diff --git a/libp2p/network/network_interface.py b/libp2p/network/network_interface.py
index f34a752..1a84c0e 100644
--- a/libp2p/network/network_interface.py
+++ b/libp2p/network/network_interface.py
@@ -67,7 +67,7 @@ class INetwork(ABC):
         """
 
     @abstractmethod
-    async def listen(self, *args: Multiaddr) -> bool:
+    async def listen(self, *args: Sequence[Multiaddr]) -> bool:
         """
         :param *args: one or many multiaddrs to start listening on
         :return: True if at least one success
diff --git a/libp2p/network/swarm.py b/libp2p/network/swarm.py
index bedba40..a259fce 100644
--- a/libp2p/network/swarm.py
+++ b/libp2p/network/swarm.py
@@ -18,11 +18,11 @@ from libp2p.protocol_muxer.multiselect import Multiselect
 from libp2p.protocol_muxer.multiselect_client import MultiselectClient
 from libp2p.routing.interfaces import IPeerRouting
 from libp2p.stream_muxer.muxed_connection_interface import IMuxedConn
+from libp2p.stream_muxer.muxed_stream_interface import IMuxedStream
 from libp2p.transport.upgrader import TransportUpgrader
 from libp2p.transport.transport_interface import ITransport
 from libp2p.transport.listener_interface import IListener
 
-from libp2p.stream_muxer.mplex.mplex_stream import MplexStream
 
 from .network_interface import INetwork
 from .notifee_interface import INotifee
@@ -168,7 +168,7 @@ class Swarm(INetwork):
 
         return net_stream
 
-    async def listen(self, *args: Multiaddr) -> bool:
+    async def listen(self, *args: Sequence[Multiaddr]) -> bool:
         """
         :param *args: one or many multiaddrs to start listening on
         :return: true if at least one success
@@ -252,7 +252,7 @@ class Swarm(INetwork):
     # TODO: `disconnect`?
 
 
-GenericProtocolHandlerFn = Callable[[MplexStream], Awaitable[None]]
+GenericProtocolHandlerFn = Callable[[IMuxedStream], Awaitable[None]]
 
 
 def create_generic_protocol_handler(swarm: Swarm) -> GenericProtocolHandlerFn:
@@ -264,7 +264,7 @@ def create_generic_protocol_handler(swarm: Swarm) -> GenericProtocolHandlerFn:
     """
     multiselect = swarm.multiselect
 
-    async def generic_protocol_handler(muxed_stream: MplexStream) -> None:
+    async def generic_protocol_handler(muxed_stream: IMuxedStream) -> None:
         # Perform protocol muxing to determine protocol to use
         protocol, handler = await multiselect.negotiate(muxed_stream)
 
diff --git a/libp2p/peer/id.py b/libp2p/peer/id.py
index 4e82ca2..8561e0e 100644
--- a/libp2p/peer/id.py
+++ b/libp2p/peer/id.py
@@ -20,16 +20,15 @@ MAX_INLINE_KEY_LENGTH = 42
 
 class ID:
 
-    _id_str: str
+    _id_str: bytes
 
-    def __init__(self, id_str: str) -> None:
+    def __init__(self, id_str: bytes) -> None:
         self._id_str = id_str
 
-    # FIXME: Should return type `bytes`
-    def to_bytes(self) -> str:
+    def to_bytes(self) -> bytes:
         return self._id_str
 
-    def get_raw_id(self) -> str:
+    def get_raw_id(self) -> bytes:
         return self._id_str
 
     def pretty(self) -> str:
@@ -86,6 +85,6 @@ def id_from_private_key(key: RsaKey) -> ID:
     return id_from_public_key(key.publickey())
 
 def digest(data: Union[str, bytes]) -> bytes:
-    if not isinstance(data, bytes):
-        data = str(data).encode('utf8')
+    if isinstance(data, str):
+        data = data.encode('utf8')
     return hashlib.sha1(data).digest()
diff --git a/libp2p/peer/peerdata.py b/libp2p/peer/peerdata.py
index bb4962b..68749c8 100644
--- a/libp2p/peer/peerdata.py
+++ b/libp2p/peer/peerdata.py
@@ -31,7 +31,7 @@ class PeerData(IPeerData):
         self.protocols = list(protocols)
 
     def add_addrs(self, addrs: Sequence[Multiaddr]) -> None:
-        self.addrs.extend(list(addrs))
+        self.addrs.extend(addrs)
 
     def get_addrs(self) -> List[Multiaddr]:
         return self.addrs
diff --git a/libp2p/pubsub/gossipsub.py b/libp2p/pubsub/gossipsub.py
index 4f2c88e..f0d903b 100644
--- a/libp2p/pubsub/gossipsub.py
+++ b/libp2p/pubsub/gossipsub.py
@@ -533,6 +533,7 @@ class GossipSub(IPubsubRouter):
         from_id_str = sender_peer_id
 
         # FIXME: Update type of message ID
+        # FIXME: Find a better way to parse the msg ids
         msg_ids: List[Any] = [literal_eval(msg) for msg in iwant_msg.messageIDs]
         msgs_to_forward: List[rpc_pb2.Message] = []
         for msg_id_iwant in msg_ids:
diff --git a/libp2p/pubsub/pubsub.py b/libp2p/pubsub/pubsub.py
index 43fc763..744c37c 100644
--- a/libp2p/pubsub/pubsub.py
+++ b/libp2p/pubsub/pubsub.py
@@ -373,6 +373,6 @@ class Pubsub:
         self.seen_messages[msg_id] = 1
 
     def _is_subscribed_to_msg(self, msg: rpc_pb2.Message) -> bool:
-        if not bool(self.my_topics):
+        if not self.my_topics:
             return False
         return all([topic in self.my_topics for topic in msg.topicIDs])
diff --git a/libp2p/stream_muxer/muxed_stream_interface.py b/libp2p/stream_muxer/muxed_stream_interface.py
index 2b4dac3..eb6f267 100644
--- a/libp2p/stream_muxer/muxed_stream_interface.py
+++ b/libp2p/stream_muxer/muxed_stream_interface.py
@@ -2,17 +2,13 @@ from abc import (
     ABC,
     abstractmethod,
 )
-from typing import (
-    TYPE_CHECKING,
-)
 
-if TYPE_CHECKING:
-    from libp2p.stream_muxer.mplex.mplex import Mplex
+from libp2p.stream_muxer.muxed_connection_interface import IMuxedConn
 
 
 class IMuxedStream(ABC):
 
-    mplex_conn: 'Mplex'
+    mplex_conn: IMuxedConn
 
     @abstractmethod
     def read(self):
diff --git a/tox.ini b/tox.ini
index 4e87b31..ea6af8f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -19,5 +19,6 @@ basepython =
 basepython = python3
 extras = dev
 commands =
+    # TODO: Add the tests/ folder back to pylint
     pylint --rcfile={toxinidir}/.pylintrc libp2p examples
     mypy -p libp2p -p examples --config-file {toxinidir}/mypy.ini