messages: clarify bytes repr errors (#9243)

Resolves #9236 Co-authored-by: hauntsaninja <>
python · Aug 2, 2020 · a9c9a41 · a9c9a41
1 parent e6615c9
commit a9c9a41
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Showing 3 changed files with 17 additions and 15 deletions.
diff --git a/mypy/checkstrformat.py b/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

diff --git a/test-data/unit/check-errorcodes.test b/test-data/unit/check-errorcodes.test
@@ -617,8 +617,8 @@ def g() -> int:
 '%d' % 'no' # E: Incompatible types in string interpolation (expression has type "str", placeholder has type "Union[int, float, SupportsInt]") [str-format]
 '%d + %d' % (1, 2, 3) # E: Not all arguments converted during string formatting [str-format]
 
-'{}'.format(b'abc') # E: On Python 3 '{}'.format(b'abc') produces "b'abc'"; use !r if this is a desired behavior [str-bytes-safe]
-'%s' % b'abc' # E: On Python 3 '%s' % b'abc' produces "b'abc'"; use %r if this is a desired behavior [str-bytes-safe]
+'{}'.format(b'abc') # E: On Python 3 '{}'.format(b'abc') produces "b'abc'", not 'abc'; use '{!r}'.format(b'abc') if this is desired behavior [str-bytes-safe]
+'%s' % b'abc' # E: On Python 3 '%s' % b'abc' produces "b'abc'", not 'abc'; use '%r' % b'abc' if this is desired behavior [str-bytes-safe]
 [builtins fixtures/primitives.pyi]
 [typing fixtures/typing-medium.pyi]
 

diff --git a/test-data/unit/check-expressions.test b/test-data/unit/check-expressions.test
@@ -1183,8 +1183,8 @@ xb: bytes
 xs: str
 
 '%s' % xs # OK
-'%s' % xb # E: On Python 3 '%s' % b'abc' produces "b'abc'"; use %r if this is a desired behavior
-'%(name)s' % {'name': b'value'} # E: On Python 3 '%s' % b'abc' produces "b'abc'"; use %r if this is a desired behavior
+'%s' % xb # E: On Python 3 '%s' % b'abc' produces "b'abc'", not 'abc'; use '%r' % b'abc' if this is desired behavior
+'%(name)s' % {'name': b'value'} # E: On Python 3 '%s' % b'abc' produces "b'abc'", not 'abc'; use '%r' % b'abc' if this is desired behavior
 [builtins fixtures/primitives.pyi]
 
 [case testStringInterpolationSBytesVsStrResultsPy2]
@@ -1576,17 +1576,17 @@ N = NewType('N', bytes)
 n: N
 
 '{}'.format(a)
-'{}'.format(b) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'"; use !r if this is a desired behavior
-'{}'.format(x) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'"; use !r if this is a desired behavior
-'{}'.format(n) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'"; use !r if this is a desired behavior
+'{}'.format(b) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'", not 'abc'; use '{!r}'.format(b'abc') if this is desired behavior
+'{}'.format(x) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'", not 'abc'; use '{!r}'.format(b'abc') if this is desired behavior
+'{}'.format(n) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'", not 'abc'; use '{!r}'.format(b'abc') if this is desired behavior
 
 class C(Generic[B]):
  x: B
  def meth(self) -> None:
- '{}'.format(self.x) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'"; use !r if this is a desired behavior
+ '{}'.format(self.x) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'", not 'abc'; use '{!r}'.format(b'abc') if this is desired behavior
 
 def func(x: A) -> A:
- '{}'.format(x) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'"; use !r if this is a desired behavior
+ '{}'.format(x) # E: On Python 3 '{}'.format(b'abc') produces "b'abc'", not 'abc'; use '{!r}'.format(b'abc') if this is desired behavior
  return x
 
 '{!r}'.format(b)