respect header flag in csv type inference and writing

polars/polars-io/src/csv.rs

@@ -17,7 +17,7 @@
 //!     let mut file = File::create("example.csv").expect("could not create file");
 //!
 //!     CsvWriter::new(&mut file)
-//!     .has_headers(true)
+//!     .has_header(true)
 //!     .with_delimiter(b',')
 //!     .finish(df)
 //! }
@@ -60,6 +60,7 @@ pub struct CsvWriter<W: Write> {
     writer_builder: write::WriterBuilder,
     /// arrow specific options
     options: write::SerializeOptions,
+    header: bool,
 }
 
 impl<W> SerWriter<W> for CsvWriter<W>
@@ -81,13 +82,16 @@ where
             buffer,
             writer_builder: write::WriterBuilder::new(),
             options,
+            header: true,
         }
     }
 
     fn finish(self, df: &DataFrame) -> Result<()> {
         let mut writer = self.writer_builder.from_writer(self.buffer);
         let iter = df.iter_record_batches();
-        write::write_header(&mut writer, &df.schema().to_arrow())?;
+        if self.header {
+            write::write_header(&mut writer, &df.schema().to_arrow())?;
+        }
         for batch in iter {
             write::write_batch(&mut writer, &batch, &self.options)?;
         }
@@ -100,8 +104,8 @@ where
     W: Write,
 {
     /// Set whether to write headers
-    pub fn has_headers(mut self, has_headers: bool) -> Self {
-        self.writer_builder.has_headers(has_headers);
+    pub fn has_header(mut self, has_header: bool) -> Self {
+        self.header = has_header;
         self
     }
 
@@ -595,11 +599,19 @@ mod test {
         let df = create_df();
 
         CsvWriter::new(&mut buf)
-            .has_headers(true)
+            .has_header(true)
             .finish(&df)
             .expect("csv written");
         let csv = std::str::from_utf8(&buf).unwrap();
         assert_eq!("days,temp\n0,22.1\n1,19.9\n2,7.0\n3,2.0\n4,3.0\n", csv);
+
+        let mut buf: Vec<u8> = Vec::new();
+        CsvWriter::new(&mut buf)
+            .has_header(false)
+            .finish(&df)
+            .expect("csv written");
+        let csv = std::str::from_utf8(&buf).unwrap();
+        assert_eq!("0,22.1\n1,19.9\n2,7.0\n3,2.0\n4,3.0\n", csv);
     }
 
     #[test]
@@ -1183,4 +1195,16 @@ linenum,last_name,first_name
 
         Ok(())
     }
+
+    #[test]
+    fn test_header_inference() -> Result<()> {
+        let csv = r#"not_a_header,really,even_if,it_looks_like_one
+1,2,3,4
+4,3,2,1
+"#;
+        let file = Cursor::new(csv);
+        let df = CsvReader::new(file).has_header(false).finish()?;
+        assert_eq!(df.dtypes(), vec![DataType::Utf8; 4]);
+        Ok(())
+    }
 }

polars/polars-io/src/csv_core/utils.rs

@@ -167,6 +167,10 @@ pub fn infer_file_schema(
     } else {
         return Err(PolarsError::NoData("empty csv".into()));
     };
+    if !has_header {
+        // re-init lines so that the header is included in type inference.
+        lines = SplitLines::new(bytes, b'\n').skip(skip_rows);
+    }
 
     let header_length = headers.len();
     // keep track of inferred field types