messages: clarify bytes repr errors

Resolves python#9236

mypy/checkstrformat.py

@@ -384,9 +384,10 @@ def perform_special_format_checks(self, spec: ConversionSpecifier, call: CallExp
  if self.chk.options.python_version >= (3, 0):
  if (has_type_component(actual_type, 'builtins.bytes') and
  not custom_special_method(actual_type, '__str__')):
- self.msg.fail("On Python 3 '{}'.format(b'abc') produces \"b'abc'\";"
- " use !r if this is a desired behavior", call,
- code=codes.STR_BYTES_PY3)
+ self.msg.fail(
+ "On Python 3 '{}'.format(b'abc') produces \"b'abc'\", not 'abc'; "
+ "use '{!r}'.format(b'abc') if this is desired behavior",
+ call, code=codes.STR_BYTES_PY3)
  if spec.flags:
  numeric_types = UnionType([self.named_type('builtins.int'),
  self.named_type('builtins.float')])
@@ -843,9 +844,10 @@ def check_s_special_cases(self, expr: FormatStringExpr, typ: Type, context: Cont
  # Couple special cases for string formatting.
  if self.chk.options.python_version >= (3, 0):
  if has_type_component(typ, 'builtins.bytes'):
- self.msg.fail("On Python 3 '%s' % b'abc' produces \"b'abc'\";"
- " use %r if this is a desired behavior", context,
- code=codes.STR_BYTES_PY3)
+ self.msg.fail(
+ "On Python 3 '%s' % b'abc' produces \"b'abc'\", not 'abc'; "
+ "use '%r' % b'abc' if this is desired behavior",
+ context, code=codes.STR_BYTES_PY3)
  if self.chk.options.python_version < (3, 0):
  if has_type_component(typ, 'builtins.unicode'):
  self.unicode_upcast = True