[Python] Remove mutable default argument (#4665)

samples/client/petstore/python/petstore_api/configuration.py

@@ -300,41 +300,37 @@ class Configuration(object):
             }
         ]
 
-    def get_host_from_settings(self, index, variables={}):
+    def get_host_from_settings(self, index, variables=None):
         """Gets host URL based on the index and variables
         :param index: array index of the host settings
         :param variables: hash of variable and the corresponding value
         :return: URL based on host settings
         """
-
+        variables = {} if variables is None else variables
         servers = self.get_host_settings()
 
-        # check array index out of bound
-        if index < 0 or index >= len(servers):
+        try:
+            server = servers[index]
+        except IndexError:
             raise ValueError(
-                "Invalid index {} when selecting the host settings. Must be less than {}"  # noqa: E501
-                .format(index, len(servers)))
+                "Invalid index {0} when selecting the host settings. "
+                "Must be less than {1}".format(index, len(servers)))
 
-        server = servers[index]
         url = server['url']
 
-        # go through variable and assign a value
-        for variable_name in server['variables']:
-            if variable_name in variables:
-                if variables[variable_name] in server['variables'][
-                        variable_name]['enum_values']:
-                    url = url.replace("{" + variable_name + "}",
-                                      variables[variable_name])
-                else:
-                    raise ValueError(
-                        "The variable `{}` in the host URL has invalid value {}. Must be {}."  # noqa: E501
-                        .format(
-                            variable_name, variables[variable_name],
-                            server['variables'][variable_name]['enum_values']))
-            else:
-                # use default value
-                url = url.replace(
-                    "{" + variable_name + "}",
-                    server['variables'][variable_name]['default_value'])
+        # go through variables and replace placeholders
+        for variable_name, variable in server['variables'].items():
+            used_value = variables.get(
+                variable_name, variable['default_value'])
+
+            if 'enum_values' in variable \
+                    and used_value not in variable['enum_values']:
+                raise ValueError(
+                    "The variable `{0}` in the host URL has invalid value "
+                    "{1}. Must be {2}.".format(
+                        variable_name, variables[variable_name],
+                        variable['enum_values']))
+
+            url = url.replace("{" + variable_name + "}", used_value)
 
         return url