code: def execute(self):
      Run Checkstyle on all found non-synthetic source files."""
      python_tgts = self.context.targets(
        lambda tgt: isinstance(tgt, (PythonTarget))
      )
      if not python_tgts:
        return 0
      interpreter_cache = PythonInterpreterCache.global_instance()
      with self.invalidated(self.get_targets(self._is_checked)) as invalidation_check:
        failure_count = 0
        tgts_by_compatibility, _ = interpreter_cache.partition_targets_by_compatibility(
          [vt.target for vt in invalidation_check.invalid_vts]
        )
        for filters, targets in tgts_by_compatibility.items():
          sources = self.calculate_sources([tgt for tgt in targets])
          if sources:
            allowed_interpreters = set(interpreter_cache.setup(filters=filters))
            if not allowed_interpreters:
              raise TaskError('No valid interpreters found for targets: {}\n(filters: {})'
                              .format(targets, filters))
            interpreter = min(allowed_interpreters)
            failure_count += self.checkstyle(interpreter, sources)
        if failure_count > 0 and self.get_options().fail:
          raise TaskError('{} Python Style issues found. You may try `./pants fmt <targets>`'
                          .format(failure_count))
        return failure_count
docs: Run Checkstyle on all found non-synthetic source files.