Calculator with pyQT

Requirements

1. +,-,*,/
2. sign change +,-
3. dot
4. renew
5. go back

Rules

1. start with '0'
2. operator only can be next to numbers
3. 'AC' : renew
4. 'CE' : go back to last change
5. '.'(dot) shows '0.' automatically
6. '.'(dot) only used once

additional

7. '-' shows (-)
8. double '-' means ' '(none)
9. numbers put in directly between '(' and ')'
10. operator adds next to ')'