core: parse RPL_ISUPPORT into bot.server._isupport

Parses one or more `RPL_ISUPPORT` messages from the IRC server for
parameters found in: https://modern.ircdocs.horse/.

While the documentation above does not explicitly list the parameters
mentioned in sopel-irc#1082, each parameter listed there would be parsed into
`bot.server._isupport`.

sopel/bot.py

@@ -62,10 +62,13 @@ class _ServerInfo(object):  # Eventually this can become a Python 3 data class
         set.
 
     hostname: A string containing 'servername' from RPL_MYINFO
+
+    _isupport: A dict containing parsed values from RPL_ISUPPORT.
     """
     def __init__(self):
         self.capabilities = {}
-        self.hostname = None  # This attribute should be set only by the bot (not modules)
+        self.hostname = None
+        self._isupport = collections.defaultdict(lambda: None)  # Will be falsy when parameter not in RPL_ISUPPORT
 
 
 class Sopel(irc.Bot):

sopel/coretasks.py

@@ -176,6 +176,101 @@ def parse_reply_myinfo(bot, trigger):
     bot.server.hostname = trigger.args[1]
 
 
+@sopel.module.rule('.*')
+@sopel.module.event(events.RPL_ISUPPORT)
+@sopel.module.priority('high')
+@sopel.module.thread(False)
+@sopel.module.unblockable
+def parse_reply_isupport(bot, trigger):
+    """Parse RPL_ISUPPORT.
+
+    005 RPL_ISUPPORT "<client> <1-13 tokens> :are supported by this server"
+    """
+    DEFAULT_VALUES = {
+        'EXCEPTS': 'e',
+        'INVEX': 'I',
+    }
+
+    CAST_TO = {
+        'AWAYLEN': int,
+        'CHANNELLEN': int,
+        'HOSTLEN': int,
+        'KICKLEN': int,
+        'MODES': int,
+        'NICKLEN': int,
+        'TOPICLEN': int,
+        'USERLEN': int,
+    }
+
+    def parse_value(key, value):
+        """Parse the value in parameter[=value] from RPL_ISUPPORT.
+
+        Args:
+            key (string): used to determine parsed value type
+            value (string): format <value_type>:[limit] OR <value>
+
+        Returns:
+            object: a tuple of (value_type, limit (int)) if value argument has
+                ':', a string of parsed value otherwise. If limit is not
+                provided after ":", None is returned to indicate no limit.
+        """
+        if ':' in value:
+            parts = value.split(':')
+            return (parts[0], int(parts[1]) if parts[1] else None)
+
+        return value if key not in CAST_TO else CAST_TO[key](value)
+
+    def handle_value(key, value, is_single=True):
+        """Properly store the value in the `bot.server._isupport` dict.
+
+        Args:
+            key (string): the ISUPPORT parameter name
+            value (string): the raw parameter value string
+            is_single (bool): is the parameter value a single entry, or not
+                (is a list of items)?
+        """
+        # Single parameter value
+        parsed_value = parse_value(key, value)
+        if type(parsed_value) is tuple:
+            # A value_type:limit value
+            if key not in bot.server._isupport:
+                # Not using defaultdict here; don't assume no limit;
+                # explicitly check for limit, if needed.
+                bot.server._isupport[key] = {}
+            bot.server._isupport[key][parsed_value[0]] = parsed_value[1]
+            return
+
+        # Else, simple value
+        if not is_single:
+            # A value with multiple entries (e.g., CHANMODES)
+            if key not in bot.server._isupport:
+                bot.server._isupport[key] = []
+            bot.server._isupport[key].append(parsed_value)
+            return
+
+        # A value with a single entry (e.g., NETWORK)
+        bot.server._isupport[key] = parsed_value
+        return
+
+    # Last arg is the 'are supported by this server' string; first arg is the client.
+    parameters = trigger.args[1:-1]
+    for param in parameters:
+        if '=' not in param:
+            # Simple flag (e.g,. SAFELIST)
+            val = True if param not in DEFAULT_VALUES else DEFAULT_VALUES[param]
+            bot.server._isupport[param] = val  # Default values should be correct type
+            continue
+
+        key, raw_value = param.split('=')
+        if ',' not in raw_value:
+            handle_value(key, raw_value)
+            continue
+
+        values = raw_value.split(',')
+        for val in values:
+            handle_value(key, val, is_single=False)
+
+
 @sopel.module.rule('(.*)')
 @sopel.module.event(events.RPL_NAMREPLY)
 @sopel.module.priority('high')