chore: add some more documentation to the new QueryBuilder interface

Signed-off-by: R. Tyler Croy <[email protected]>

python/deltalake/query.py

@@ -9,8 +9,15 @@
 from deltalake.table import DeltaTable
 from deltalake.warnings import ExperimentalWarning
 
-
 class QueryBuilder:
+    """
+    QueryBuilder is an experimental API which exposes Apache DataFusion SQL to Python users of the deltalake library.
+
+    This API is subject to change.
+
+    >>> qb = QueryBuilder()
+    """
+
     def __init__(self) -> None:
         warnings.warn(
             "QueryBuilder is experimental and subject to change",
@@ -19,13 +26,38 @@ def __init__(self) -> None:
         self._query_builder = PyQueryBuilder()
 
     def register(self, table_name: str, delta_table: DeltaTable) -> QueryBuilder:
-        """Add a table to the query builder."""
+        """
+        Add a table to the query builder instance by name. The `table_name`
+        will be how the referenced `DeltaTable` can be referenced in SQL
+        queries.
+
+        For example:
+
+        >>> tmp = getfixture('tmp_path')
+        >>> import pyarrow as pa
+        >>> from deltalake import DeltaTable, QueryBuilder
+        >>> dt = DeltaTable.create(table_uri=tmp, schema=pa.schema([pa.field('name', pa.string())]))
+        >>> qb = QueryBuilder().register('test', dt)
+        >>> assert qb is not None
+        """
         self._query_builder.register(
             table_name=table_name,
             delta_table=delta_table._table,
         )
         return self
 
     def execute(self, sql: str) -> List[pyarrow.RecordBatch]:
-        """Execute the query and return a list of record batches."""
+        """
+        Execute the query and return a list of record batches
+
+        For example:
+
+        >>> tmp = getfixture('tmp_path')
+        >>> import pyarrow as pa
+        >>> from deltalake import DeltaTable, QueryBuilder
+        >>> dt = DeltaTable.create(table_uri=tmp, schema=pa.schema([pa.field('name', pa.string())]))
+        >>> qb = QueryBuilder().register('test', dt)
+        >>> results = qb.execute('SELECT * FROM test')
+        >>> assert results is not None
+        """
         return self._query_builder.execute(sql)

python/src/filesystem.rs

@@ -13,7 +13,7 @@ use std::sync::Arc;
 
 const DEFAULT_MAX_BUFFER_SIZE: usize = 5 * 1024 * 1024;
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
 pub(crate) struct FsConfig {
     pub(crate) root_url: String,
     pub(crate) options: HashMap<String, String>,

python/src/query.rs

@@ -9,21 +9,31 @@ use pyo3::prelude::*;
 
 use crate::{error::PythonError, utils::rt, RawDeltaTable};
 
+/// PyQueryBuilder supports the _experimental_ `QueryBuilder` Pythoh interface which allows users
+/// to take advantage of the [Apache DataFusion](https://datafusion.apache.org) engine already
+/// present in the Python package.
 #[pyclass(module = "deltalake._internal")]
+#[derive(Default)]
 pub(crate) struct PyQueryBuilder {
-    _ctx: SessionContext,
+    /// DataFusion [SessionContext] to hold mappings of registered tables
+    ctx: SessionContext,
 }
 
 #[pymethods]
 impl PyQueryBuilder {
     #[new]
     pub fn new() -> Self {
         let config = DeltaSessionConfig::default().into();
-        let _ctx = SessionContext::new_with_config(config);
+        let ctx = SessionContext::new_with_config(config);
 
-        PyQueryBuilder { _ctx }
+        PyQueryBuilder { ctx }
     }
 
+    /// Register the given [RawDeltaTable] into the [SessionContext] using the provided
+    /// `table_name`
+    ///
+    /// Once called, the provided `delta_table` will be referencable in SQL queries so long as
+    /// another table of the same name is not registered over it.
     pub fn register(&self, table_name: &str, delta_table: &RawDeltaTable) -> PyResult<()> {
         let snapshot = delta_table._table.snapshot().map_err(PythonError::from)?;
         let log_store = delta_table._table.log_store();
@@ -37,17 +47,22 @@ impl PyQueryBuilder {
                 .map_err(PythonError::from)?,
         );
 
-        self._ctx
+        self.ctx
             .register_table(table_name, provider)
             .map_err(PythonError::from)?;
 
         Ok(())
     }
 
+    /// Execute the given SQL command within the [SessionContext] of this instance
+    ///
+    /// **NOTE:** Since this function returns a materialized Python list of `RecordBatch`
+    /// instances, it may result unexpected memory consumption for queries which return large data
+    /// sets.
     pub fn execute(&self, py: Python, sql: &str) -> PyResult<PyObject> {
         let batches = py.allow_threads(|| {
             rt().block_on(async {
-                let df = self._ctx.sql(sql).await?;
+                let df = self.ctx.sql(sql).await?;
                 df.collect().await
             })
             .map_err(PythonError::from)