Fix outstanding Mypy errors

- Fix wrong import path for napalm.base.exceptions.CommitError
- Re-write convert method to properly utilize typing

napalm/base/helpers.py

@@ -302,16 +302,36 @@ def find_txt(
     return str(value)
 
 
-def convert(to: Callable[[T], R], who: Optional[T], default: R = "") -> R:
+def convert(to: Callable[[T], R], who: Optional[T], default: Optional[R] = None) -> R:
     """
     Converts data to a specific datatype.
     In case of error, will return a default value.
 
     :param to:      datatype to be casted to.
     :param who:     value to cast.
-    :param default: value to return in case of error.
-    :return: a str value.
+    :param default: default value to return in case of an error with the conversion function.
+    :return:        the result of the cast or a default value.
     """
+    if default is None:
+        # Mypy is currently unable to resolve the Optional[R] correctly, therefore the following
+        # assignments to 'default' need a 'type: ignore' statement.
+        # Ref: https://github.com/python/mypy/issues/8708
+        if to in [str, ip, mac]:
+            default = ""  # type: ignore
+        elif to in [float, int]:
+            default = 0  # type: ignore
+        elif to == bool:
+            default = False  # type: ignore
+        elif to == list:
+            default = []  # type: ignore
+        else:
+            raise ValueError(
+                f"Can't convert with callable {to} - no default is defined for this type."
+            )
+
+    # This is safe because the None-case if handled above
+    assert default is not None
+
     if who is None:
         return default
     try:

napalm/base/mock.py

@@ -198,7 +198,9 @@ def commit_config(self, message: str = "", revert_in: Optional[int] = None) -> N
         self._raise_if_closed()
         if revert_in is not None:
             if self.has_pending_commit():
-                raise napalm.CommitError("Pending commit confirm already in process!")
+                raise napalm.base.exceptions.CommitError(
+                    "Pending commit confirm already in process!"
+                )
             else:
                 self._pending_commits = True
         self.merge = None

napalm/nxos/nxos.py

@@ -862,7 +862,7 @@ def _compute_timestamp(stupid_cisco_output: str) -> float:
             for key in things_keys:
                 if key in part:
                     things[key]["count"] = napalm.base.helpers.convert(
-                        int, part.replace(key, ""), 0
+                        int, part.replace(key, "")
                     )
 
         delta = sum([det.get("count", 0) * det["weight"] for det in things.values()])
@@ -1025,7 +1025,7 @@ def get_interfaces(self) -> Dict[str, models.InterfaceDict]:
                 "speed": interface_speed,
                 "mtu": interface_mtu,
                 "mac_address": napalm.base.helpers.convert(
-                    napalm.base.helpers.mac, mac_address, ""
+                    napalm.base.helpers.mac, mac_address
                 ),
             }
         return interfaces
@@ -1161,7 +1161,7 @@ def get_arp_table(self, vrf: str = "") -> List[models.ARPTableDict]:
                 {
                     "interface": interface,
                     "mac": napalm.base.helpers.convert(
-                        napalm.base.helpers.mac, raw_mac, raw_mac
+                        napalm.base.helpers.mac, raw_mac
                     ),
                     "ip": napalm.base.helpers.ip(raw_ip),
                     "age": age_sec,