Auto merge of #400 - pimutils:not-implemented-discovery, r=untitaker

Nicer output for unsupported discovery

Fix #347

tests/cli/test_discover.py

@@ -60,3 +60,24 @@ def test_discover_command(tmpdir, runner):
         .join('status') \
         .join('foobar.collections') \
         .read()
+
+
+def test_discover_on_unsupported_storage(tmpdir, runner):
+    runner.write_with_general(dedent('''
+    [storage foo]
+    type = http
+    url = https://example.com/foo.ics
+
+    [storage bar]
+    type = memory
+    fileext = .txt
+
+    [pair foobar]
+    a = foo
+    b = bar
+    collections = ["from a"]
+    ''').format(str(tmpdir)))
+
+    result = runner.invoke(['discover'])
+    assert result.exception
+    assert 'doesn\'t support collection discovery' in result.output

vdirsyncer/cli/utils.py

@@ -223,7 +223,14 @@ def _discover_from_config(config):
     cls, config = storage_class_from_config(config)
 
     try:
-        discovered = list(cls.discover(**config))
+        try:
+            discovered = list(cls.discover(**config))
+        except NotImplementedError:
+            raise exceptions.UserError(
+                'The storage {} (type {}) doesn\'t support collection '
+                'discovery. You can only use `collections = null` with it.'
+                .format(config.get('instance_name', '???'), storage_type)
+            )
     except Exception:
         return handle_storage_init_error(cls, config)
     else: