Porting code from Python 2 to Python 3 can be a daunting task. Tools like Futureize or Modernize can do most of the mechanical work for you, and Pylint can find obvious problems with code that's meant to be 2and3 compatible. You should absolutely be using these tools as they identify the lion's share of compatibility problems. Thanks to this work, it's really never been easier to port a large codebase to Python 3.
Even with these tools, however, porting code in a way that ensures identical behavior in Python 2 and Python 3 is tough. Python is a highly dynamic language and there is a huge breadth of changes between Python 2 and Python 3. Also, while we'd all love to work in code bases with 100% unit test coverage, the reality is unfortunately often very different. Given this, it's hard if not impossible for a static analysis tool t