old_contents: from .base import Base
  from .. import identity_str, identity_bytes, identity_mmap_str, identity_mmap_bytes
  
  class TestIdentityStr(Base):
      serializer = identity_str
      obj = 'abc'
      dumped_obj = 'abc'
  
  class TestIdentityBytes(Base):
      serializer = identity_bytes
      obj = 'abc'.encode('ascii')
      dumped_obj = 'abc'.encode('ascii')
  
  class TestIdentityMmapStr(TestIdentityStr):
      serializer = identity_mmap_str
  
  class TestIdentityMmapBytes(TestIdentityBytes):
      serializer = identity_mmap_bytes
  
  class TestIdentityMmapEmpty(Base):
      serializer = identity_mmap_bytes
      obj = ''.encode('ascii')
      dumped_obj = ''.encode('ascii')
  
new_contents: from .base import Base
  from .. import identity_str, identity_bytes, identity_mmap_str, identity_mmap_bytes
  
  class TestIdentityStr(Base):
      serializer = identity_str
      obj = 'abc'
      dumped_obj = 'abc'
  
  class TestIdentityBytes(Base):
      serializer = identity_bytes
      obj = 'abc'.encode('ascii')
      dumped_obj = 'abc'.encode('ascii')
  
  class TestIdentityMmapStr(Base):
      serializer = identity_mmap_str
      obj = 'abc'
      dumped_obj = 'abc'.encode('ascii')
  
  class TestIdentityMmapBytes(Base):
      serializer = identity_mmap_bytes
      obj = 'abc'.encode('ascii')
      dumped_obj = 'abc'.encode('ascii')
  
  class TestIdentityMmapEmpty(Base):
      serializer = identity_mmap_bytes
      obj = ''.encode('ascii')
      dumped_obj = ''.encode('ascii')
  
subject: fix test case slightly