fix type

Author: ion-elgreco

python/datafusion/dataframe.py

@@ -452,6 +452,10 @@ def join(
             raise ValueError(
                 "either `on` or `left_on` and `right_on` should be provided."
             )
+        if isinstance(left_on, str):
+            left_on = [left_on]
+        if isinstance(right_on, str):
+            right_on = [right_on]
 
         return DataFrame(self.df.join(right.df, how, left_on, right_on))
 

src/dataframe.rs

@@ -290,14 +290,8 @@ impl PyDataFrame {
             }
         };
 
-        let left_keys = left_on
-            .iter()
-            .map(|s| s.as_ref())
-            .collect::<Vec<&str>>();
-        let right_keys = right_on
-            .iter()
-            .map(|s| s.as_ref())
-            .collect::<Vec<&str>>();
+        let left_keys = left_on.iter().map(|s| s.as_ref()).collect::<Vec<&str>>();
+        let right_keys = right_on.iter().map(|s| s.as_ref()).collect::<Vec<&str>>();
 
         let df = self.df.as_ref().clone().join(
             right.df.as_ref().clone(),