Fixes several points found by Cammille.

pes_to_spec/config.py

@@ -23,13 +23,15 @@ class VSConfig(object):
                    "SA3_XTD10_PES/DCTRL/V30330S_XENON",
                   ]
 
-    def __init__(self, other=None):
+    def __init__(self, other: object=None):
         self.voltage: Dict[str, float] = dict()
         self.voltage_tol: Dict[str, float] = dict()
         self.pressure: float = np.nan
         self.pressure_tol: float = np.nan
         self.gas: Set[str] = set()
         if other is not None:
+            if not isinstance(other, VSConfig):
+                raise ValueError("The argument given in the constructor must be another class of type VSConfig.")
             self.voltage = deepcopy(other.voltage)
             self.voltage_tol = deepcopy(other.voltage_tol)
             self.pressure = deepcopy(other.pressure)
@@ -92,16 +94,18 @@ class VSConfig(object):
             errors += [f"Multiple gas active during the data: {self.gas}."]
         return errors
 
-    def __sub__(self, other: 'VSConfig') -> 'VSConfig':
+    def __sub__(self, other: object) -> 'VSConfig':
         """
         Take difference between two configuratons.
         """
+        if not isinstance(other, VSConfig):
+            raise ValueError("I can only subtract with another VSConfig object.")
         diff = VSConfig(self)
-        diff.voltage = {k: self.voltage[k] - other.voltage[k]
-                             for k in self.voltage}
+        diff.voltage = {k: v - other.voltage[k]
+                        for k, v in self.voltage.items()}
         diff.pressure = self.pressure - other.pressure
-        diff.voltage_tol = {k: np.sqrt(self.voltage_tol[k]**2 + other.voltage_tol[k]**2)
-                             for k in self.voltage_tol}
+        diff.voltage_tol = {k: np.sqrt(v**2 + other.voltage_tol[k]**2)
+                            for k in self.voltage_tol.items()}
         diff.pressure_tol = np.sqrt(self.pressure_tol**2 + other.pressure_tol**2)
         diff.gas = set(self.gas).difference(set(other.gas))
         return diff
@@ -113,7 +117,7 @@ class VSConfig(object):
         if not isinstance(other, VSConfig):
             raise ValueError("I can only check equality with another VSConfig object.")
         diff = self - other
-        for k, v in diff.voltage.items():
+        for v in diff.voltage.values():
             if np.fabs(v) > 0.1:
                 return False
         if np.fabs(diff.pressure) > diff.pressure_tol: