The source code for the filedialog module can be found within the Tkinter source code in a separate file named filedialog.py. You are encouraged to take a look at its implementation.

If you are feeling adventurous and want to further explore the Text Editor program, I encourage you to have a look at the source code for Python's built-in editor named IDLE, which is written in Tkinter. The source code for IDLE can be found in your local Python library directory in a folder called idlelib. On Linux Mint, this is located at /usr/lib/python3.4/idlelib.

Read the official Python styling guide, which is specified in the PEP8 documentation at https://www.python.org/dev/peps/pep-0008.

If you like, try to implement syntax highlighting of Python code in the text editor. A naive implementation would first involve defining a list of keywords. Then we can bind the <KeyRelease> event to check whether the typed word is one of the keywords. We can then add a custom tag to the word using tag_add. Finally, we can change its color by using code such as textarea.tag_config("the_keyword_tag", foreground="blue").

A slightly advanced idea to read up on and implement is called lazy loading.  This is particularly helpful if you want to open a very large file in the text editor. In the present implementation, it may take very long time to open a very large file. In contrast, lazy loading will read only the section of the file that is currently visible in the text editor, thus making the program much more responsive.