[commands] Add Command.signature

This replaces HelpFormatter.get_command_signature for the most part.

discord/ext/commands/core.py

@@ -544,6 +544,45 @@ class Command:
             return self.help.split('\n', 1)[0]
         return ''
 
+    @property
+    def signature(self):
+        """Returns a POSIX-like signature useful for help command output."""
+        result = []
+        parent = self.full_parent_name
+        if len(self.aliases) > 0:
+            aliases = '|'.join(self.aliases)
+            fmt = '[%s|%s]' % (self.name, aliases)
+            if parent:
+                fmt = parent + fmt
+            result.append(fmt)
+        else:
+            name = self.name if not parent else parent + ' ' + self.name
+            result.append(name)
+
+        if self.usage:
+            result.append(self.usage)
+            return ' '.join(result)
+
+        params = self.clean_params
+        if not params:
+            return ' '.join(result)
+
+        for name, param in params.items():
+            if param.default is not param.empty:
+                # We don't want None or '' to trigger the [name=value] case and instead it should
+                # do [name] since [name=None] or [name=] are not exactly useful for the user.
+                should_print = param.default if isinstance(param.default, str) else param.default is not None
+                if should_print:
+                    result.append('[%s=%s]' % (name, param.default))
+                else:
+                    result.append('[%s]' % name)
+            elif param.kind == param.VAR_POSITIONAL:
+                result.append('[%s...]' % name)
+            else:
+                result.append('<%s>' % name)
+
+        return ' '.join(result)
+
     @asyncio.coroutine
     def can_run(self, ctx):
         """|coro|