improve the performance of base.py (#8610)

This removes the use of the intermediate df list and directly
concatenates the dataframes if path is a list of strings. The pd.concat
function combines the dataframes efficiently, making it faster and more
memory-efficient compared to appending dataframes to a list.

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---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

libs/langchain/langchain/agents/agent_toolkits/csv/base.py

@@ -24,11 +24,13 @@ def create_csv_agent(
     if isinstance(path, (str, IOBase)):
         df = pd.read_csv(path, **_kwargs)
     elif isinstance(path, list):
-        df = []
-        for item in path:
-            if not isinstance(item, (str, IOBase)):
-                raise ValueError(f"Expected str or file-like object, got {type(path)}")
-            df.append(pd.read_csv(item, **_kwargs))
+        if not all(isinstance(item, (str, IOBase)) for item in path):
+            raise ValueError(
+                f"Expected all elements in the list to be strings, got {type(path)}."
+            )
+        dfs = [pd.read_csv(item, **_kwargs) for item in path]
+        df = pd.concat(dfs, ignore_index=True)
     else:
-        raise ValueError(f"Expected str, list, or file-like object, got {type(path)}")
+        raise ValueError(f"Expected str or list, got {type(path)}")
+
     return create_pandas_dataframe_agent(llm, df, **kwargs)