select dataframe columns with AsRef trait and df.dtypes() method

polars/src/frame/mod.rs

@@ -115,6 +115,11 @@ impl DataFrame {
         self.columns.iter().map(|s| s.name()).collect()
     }
 
+    /// Get the data types of the columns in the DataFrame.
+    pub fn dtypes(&self) -> Vec<ArrowDataType> {
+        self.columns.iter().map(|s| s.dtype().clone()).collect()
+    }
+
     /// The number of chunks per column
     pub fn n_chunks(&self) -> Result<usize> {
         Ok(self

polars/src/frame/select.rs

@@ -8,21 +8,18 @@ impl<'a> Selection<'a> for &'a str {
     }
 }
 
-impl<'a> Selection<'a> for &'a String {
-    fn to_selection_vec(self) -> Vec<&'a str> {
-        vec![self.as_str()]
-    }
-}
-
 impl<'a> Selection<'a> for Vec<&'a str> {
     fn to_selection_vec(self) -> Vec<&'a str> {
         self
     }
 }
 
-impl<'a> Selection<'a> for &'a Vec<String> {
+impl<'a, T> Selection<'a> for &T
+where
+    T: AsRef<[&'a str]>,
+{
     fn to_selection_vec(self) -> Vec<&'a str> {
-        self.iter().map(|s| s.as_str()).collect()
+        self.as_ref().iter().copied().collect()
     }
 }