passive checker of Python 3 programs

 Pyflakes is program to analyze Python programs and detect various
 errors. It works by parsing the source file, not importing it, so it
 is safe to use on modules with side effects. It's also much faster.
 .
 Unlike PyLint, Pyflakes checks only for logical errors in programs;
 it does not perform any checks on style.
 .
 This is the Python 3 command line `pyflakes3` program.