Show error for Python 2 users (#441)

* Show error for Python 2 users

The error can be disabled with a config option.

See #219

CHANGELOG.rst

@@ -15,6 +15,7 @@ Version 0.11.0
 - Discovery is no longer automatically done when running ``vdirsyncer sync``.
   ``vdirsyncer discover`` now has to be explicitly called.
 - Add a ``.plist`` example for Mac OS X.
+- Usage under Python 2 now requires a special config parameter to be set.
 
 Version 0.10.0
 ==============

tests/conftest.py

@@ -19,7 +19,7 @@ def setup_logging():
 
 @pytest.fixture(autouse=True)
 def suppress_py2_warning(monkeypatch):
-    monkeypatch.setattr('vdirsyncer.cli._check_python2', lambda: None)
+    monkeypatch.setattr('vdirsyncer.cli._check_python2', lambda _: None)
 
 
 try:

vdirsyncer/cli/__init__.py

@@ -8,7 +8,7 @@ import click
 
 import click_log
 
-from .. import PROJECT_HOME, __version__
+from .. import PROJECT_HOME, __version__, exceptions
 from ..utils.compat import PY2
 
 
@@ -38,12 +38,27 @@ def catch_errors(f):
     return inner
 
 
-def _check_python2():
-    if PY2:
-        cli_logger.warning('Python 2 support will be dropped. Please switch '
-                           'to at least Python 3.3 as soon as possible. See '
-                           '{home}/issues/219 for more information.'
-                           .format(home=PROJECT_HOME))
+def _check_python2(config):
+    # XXX: Py2
+    if not PY2:
+        return
+
+    msg = (
+        'Python 2 support will be dropped. Please switch '
+        'to at least Python 3.3 as soon as possible. See '
+        '{home}/issues/219 for more information.'
+        .format(home=PROJECT_HOME)
+    )
+
+    if not config.general.get('python2', False):
+        raise exceptions.UserError(
+            msg + (
+                '\nSet python2 = true in the [general] section to get rid of '
+                'this error for now.'
+            )
+        )
+    else:
+        cli_logger.warning(msg)
 
 
 @click.group()
@@ -57,11 +72,11 @@ def app(ctx, config):
     '''
     vdirsyncer -- synchronize calendars and contacts
     '''
-    _check_python2()
     from .config import load_config
 
     if not ctx.config:
         ctx.config = load_config(config)
+    _check_python2(ctx.config)
 
 main = app
 

vdirsyncer/cli/config.py

@@ -14,7 +14,7 @@ try:
 except ImportError:
     from configparser import RawConfigParser
 
-GENERAL_ALL = frozenset(['status_path'])
+GENERAL_ALL = frozenset(['status_path', 'python2'])  # XXX: Py2
 GENERAL_REQUIRED = frozenset(['status_path'])
 SECTION_NAME_CHARS = frozenset(chain(string.ascii_letters, string.digits, '_'))