Allow using config.bind(key, None) but deprecate it

Using config.bind(key, None) in a config.py file never was supposed to be
possible. However, it worked by accident until v1.7.0, where it then broke
because of 9ed6aaaa1646fad0710ba28bf12bd8eb9e011a72.

Also, note that :config-py-write also emitted such invalid lines, which was
fixed in v1.8.0 in #4949.

(cherry picked from commit 2033ed1955b642cbcd2bc4ea88d361c340fb3d38)

2 files changed, 25 insertions, 2 deletions

diff --git a/doc/changelog.asciidoc b/doc/changelog.asciidoc
index de6a2f33e..dd183a29c 100644
--- a/doc/changelog.asciidoc
+++ b/doc/changelog.asciidoc
@@ -15,6 +15,19 @@ breaking changes (such as renamed commands) can happen in minor releases.
 // `Fixed` for any bug fixes.
 // `Security` to invite users to upgrade in case of vulnerabilities.
 
+v1.9.0 (unreleased)
+-------------------
+
+Fixed
+~~~~~
+
+- dictcli.py now works correctly on Windows again.
+- Unbinding keys via `config.bind(key, None)` accidentally worked in
+  v1.7.0 but raises an exception in v1.8.0. It now works again, but is
+  deprecated and shows an error. Note that `:config-py-write` did write
+  such invalid lines before v1.8.0, so existing config files might need
+  adjustments.
+
 v1.8.1 (2019-09-27)
 -------------------
 
diff --git a/qutebrowser/config/configfiles.py b/qutebrowser/config/configfiles.py
index e7066fa11..f220d2ca4 100644
--- a/qutebrowser/config/configfiles.py
+++ b/qutebrowser/config/configfiles.py
@@ -442,11 +442,21 @@ class ConfigAPI:
             urlpattern = urlmatch.UrlPattern(pattern) if pattern else None
             self._config.set_obj(name, value, pattern=urlpattern)
 
-    def bind(self, key: str, command: str, mode: str = 'normal') -> None:
+    def bind(self, key: str,
+             command: typing.Optional[str],
+             mode: str = 'normal') -> None:
         """Bind a key to a command, with an optional key mode."""
         with self._handle_error('binding', key):
             seq = keyutils.KeySequence.parse(key)
-            self._keyconfig.bind(seq, command, mode=mode)
+            if command is None:
+                text = ("Unbinding commands with config.bind('{key}', None) "
+                        "is deprecated. Use config.unbind('{key}') instead."
+                        .format(key=key))
+                self.errors.append(configexc.ConfigErrorDesc(
+                    "While unbinding '{}'".format(key), text))
+                self._keyconfig.unbind(seq, mode=mode)
+            else:
+                self._keyconfig.bind(seq, command, mode=mode)
 
     def unbind(self, key: str, mode: str = 'normal') -> None:
         """Unbind a key from a command, with an optional key mode."""