Extract year and group validation to function

app.py

@@ -27,19 +27,8 @@ def hello():
     year = request.args.get('year', '')
     group = request.args.get('group', '')
 
-    try:
-        int(year)
-        if year not in ALLOWED_YEARS:
-            raise ValueError('year not allowed')
-    except ValueError:
-        year = YEAR
-
-    try:
-        int(group)
-        if group not in ALLOWED_GROUPS:
-            raise ValueError('group not allowed')
-    except ValueError:
-        group = GROUP
+    year = validate(year, ALLOWED_YEARS, YEAR)
+    group = validate(group, ALLOWED_GROUPS, GROUP)
 
     url += "&promo={year}&groupe={group}".format(year=year, group=group)
     cal_str = urllib.request.urlopen(url).read()
@@ -93,5 +82,27 @@ def should_add(event):
         course_code not in excluded_courses \
         and course_type not in excluded_types
 
+
+def validate(item, validation_list, default):
+    """
+    Validates an item from user input against a list of possible values.
+
+    :param item: The item to validate.
+    :type item: object
+    :param validation_list: The list of possible values.
+    :type item: list(object)
+    :param default: The default value to fallback on if item is not in list.
+    :type default: object
+    :return: The item if it is valid (in the list), or default
+    :rtype: object
+    """
+    try:
+        int(item)
+        if item not in validation_list:
+            raise ValueError(item.__name__ + ' not allowed')
+    except ValueError:
+        return default
+    return item
+
 if __name__ == "__main__":
     app.run('0.0.0.0', PORT)