New --plugins-dir=plugins/ option (#212)

* New --plugins-dir=plugins/ option

New option causing Datasette to load and evaluate all of the Python files in
the specified directory and register any plugins that are defined in those
files.

This new option is available for the following commands:

    datasette serve mydb.db --plugins-dir=plugins/
    datasette publish now/heroku mydb.db --plugins-dir=plugins/
    datasette package mydb.db --plugins-dir=plugins/

* Unit tests for --plugins-dir=plugins/

Closes #211

tests/fixtures.py

@@ -15,12 +15,16 @@ def app_client():
         conn = sqlite3.connect(filepath)
         conn.executescript(TABLES)
         os.chdir(os.path.dirname(filepath))
+        plugins_dir = os.path.join(tmpdir, 'plugins')
+        os.mkdir(plugins_dir)
+        open(os.path.join(plugins_dir, 'my_plugin.py'), 'w').write(PLUGIN)
         ds = Datasette(
             [filepath],
             page_size=50,
             max_returned_rows=100,
             sql_time_limit_ms=20,
             metadata=METADATA,
+            plugins_dir=plugins_dir,
         )
         ds.sqlite_functions.append(
             ('sleep', 1, lambda n: time.sleep(float(n))),
@@ -90,6 +94,20 @@ METADATA = {
     }
 }
 
+PLUGIN = '''
+from datasette import hookimpl
+import pint
+
+ureg = pint.UnitRegistry()
+
+
+@hookimpl
+def prepare_connection(conn):
+    def convert_units(amount, from_, to_):
+        "select convert_units(100, 'm', 'ft');"
+        return (amount * ureg(from_)).to(to_).to_tuple()[0]
+    conn.create_function('convert_units', 3, convert_units)
+'''
 
 TABLES = '''
 CREATE TABLE simple_primary_key (

tests/test_api.py

@@ -588,8 +588,10 @@ def test_row_foreign_key_tables(app_client):
 
 
 def test_unit_filters(app_client):
-    response = app_client.get('/test_tables/units.json?distance__lt=75km&frequency__gt=1kHz',
-                              gather_request=False)
+    response = app_client.get(
+        '/test_tables/units.json?distance__lt=75km&frequency__gt=1kHz',
+        gather_request=False
+    )
     assert response.status == 200
     data = response.json
 
@@ -598,3 +600,11 @@ def test_unit_filters(app_client):
 
     assert len(data['rows']) == 1
     assert data['rows'][0][0] == 2
+
+
+def test_plugins_dir_plugin(app_client):
+    response = app_client.get(
+        "/test_tables.json?sql=select+convert_units(100%2C+'m'%2C+'ft')",
+        gather_request=False
+    )
+    assert pytest.approx(328.0839) == response.json['rows'][0][0]

tests/test_utils.py

@@ -195,6 +195,7 @@ def test_temporary_docker_directory_uses_hard_link():
             extra_options=None,
             branch=None,
             template_dir=None,
+            plugins_dir=None,
             static=[],
         ) as temp_docker:
             hello = os.path.join(temp_docker, 'hello')
@@ -218,6 +219,7 @@ def test_temporary_docker_directory_uses_copy_if_hard_link_fails(mock_link):
             extra_options=None,
             branch=None,
             template_dir=None,
+            plugins_dir=None,
             static=[],
         ) as temp_docker:
             hello = os.path.join(temp_docker, 'hello')