nu_plugin_polars: add polars into-repr to display dataframe in portable repr format (#14917)

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# Description
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This PR adds a new command that outputs a NuDataFrame or NuLazyFrame in
its repr format, which can then be ingested in another polars instance.
Advantages of serializing a dataframe in this format are that it can be
viewed as a table, carries type information, and can easily be copied to
the clipboard.

```nushell
# In Nushell
> [[a b]; [2025-01-01 2] [2025-01-02 4]] | polars into-df | polars into-lazy | polars into-repr

shape: (2, 2)
┌─────────────────────┬─────┐
│ a                   ┆ b   │
│ ---                 ┆ --- │
│ datetime[ns]        ┆ i64 │
╞═════════════════════╪═════╡
│ 2025-01-01 00:00:00 ┆ 2   │
│ 2025-01-02 00:00:00 ┆ 4   │
└─────────────────────┴─────┘
```

```python
# In python
>>> import polars as pl
>>> df = pl.from_repr("""
... shape: (2, 2)
... ┌─────────────────────┬─────┐
... │ a                   ┆ b   │
... │ ---                 ┆ --- │
... │ datetime[ns]        ┆ i64 │
... ╞═════════════════════╪═════╡
... │ 2025-01-01 00:00:00 ┆ 2   │
... │ 2025-01-02 00:00:00 ┆ 4   │
... └─────────────────────┴─────┘""")
shape: (2, 2)
┌─────────────────────┬─────┐
│ a                   ┆ b   │
│ ---                 ┆ --- │
│ datetime[ns]        ┆ i64 │
╞═════════════════════╪═════╡
│ 2025-01-01 00:00:00 ┆ 2   │
│ 2025-01-02 00:00:00 ┆ 4   │
└─────────────────────┴─────┘

>>> df.select(pl.col("a").dt.offset_by("12m"))
shape: (2, 1)
┌─────────────────────┐
│ a                   │
│ ---                 │
│ datetime[ns]        │
╞═════════════════════╡
│ 2025-01-01 00:12:00 │
│ 2025-01-02 00:12:00 │
└─────────────────────┘
```

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
A new command `polars into-repr` is added. No other commands are
impacted by the changes in this PR.

# Tests + Formatting
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> **Note**
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> ```
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Examples were added in the command definition.

# After Submitting
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crates/nu_plugin_polars/src/dataframe/command/core/mod.rs

@@ -10,6 +10,7 @@ mod summary;
 mod to_df;
 mod to_lazy;
 mod to_nu;
+mod to_repr;
 
 use crate::PolarsPlugin;
 use nu_plugin::PluginCommand;
@@ -22,6 +23,7 @@ pub use summary::Summary;
 pub use to_df::ToDataFrame;
 pub use to_lazy::ToLazyFrame;
 pub use to_nu::ToNu;
+pub use to_repr::ToRepr;
 
 pub(crate) fn core_commands() -> Vec<Box<dyn PluginCommand<Plugin = PolarsPlugin>>> {
     vec![
@@ -37,5 +39,6 @@ pub(crate) fn core_commands() -> Vec<Box<dyn PluginCommand<Plugin = PolarsPlugin
         Box::new(ToDataFrame),
         Box::new(save::SaveDF),
         Box::new(ToLazyFrame),
+        Box::new(ToRepr),
     ]
 }

crates/nu_plugin_polars/src/dataframe/command/core/to_repr.rs

@@ -0,0 +1,115 @@
+use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
+use nu_protocol::{
+    Category, Example, LabeledError, PipelineData, ShellError, Signature, Span, Type, Value,
+};
+
+use crate::{
+    values::{cant_convert_err, CustomValueSupport, NuLazyFrame, PolarsPluginType},
+    PolarsPlugin,
+};
+
+use crate::values::NuDataFrame;
+
+#[derive(Clone)]
+pub struct ToRepr;
+
+impl PluginCommand for ToRepr {
+    type Plugin = PolarsPlugin;
+
+    fn name(&self) -> &str {
+        "polars into-repr"
+    }
+
+    fn description(&self) -> &str {
+        "Display a dataframe in its repr format."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .input_output_types(vec![(Type::Custom("dataframe".into()), Type::String)])
+            .category(Category::Custom("dataframe".into()))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Shows dataframe in repr format",
+                example:
+                    "[[a b]; [2025-01-01 2] [2025-01-02 4]] | polars into-df | polars into-repr",
+                result: Some(Value::string(
+                    r#"
+shape: (2, 2)
+┌─────────────────────┬─────┐
+│ a                   ┆ b   │
+│ ---                 ┆ --- │
+│ datetime[ns]        ┆ i64 │
+╞═════════════════════╪═════╡
+│ 2025-01-01 00:00:00 ┆ 2   │
+│ 2025-01-02 00:00:00 ┆ 4   │
+└─────────────────────┴─────┘"#
+                        .trim(),
+                    Span::test_data(),
+                )),
+            },
+            Example {
+                description: "Shows lazy dataframe in repr format",
+                example:
+                    "[[a b]; [2025-01-01 2] [2025-01-02 4]] | polars into-df | polars into-lazy | polars into-repr",
+                result: Some(Value::string(
+                    r#"
+shape: (2, 2)
+┌─────────────────────┬─────┐
+│ a                   ┆ b   │
+│ ---                 ┆ --- │
+│ datetime[ns]        ┆ i64 │
+╞═════════════════════╪═════╡
+│ 2025-01-01 00:00:00 ┆ 2   │
+│ 2025-01-02 00:00:00 ┆ 4   │
+└─────────────────────┴─────┘"#
+                        .trim(),
+                    Span::test_data(),
+                )),
+            },
+        ]
+    }
+
+    fn run(
+        &self,
+        plugin: &Self::Plugin,
+        _engine: &EngineInterface,
+        call: &EvaluatedCall,
+        input: PipelineData,
+    ) -> Result<PipelineData, LabeledError> {
+        let value = input.into_value(call.head)?;
+        if NuDataFrame::can_downcast(&value) || NuLazyFrame::can_downcast(&value) {
+            dataframe_command(plugin, call, value)
+        } else {
+            Err(cant_convert_err(
+                &value,
+                &[PolarsPluginType::NuDataFrame, PolarsPluginType::NuLazyFrame],
+            ))
+        }
+        .map_err(|e| e.into())
+    }
+}
+
+fn dataframe_command(
+    plugin: &PolarsPlugin,
+    call: &EvaluatedCall,
+    input: Value,
+) -> Result<PipelineData, ShellError> {
+    let df = NuDataFrame::try_from_value_coerce(plugin, &input, call.head)?;
+    let value = Value::string(format!("{}", df), call.head);
+    Ok(PipelineData::Value(value, None))
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::test::test_polars_plugin_command;
+
+    #[test]
+    fn test_examples() -> Result<(), ShellError> {
+        test_polars_plugin_command(&ToRepr)
+    }
+}

crates/nu_plugin_polars/src/dataframe/values/nu_dataframe/mod.rs

@@ -13,6 +13,7 @@ use polars::prelude::{
 };
 use polars_plan::prelude::{lit, Expr, Null};
 use polars_utils::total_ord::{TotalEq, TotalHash};
+use std::fmt;
 use std::{
     cmp::Ordering,
     collections::HashSet,
@@ -118,6 +119,12 @@ impl From<DataFrame> for NuDataFrame {
     }
 }
 
+impl fmt::Display for NuDataFrame {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.df)
+    }
+}
+
 impl NuDataFrame {
     pub fn new(from_lazy: bool, df: DataFrame) -> Self {
         let id = Uuid::new_v4();