Fixed issues: grammatical in service_metadata.md and logical in calcu…

…lator.py

docs/metadata_provider/service_metadata.md

@@ -104,7 +104,7 @@ This class represents the service metadata.
 
 #### `__init__`
 
-Initializes a new instance of the class. Initializes the `m` dict with empty of default values.
+Initializes a new instance of the class. Initializes the `m` dict with empty or default values.
 
 ###### returns:
 
@@ -272,7 +272,7 @@ If the asset type is not supported, an exception is raised.
 #### `add_endpoint_to_group`
 
 Checks the endpoint is valid and adds it to the `endpoints` field of the specified payment group in the `m` dict. 
-If the endpoint is not valid of if the group does not exist or if the endpoint is already present, an exception is 
+If the endpoint is not valid or if the group does not exist or if the endpoint is already present, an exception is 
 raised.
 
 ###### args:

examples/calculator.py

@@ -22,11 +22,12 @@ def parse_expression(expression):
     if len(elements) != 3:
         raise Exception(f"Invalid expression '{expression}'. Three items required.")
 
-    a = int(elements[0])
-    b = int(elements[2])
     if elements[1] not in ["+", "-", "*", "/"]:
-        raise Exception(f"Invalid expression '{expression}'. Operation must be '+' or '-' or '*' or '/'")
-    elif not isinstance(a, (float, int)) or not isinstance(b, (float, int)):
+        raise Exception(f"Invalid expression '{expression}'. Operation must be '+' or '-' or '*' or '/'.")
+    try:
+        a = float(elements[0])
+        b = float(elements[2])
+    except ValueError:
         raise Exception(f"Invalid expression '{expression}'. Operands must be integers or floating point numbers.")
     op = elements[1]