New 'datasette --pdb' option, closes #962

datasette/app.py

@@ -191,10 +191,12 @@ class Datasette:
         secret=None,
         version_note=None,
         config_dir=None,
+        pdb=False,
     ):
         assert config_dir is None or isinstance(
             config_dir, Path
         ), "config_dir= should be a pathlib.Path"
+        self.pdb = pdb
         self._secret = secret or secrets.token_hex(32)
         self.files = tuple(files) + tuple(immutables or [])
         if config_dir:
@@ -1057,6 +1059,11 @@ class DatasetteRouter:
                 await self.handle_500(request, send, exception or NotFound("404"))
 
     async def handle_500(self, request, send, exception):
+        if self.ds.pdb:
+            import pdb
+
+            pdb.post_mortem(exception.__traceback__)
+
         title = None
         if isinstance(exception, NotFound):
             status = 404

datasette/cli.py

@@ -352,6 +352,7 @@ def uninstall(packages, yes):
 )
 @click.option("--version-note", help="Additional note to show on /-/versions")
 @click.option("--help-config", is_flag=True, help="Show available config options")
+@click.option("--pdb", is_flag=True, help="Launch debugger on any errors")
 def serve(
     files,
     immutable,
@@ -373,6 +374,7 @@ def serve(
     get,
     version_note,
     help_config,
+    pdb,
     return_instance=False,
 ):
     """Serve up specified SQLite database files with a web UI"""
@@ -418,6 +420,7 @@ def serve(
         memory=memory,
         secret=secret,
         version_note=version_note,
+        pdb=pdb,
     )
 
     # if files is a single directory, use that as config_dir=

docs/datasette-serve-help.txt

@@ -38,4 +38,5 @@ Options:
 
   --version-note TEXT       Additional note to show on /-/versions
   --help-config             Show available config options
+  --pdb                     Launch debugger on any errors
   --help                    Show this message and exit.

tests/test_cli.py

@@ -104,6 +104,7 @@ def test_metadata_yaml():
         version_note=None,
         get=None,
         help_config=False,
+        pdb=False,
         return_instance=True,
     )
     client = _TestClient(ds.app())