Add "--as-columns" flag to polars into-df (#13449)

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# Description
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To facilitate column-oriented dataframe construction, this PR added a
`--as-columns` flag to `polars into-df` command so that when specified,
and when input shape is record of lists, each list will be treated as a
column rather than a cell value, i.e. `{a: [1 3], b: [2 4]} | polars
into-df --as-columns` returns the same dataframe as `[[a b];[1 2] [3 4]]
| polars into-df`


# User-Facing Changes
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A new flag `--as-columns`, no change of semantics if this flag is
unspecified.

# Tests + Formatting
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---------

Co-authored-by: Ben Yang <ben@ya.ng>

crates/nu_plugin_polars/src/dataframe/eager/to_df.rs

@@ -37,6 +37,11 @@ impl PluginCommand for ToDataFrame {
                 r#"Polars Schema in format [{name: str}]. CSV, JSON, and JSONL files"#,
                 Some('s'),
             )
+            .switch(
+                "as-columns",
+                r#"When input shape is record of lists, treat each list as column values."#,
+                Some('c'),
+            )
             .input_output_type(Type::Any, Type::Custom("dataframe".into()))
             .category(Category::Custom("dataframe".into()))
     }
@@ -64,6 +69,27 @@ impl PluginCommand for ToDataFrame {
                     .into_value(Span::test_data()),
                 ),
             },
+            Example {
+                description: "Takes a record of lists and creates a dataframe",
+                example: "{a: [1 3], b: [2 4]} | polars into-df --as-columns",
+                result: Some(
+                    NuDataFrame::try_from_columns(
+                        vec![
+                            Column::new(
+                                "a".to_string(),
+                                vec![Value::test_int(1), Value::test_int(3)],
+                            ),
+                            Column::new(
+                                "b".to_string(),
+                                vec![Value::test_int(2), Value::test_int(4)],
+                            ),
+                        ],
+                        None,
+                    )
+                    .expect("simple df for test should not fail")
+                    .into_value(Span::test_data()),
+                ),
+            },
             Example {
                 description: "Takes a list of tables and creates a dataframe",
                 example: "[[1 2 a] [3 4 b] [5 6 c]] | polars into-df",
@@ -182,7 +208,39 @@ impl PluginCommand for ToDataFrame {
             .map(|schema| NuSchema::try_from(&schema))
             .transpose()?;
 
-        let df = NuDataFrame::try_from_iter(plugin, input.into_iter(), maybe_schema.clone())?;
+        let maybe_as_columns = call.has_flag("as-columns")?;
+
+        let df = if !maybe_as_columns {
+            NuDataFrame::try_from_iter(plugin, input.into_iter(), maybe_schema.clone())?
+        } else {
+            match &input {
+                PipelineData::Value(Value::Record { val, .. }, _) => {
+                    let items: Result<Vec<(String, Vec<Value>)>, &str> = val
+                        .iter()
+                        .map(|(k, v)| match v.to_owned().into_list() {
+                            Ok(v) => Ok((k.to_owned(), v)),
+                            _ => Err("error"),
+                        })
+                        .collect();
+                    match items {
+                        Ok(items) => {
+                            let columns = items
+                                .iter()
+                                .map(|(k, v)| Column::new(k.to_owned(), v.to_owned()))
+                                .collect::<Vec<Column>>();
+                            NuDataFrame::try_from_columns(columns, maybe_schema)?
+                        }
+                        Err(_) => NuDataFrame::try_from_iter(
+                            plugin,
+                            input.into_iter(),
+                            maybe_schema.clone(),
+                        )?,
+                    }
+                }
+                _ => NuDataFrame::try_from_iter(plugin, input.into_iter(), maybe_schema.clone())?,
+            }
+        };
+
         df.to_pipeline_data(plugin, engine, call.head)
             .map_err(LabeledError::from)
     }

crates/nu_plugin_polars/src/dataframe/eager/to_nu.rs

@@ -23,7 +23,7 @@ impl PluginCommand for ToNu {
     }
 
     fn usage(&self) -> &str {
-        "Converts a dataframe or an expression into into nushell value for access and exploration."
+        "Converts a dataframe or an expression into nushell value for access and exploration."
     }
 
     fn signature(&self) -> Signature {