fix: Raise proper error for mismatching parquet schema instead of pan…

…icking (pola-rs#17321)

crates/polars-arrow/src/datatypes/schema.rs

@@ -1,5 +1,6 @@
 use std::sync::Arc;
 
+use polars_error::{polars_bail, PolarsResult};
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
@@ -62,6 +63,24 @@ impl ArrowSchema {
             metadata: self.metadata,
         }
     }
+
+    pub fn try_project(&self, indices: &[usize]) -> PolarsResult<Self> {
+        let fields = indices.iter().map(|&i| {
+            let Some(out) = self.fields.get(i) else {
+                polars_bail!(
+                    SchemaFieldNotFound: "projection index {} is out of bounds for schema of length {}",
+                    i, self.fields.len()
+                );
+            };
+
+            Ok(out.clone())
+        }).collect::<PolarsResult<Vec<_>>>()?;
+
+        Ok(ArrowSchema {
+            fields,
+            metadata: self.metadata.clone(),
+        })
+    }
 }
 
 impl From<Vec<Field>> for ArrowSchema {

crates/polars-core/src/schema.rs

@@ -446,6 +446,9 @@ pub trait IndexOfSchema: Debug {
     /// Get a vector of all column names.
     fn get_names(&self) -> Vec<&str>;
 
+    /// Get a vector of (name, dtype) pairs
+    fn get_names_and_dtypes(&'_ self) -> Vec<(&'_ str, DataType)>;
+
     fn try_index_of(&self, name: &str) -> PolarsResult<usize> {
         self.index_of(name).ok_or_else(|| {
             polars_err!(
@@ -464,6 +467,13 @@ impl IndexOfSchema for Schema {
     fn get_names(&self) -> Vec<&str> {
         self.iter_names().map(|name| name.as_str()).collect()
     }
+
+    fn get_names_and_dtypes(&'_ self) -> Vec<(&'_ str, DataType)> {
+        self.inner
+            .iter()
+            .map(|(name, dtype)| (name.as_str(), dtype.clone()))
+            .collect()
+    }
 }
 
 impl IndexOfSchema for ArrowSchema {
@@ -474,6 +484,45 @@ impl IndexOfSchema for ArrowSchema {
     fn get_names(&self) -> Vec<&str> {
         self.fields.iter().map(|f| f.name.as_str()).collect()
     }
+
+    fn get_names_and_dtypes(&'_ self) -> Vec<(&'_ str, DataType)> {
+        self.fields
+            .iter()
+            .map(|x| (x.name.as_str(), DataType::from_arrow(&x.data_type, true)))
+            .collect()
+    }
+}
+
+pub trait SchemaNamesAndDtypes {
+    const IS_ARROW: bool;
+    type DataType: Debug + PartialEq;
+
+    /// Get a vector of (name, dtype) pairs
+    fn get_names_and_dtypes(&'_ self) -> Vec<(&'_ str, Self::DataType)>;
+}
+
+impl SchemaNamesAndDtypes for Schema {
+    const IS_ARROW: bool = false;
+    type DataType = DataType;
+
+    fn get_names_and_dtypes(&'_ self) -> Vec<(&'_ str, Self::DataType)> {
+        self.inner
+            .iter()
+            .map(|(name, dtype)| (name.as_str(), dtype.clone()))
+            .collect()
+    }
+}
+
+impl SchemaNamesAndDtypes for ArrowSchema {
+    const IS_ARROW: bool = true;
+    type DataType = ArrowDataType;
+
+    fn get_names_and_dtypes(&'_ self) -> Vec<(&'_ str, Self::DataType)> {
+        self.fields
+            .iter()
+            .map(|x| (x.name.as_str(), x.data_type.clone()))
+            .collect()
+    }
 }
 
 impl From<&ArrowSchema> for Schema {
@@ -498,3 +547,51 @@ impl From<&ArrowSchemaRef> for Schema {
         Self::from(value.as_ref())
     }
 }
+
+pub fn ensure_matching_schema<S: SchemaNamesAndDtypes>(lhs: &S, rhs: &S) -> PolarsResult<()> {
+    let lhs = lhs.get_names_and_dtypes();
+    let rhs = rhs.get_names_and_dtypes();
+
+    if lhs.len() != rhs.len() {
+        polars_bail!(
+            SchemaMismatch:
+            "schemas contained differing number of columns: {} != {}",
+            lhs.len(), rhs.len(),
+        );
+    }
+
+    for (i, ((l_name, l_dtype), (r_name, r_dtype))) in lhs.iter().zip(&rhs).enumerate() {
+        if l_name != r_name {
+            polars_bail!(
+                SchemaMismatch:
+                "schema names differ at index {}: {} != {}",
+                i, l_name, r_name
+            )
+        }
+        if l_dtype != r_dtype
+            && (!S::IS_ARROW
+                || unsafe {
+                    // For timezone normalization. Easier than writing out the entire PartialEq.
+                    DataType::from_arrow(
+                        std::mem::transmute::<&<S as SchemaNamesAndDtypes>::DataType, &ArrowDataType>(
+                            l_dtype,
+                        ),
+                        true,
+                    ) != DataType::from_arrow(
+                        std::mem::transmute::<&<S as SchemaNamesAndDtypes>::DataType, &ArrowDataType>(
+                            r_dtype,
+                        ),
+                        true,
+                    )
+                })
+        {
+            polars_bail!(
+                SchemaMismatch:
+                "schema dtypes differ at index {} for column {}: {:?} != {:?}",
+                i, l_name, l_dtype, r_dtype
+            )
+        }
+    }
+
+    Ok(())
+}

crates/polars-io/src/parquet/read/reader.rs

@@ -80,22 +80,38 @@ impl<R: MmapBytesReader> ParquetReader<R> {
         self
     }
 
-    /// Set the [`Schema`] if already known. This must be exactly the same as
-    /// the schema in the file itself.
-    pub fn with_schema(mut self, schema: Option<ArrowSchemaRef>) -> Self {
-        self.schema = schema;
-        self
+    /// Ensure the schema of the file matches the given schema. Calling this
+    /// after setting the projection will ensure only the projected indices
+    /// are checked.
+    pub fn check_schema(mut self, schema: &ArrowSchema) -> PolarsResult<Self> {
+        let self_schema = self.schema()?;
+        let self_schema = self_schema.as_ref();
+
+        if let Some(ref projection) = self.projection {
+            let projection = projection.as_slice();
+
+            ensure_matching_schema(
+                &schema.try_project(projection)?,
+                &self_schema.try_project(projection)?,
+            )?;
+        } else {
+            ensure_matching_schema(schema, self_schema)?;
+        }
+
+        Ok(self)
     }
 
     /// [`Schema`] of the file.
     pub fn schema(&mut self) -> PolarsResult<ArrowSchemaRef> {
-        match &self.schema {
-            Some(schema) => Ok(schema.clone()),
+        self.schema = Some(match &self.schema {
+            Some(schema) => schema.clone(),
             None => {
                 let metadata = self.get_metadata()?;
-                Ok(Arc::new(read::infer_schema(metadata)?))
+                Arc::new(read::infer_schema(metadata)?)
             },
-        }
+        });
+
+        Ok(self.schema.clone().unwrap())
     }
 
     /// Use statistics in the parquet to determine if pages
@@ -226,7 +242,6 @@ impl ParquetAsyncReader {
     pub async fn from_uri(
         uri: &str,
         cloud_options: Option<&CloudOptions>,
-        schema: Option<ArrowSchemaRef>,
         metadata: Option<FileMetaDataRef>,
     ) -> PolarsResult<ParquetAsyncReader> {
         Ok(ParquetAsyncReader {
@@ -238,20 +253,40 @@ impl ParquetAsyncReader {
             predicate: None,
             use_statistics: true,
             hive_partition_columns: None,
-            schema,
+            schema: None,
             parallel: Default::default(),
         })
     }
 
+    pub async fn check_schema(mut self, schema: &ArrowSchema) -> PolarsResult<Self> {
+        let self_schema = self.schema().await?;
+        let self_schema = self_schema.as_ref();
+
+        if let Some(ref projection) = self.projection {
+            let projection = projection.as_slice();
+
+            ensure_matching_schema(
+                &schema.try_project(projection)?,
+                &self_schema.try_project(projection)?,
+            )?;
+        } else {
+            ensure_matching_schema(schema, self_schema)?;
+        }
+
+        Ok(self)
+    }
+
     pub async fn schema(&mut self) -> PolarsResult<ArrowSchemaRef> {
-        Ok(match self.schema.as_ref() {
+        self.schema = Some(match self.schema.as_ref() {
             Some(schema) => Arc::clone(schema),
             None => {
                 let metadata = self.reader.get_metadata().await?;
                 let arrow_schema = polars_parquet::arrow::read::infer_schema(metadata)?;
                 Arc::new(arrow_schema)
             },
-        })
+        });
+
+        Ok(self.schema.clone().unwrap())
     }
 
     pub async fn num_rows(&mut self) -> PolarsResult<usize> {

crates/polars-io/src/utils.rs

@@ -270,53 +270,6 @@ pub fn materialize_projection(
     }
 }
 
-pub fn check_projected_schema_impl(
-    a: &Schema,
-    b: &Schema,
-    projected_names: Option<&[String]>,
-    msg: &str,
-) -> PolarsResult<()> {
-    if !projected_names
-        .map(|projected_names| {
-            projected_names
-                .iter()
-                .all(|name| a.get(name) == b.get(name))
-        })
-        .unwrap_or_else(|| a == b)
-    {
-        polars_bail!(ComputeError: "{msg}\n\n\
-                    Expected: {:?}\n\n\
-                    Got: {:?}", a, b)
-    }
-    Ok(())
-}
-
-/// Checks if the projected columns are equal
-pub fn check_projected_arrow_schema(
-    a: &ArrowSchema,
-    b: &ArrowSchema,
-    projected_names: Option<&[String]>,
-    msg: &str,
-) -> PolarsResult<()> {
-    if a != b {
-        let a = Schema::from(a);
-        let b = Schema::from(b);
-        check_projected_schema_impl(&a, &b, projected_names, msg)
-    } else {
-        Ok(())
-    }
-}
-
-/// Checks if the projected columns are equal
-pub fn check_projected_schema(
-    a: &Schema,
-    b: &Schema,
-    projected_names: Option<&[String]>,
-    msg: &str,
-) -> PolarsResult<()> {
-    check_projected_schema_impl(a, b, projected_names, msg)
-}
-
 /// Split DataFrame into chunks in preparation for writing. The chunks have a
 /// maximum number of rows per chunk to ensure reasonable memory efficiency when
 /// reading the resulting file, and a minimum size per chunk to ensure