Exceptions occurring in tab completion functions are now printed to stderr before returning control back to readline

3 files changed, 41 insertions, 6 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e5142c9b..bead9ce1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,8 @@
     * `StdSim.buffer.write()` now flushes when the wrapped stream uses line buffering and the bytes being written
       contain a newline or carriage return. This helps when `pyscript` is echoing the output of a shell command
       since the output will print at the same frequency as when the command is run in a terminal.
+    * Exceptions occurring in tab completion functions are now printed to stderr before returning control back to
+    readline. This makes debugging a lot easier since readline suppresses these exceptions.
 * **Python 3.4 EOL notice**
     * Python 3.4 reached its [end of life](https://www.python.org/dev/peps/pep-0429/) on March 18, 2019
     * This is the last release of `cmd2` which will support Python 3.4
diff --git a/cmd2/cmd2.py b/cmd2/cmd2.py
index a7b60b1a..3c1c8d2c 100644
--- a/cmd2/cmd2.py
+++ b/cmd2/cmd2.py
@@ -1362,16 +1362,13 @@ class Cmd(cmd.Cmd):
             # Display matches using actual display function. This also redraws the prompt and line.
             orig_pyreadline_display(matches_to_display)
 
-    # -----  Methods which override stuff in cmd -----
-
-    def complete(self, text: str, state: int) -> Optional[str]:
-        """Override of command method which returns the next possible completion for 'text'.
+    def _complete_worker(self, text: str, state: int) -> Optional[str]:
+        """The actual worker function for tab completion which is called by complete() and returns
+        the next possible completion for 'text'.
 
         If a command has not been entered, then complete against command list.
         Otherwise try to call complete_<command> to get list of completions.
 
-        This method gets called directly by readline because it is set as the tab-completion function.
-
         This completer function is called as complete(text, state), for state in 0, 1, 2, …, until it returns a
         non-string value. It should return the next possible completion starting with text.
 
@@ -1581,6 +1578,24 @@ class Cmd(cmd.Cmd):
         except IndexError:
             return None
 
+    def complete(self, text: str, state: int) -> Optional[str]:
+        """Override of cmd2's complete method which returns the next possible completion for 'text'
+
+        This method gets called directly by readline. Since readline suppresses any exception raised
+        in completer functions, they can be difficult to debug. Therefore this function wraps the
+        actual tab completion logic and prints to stderr any exception that occurs before returning
+        control to readline.
+
+        :param text: the current word that user is typing
+        :param state: non-negative integer
+        """
+        # noinspection PyBroadException
+        try:
+            return self._complete_worker(text, state)
+        except Exception as e:
+            self.perror(e)
+            return None
+
     def _autocomplete_default(self, text: str, line: str, begidx: int, endidx: int,
                               argparser: argparse.ArgumentParser) -> List[str]:
         """Default completion function for argparse commands."""
diff --git a/tests/test_completion.py b/tests/test_completion.py
index 23843012..158856ec 100644
--- a/tests/test_completion.py
+++ b/tests/test_completion.py
@@ -77,6 +77,12 @@ class CompletionsExample(cmd2.Cmd):
         num_strs = ['2', '11', '1']
         return self.basic_complete(text, line, begidx, endidx, num_strs)
 
+    def do_test_raise_exception(self, args):
+        pass
+
+    def complete_test_raise_exception(self, text, line, begidx, endidx):
+        raise IndexError("You are out of bounds!!")
+
 
 @pytest.fixture
 def cmd2_app():
@@ -120,6 +126,18 @@ def test_complete_bogus_command(cmd2_app):
     first_match = complete_tester(text, line, begidx, endidx, cmd2_app)
     assert first_match is None
 
+def test_complete_exception(cmd2_app, capsys):
+    text = ''
+    line = 'test_raise_exception {}'.format(text)
+    endidx = len(line)
+    begidx = endidx - len(text)
+
+    first_match = complete_tester(text, line, begidx, endidx, cmd2_app)
+    out, err = capsys.readouterr()
+
+    assert first_match is None
+    assert "IndexError" in err
+
 def test_complete_macro(base_app, request):
     # Create the macro
     out, err = run_cmd(base_app, 'macro create fake pyscript {1}')