Show traceback on error

Closes: #21

web_view.py

@@ -2,16 +2,48 @@
 
 """Web page to show upcoming events."""
 
+import inspect
+import sys
+import traceback
 from datetime import date, datetime, timezone
 
-from flask import Flask, render_template
+import flask
+import werkzeug
+import werkzeug.debug.tbtools
 
 from agenda import get_data
 
-app = Flask(__name__)
+app = flask.Flask(__name__)
 app.debug = True
 
 
+@app.errorhandler(werkzeug.exceptions.InternalServerError)
+def exception_handler(e: werkzeug.exceptions.InternalServerError) -> tuple[str, int]:
+    """Handle exception."""
+    exec_type, exc_value, current_traceback = sys.exc_info()
+    assert exc_value
+    tb = werkzeug.debug.tbtools.DebugTraceback(exc_value)
+
+    summary = tb.render_traceback_html(include_title=False)
+    exc_lines = "".join(tb._te.format_exception_only())
+
+    last_frame = list(traceback.walk_tb(current_traceback))[-1][0]
+    last_frame_args = inspect.getargs(last_frame.f_code)
+
+    return (
+        flask.render_template(
+            "show_error.html",
+            plaintext=tb.render_traceback_text(),
+            exception=exc_lines,
+            exception_type=tb._te.exc_type.__name__,
+            summary=summary,
+            last_frame=last_frame,
+            last_frame_args=last_frame_args,
+        ),
+        500,
+    )
+
+
 @app.route("/")
 def index() -> str:
     """Index page."""
@@ -27,7 +59,7 @@ def index() -> str:
     def days(when: date) -> str:
         return "today" if when == today else f"{(when - today).days:,d} days"
 
-    return render_template("index.html", days=days, days_hours=days_hours, **data)
+    return flask.render_template("index.html", days=days, days_hours=days_hours, **data)
 
 
 if __name__ == "__main__":