Optionally return shell output on incorrect command (#894) (#899)

This allows the LLM to correct its previous command by looking at the
error message output to the shell.

Additionally, this uses subprocess.run because that is now recommended
over subprocess.check_output:

https://docs.python.org/3/library/subprocess.html#using-the-subprocess-module

Co-authored-by: Amos Ng <me@amos.ng>

langchain/utilities/bash.py

@@ -6,9 +6,10 @@ from typing import List, Union
 class BashProcess:
     """Executes bash commands and returns the output."""
 
-    def __init__(self, strip_newlines: bool = False):
+    def __init__(self, strip_newlines: bool = False, return_err_output: bool = False):
         """Initialize with stripping newlines."""
         self.strip_newlines = strip_newlines
+        self.return_err_output = return_err_output
 
     def run(self, commands: Union[str, List[str]]) -> str:
         """Run commands and return final output."""
@@ -16,8 +17,16 @@ class BashProcess:
             commands = [commands]
         commands = ";".join(commands)
         try:
-            output = subprocess.check_output(commands, shell=True).decode()
+            output = subprocess.run(
+                commands,
+                shell=True,
+                check=True,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.STDOUT,
+            ).stdout.decode()
         except subprocess.CalledProcessError as error:
+            if self.return_err_output:
+                return error.stdout.decode()
             return str(error)
         if self.strip_newlines:
             output = output.strip()

tests/unit_tests/test_bash.py

@@ -21,6 +21,13 @@ def test_incorrect_command() -> None:
     assert output == "Command 'invalid_command' returned non-zero exit status 127."
 
 
+def test_incorrect_command_return_err_output() -> None:
+    """Test optional returning of shell output on incorrect command."""
+    session = BashProcess(return_err_output=True)
+    output = session.run(["invalid_command"])
+    assert output == "/bin/sh: 1: invalid_command: not found\n"
+
+
 def test_create_directory_and_files(tmp_path: Path) -> None:
     """Test creation of a directory and files in a temporary directory."""
     session = BashProcess(strip_newlines=True)