diff --git a/pySim/transport/__init__.py b/pySim/transport/__init__.py
index 1dd8d18..0b50a4f 100644
--- a/pySim/transport/__init__.py
+++ b/pySim/transport/__init__.py
@@ -68,7 +68,7 @@
         self.proactive_handler = proactive_handler
 
     @abc.abstractmethod
-    def __str__(self):
+    def __str__(self) -> str:
         """Implementation specific method for printing an information to identify the device."""
 
     @abc.abstractmethod
diff --git a/pySim/transport/calypso.py b/pySim/transport/calypso.py
index b827d88..d7d9649 100644
--- a/pySim/transport/calypso.py
+++ b/pySim/transport/calypso.py
@@ -160,5 +160,5 @@
 
         return b2h(data), b2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "osmocon:%s" % (self._sock_path)
diff --git a/pySim/transport/modem_atcmd.py b/pySim/transport/modem_atcmd.py
index 58d6f9d..5e5a3dd 100644
--- a/pySim/transport/modem_atcmd.py
+++ b/pySim/transport/modem_atcmd.py
@@ -170,5 +170,5 @@
         log.debug('Command response: %s, %s',  data, sw)
         return data, sw
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "modem:%s" % self._device
diff --git a/pySim/transport/pcsc.py b/pySim/transport/pcsc.py
index 41c4c19..c7b5878 100644
--- a/pySim/transport/pcsc.py
+++ b/pySim/transport/pcsc.py
@@ -93,5 +93,5 @@
         # Return value
         return i2h(data), i2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "PCSC:%u[%s]" % (self._reader_number,  self._reader)
diff --git a/pySim/transport/serial.py b/pySim/transport/serial.py
index f4b1621..e5f7bdb 100644
--- a/pySim/transport/serial.py
+++ b/pySim/transport/serial.py
@@ -237,5 +237,5 @@
         # Return value
         return b2h(data), b2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "serial:%s" % (self._sl.name)