Improved variable names/Less code duplication

See the description of my commits. I am curious if this branch can be merged with the master branch, because I am not sure if the branching was executed correctly.

mvc/controller.py

@@ -14,8 +14,8 @@ class ControllerClass:
         self.model = model
 
         # Button bindings. Lambda is to pass argument to select_file
-        self.view.button1.bind("<Button>", lambda x: self.select_file(1))
-        self.view.button2.bind("<Button>", lambda x: self.select_file(2))
+        self.view.button_source.bind("<Button>", lambda x: self.select_file(1))
+        self.view.button_shadow.bind("<Button>", lambda x: self.select_file(2))
         self.view.comparison_button.bind("<Button>",
                                          lambda x: self.check_for_files())
 
@@ -34,17 +34,21 @@ class ControllerClass:
             label_text = "File %d: %s" % (fileno, file_path.title().split("/")[-1])
 
             if fileno == 1:
-                self.model.file_1 = file_path
+                self.model.file_path_source = file_path
                 self.view.tf1_label["text"] = label_text
             else:
-                self.model.file_2 = file_path
+                self.model.file_path_shadow = file_path
                 self.view.tf2_label["text"] = label_text
 
     def check_for_files(self):
-        if self.model.file_1 is None:
+        """Checks whether the two files for analysis have already been
+        selected. Then it either starts the analysis or tells the user to
+        choose files.
+        """
+        if self.model.file_path_source is None:
             self.view.message_label["text"] = "Cannot compare: " \
                                               "No source file selected!"
-        elif self.model.file_2 is None:
+        elif self.model.file_path_shadow is None:
             self.view.message_label["text"] = "Cannot compare: " \
                                               "No shadow file selected!"
         else: