
         skip_rows: int = 0,
         dtypes: None | (SchemaDict | Sequence[PolarsDataType]) = None,
+        schema: None | SchemaDict = None,
         null_values: str | Sequence[str] | dict[str, str] | None = None,
         missing_utf8_is_empty_string: bool = False,
         ignore_errors: bool = False,
@@ -740,6 +741,7 @@ def _read_csv(
                 quote_char=quote_char,
                 skip_rows=skip_rows,
                 dtypes=dtypes_dict,
+                schema=schema,
                 null_values=null_values,
                 missing_utf8_is_empty_string=missing_utf8_is_empty_string,
                 ignore_errors=ignore_errors,
@@ -795,6 +797,7 @@ def _read_csv(
             eol_char=eol_char,
             raise_if_empty=raise_if_empty,
             truncate_ragged_lines=truncate_ragged_lines,
+            schema=schema,
         )
         return self
 

py-polars/polars/io/_utils.py

@@ -17,7 +17,7 @@ def _is_glob_pattern(file: str) -> bool:
 
 def _is_local_file(file: str) -> bool:
     try:
-        next(glob.iglob(file, recursive=True))  # noqa: PTH207
+        next(glob.iglob(file, recursive=True))
         return True
     except StopIteration:
         return False

py-polars/polars/io/csv/functions.py

@@ -28,6 +28,7 @@ def read_csv(
     quote_char: str | None = r'"',
     skip_rows: int = 0,
     dtypes: Mapping[str, PolarsDataType] | Sequence[PolarsDataType] | None = None,
+    schema: SchemaDict | None = None,
     null_values: str | Sequence[str] | dict[str, str] | None = None,
     missing_utf8_is_empty_string: bool = False,
     ignore_errors: bool = False,
@@ -83,6 +84,10 @@ def read_csv(
         Start reading after ``skip_rows`` lines.
     dtypes
         Overwrite dtypes for specific or all columns during schema inference.
+    schema
+        Provide the schema. This means that polars doesn't do schema inference.
+        This argument expects the complete schema, whereas ``dtypes`` can be used
+        to partially overwrite a schema.
     null_values
         Values to interpret as null values. You can provide a:
 
@@ -365,6 +370,7 @@ def read_csv(
             quote_char=quote_char,
             skip_rows=skip_rows,
             dtypes=dtypes,
+            schema=schema,
             null_values=null_values,
             missing_utf8_is_empty_string=missing_utf8_is_empty_string,
             ignore_errors=ignore_errors,
@@ -691,6 +697,7 @@ def scan_csv(
     quote_char: str | None = r'"',
     skip_rows: int = 0,
     dtypes: SchemaDict | Sequence[PolarsDataType] | None = None,
+    schema: SchemaDict | None = None,
     null_values: str | Sequence[str] | dict[str, str] | None = None,
     missing_utf8_is_empty_string: bool = False,
     ignore_errors: bool = False,
@@ -741,6 +748,10 @@ def scan_csv(
         Overwrite dtypes during inference; should be a {colname:dtype,} dict or,
         if providing a list of strings to ``new_columns``, a list of dtypes of
         the same length.
+    schema
+        Provide the schema. This means that polars doesn't do schema inference.
+        This argument expects the complete schema, whereas ``dtypes`` can be used
+        to partially overwrite a schema.
     null_values
         Values to interpret as null values. You can provide a:
 
@@ -892,6 +903,7 @@ def with_column_names(_cols: list[str]) -> list[str]:
         quote_char=quote_char,
         skip_rows=skip_rows,
         dtypes=dtypes,  # type: ignore[arg-type]
+        schema=schema,
         null_values=null_values,
         missing_utf8_is_empty_string=missing_utf8_is_empty_string,
         ignore_errors=ignore_errors,

py-polars/polars/lazyframe/frame.py

@@ -327,6 +327,7 @@ def _scan_csv(
         quote_char: str | None = r'"',
         skip_rows: int = 0,
         dtypes: SchemaDict | None = None,
+        schema: SchemaDict | None = None,
         null_values: str | Sequence[str] | dict[str, str] | None = None,
         missing_utf8_is_empty_string: bool = False,
         ignore_errors: bool = False,
@@ -387,6 +388,7 @@ def _scan_csv(
             eol_char=eol_char,
             raise_if_empty=raise_if_empty,
             truncate_ragged_lines=truncate_ragged_lines,
+            schema=schema,
         )
         return self
 

py-polars/src/dataframe.rs

@@ -139,7 +139,7 @@ impl PyDataFrame {
         skip_rows, projection, separator, rechunk, columns, encoding, n_threads, path,
         overwrite_dtype, overwrite_dtype_slice, low_memory, comment_char, quote_char,
         null_values, missing_utf8_is_empty_string, try_parse_dates, skip_rows_after_header,
-        row_count, sample_size, eol_char, raise_if_empty, truncate_ragged_lines)
+        row_count, sample_size, eol_char, raise_if_empty, truncate_ragged_lines, schema)
     )]
     pub fn read_csv(
         py_f: &PyAny,
@@ -170,6 +170,7 @@ impl PyDataFrame {
         eol_char: &str,
         raise_if_empty: bool,
         truncate_ragged_lines: bool,
+        schema: Option<Wrap<Schema>>,
     ) -> PyResult<Self> {
         let null_values = null_values.map(|w| w.0);
         let comment_char = comment_char.map(|s| s.as_bytes()[0]);
@@ -219,6 +220,7 @@ impl PyDataFrame {
             .with_path(path)
             .with_dtypes(overwrite_dtype.map(Arc::new))
             .with_dtypes_slice(overwrite_dtype_slice.as_deref())
+            .with_schema(schema.map(|schema| Arc::new(schema.0)))
             .low_memory(low_memory)
             .with_null_values(null_values)
             .with_missing_is_null(!missing_utf8_is_empty_string)

py-polars/src/lazyframe.rs

@@ -147,7 +147,7 @@ impl PyLazyFrame {
     #[pyo3(signature = (path, separator, has_header, ignore_errors, skip_rows, n_rows, cache, overwrite_dtype,
         low_memory, comment_char, quote_char, null_values, missing_utf8_is_empty_string,
         infer_schema_length, with_schema_modify, rechunk, skip_rows_after_header,
-        encoding, row_count, try_parse_dates, eol_char, raise_if_empty, truncate_ragged_lines
+        encoding, row_count, try_parse_dates, eol_char, raise_if_empty, truncate_ragged_lines, schema
     )
     )]
     fn new_from_csv(
@@ -174,6 +174,7 @@ impl PyLazyFrame {
         eol_char: &str,
         raise_if_empty: bool,
         truncate_ragged_lines: bool,
+        schema: Option<Wrap<Schema>>,
     ) -> PyResult<Self> {
         let null_values = null_values.map(|w| w.0);
         let comment_char = comment_char.map(|s| s.as_bytes()[0]);
@@ -197,6 +198,7 @@ impl PyLazyFrame {
             .with_n_rows(n_rows)
             .with_cache(cache)
             .with_dtype_overwrite(overwrite_dtype.as_ref())
+            .with_schema(schema.map(|schema| Arc::new(schema.0)))
             .low_memory(low_memory)
             .with_comment_char(comment_char)
             .with_quote_char(quote_char)

py-polars/tests/unit/io/test_csv.py

@@ -1494,3 +1494,16 @@ def test_csv_ragged_lines() -> None:
             pl.read_csv(io.StringIO(s), has_header=False, truncate_ragged_lines=False)
         with pytest.raises(pl.ComputeError, match=r"found more fields than defined"):
             pl.read_csv(io.StringIO(s), has_header=False, truncate_ragged_lines=False)
+
+
+def test_provide_schema() -> None:
+    # can be used to overload schema with ragged csv files
+    assert pl.read_csv(
+        io.StringIO("A\nB,ragged\nC"),
+        has_header=False,
+        schema={"A": pl.Utf8, "B": pl.Utf8, "C": pl.Utf8},
+    ).to_dict(False) == {
+        "A": ["A", "B", "C"],
+        "B": [None, "ragged", None],
+        "C": [None, None, None],
+    }