also, in addition to @bowlofred's excellent advise - don't use globals. Make GUI using OOP approach (i.e. with class)
If you can't explain it to a six year old, you don't understand it yourself, Albert Einstein
How to Ask Questions The Smart Way: link and another link
Create MCV example
Debug small programs
How to Ask Questions The Smart Way: link and another link
Create MCV example
Debug small programs