Not mandating _filename be created for each Bits.

bitstring/bits.py

@@ -112,7 +112,6 @@ def __init__(self, auto: Optional[Union[BitsType, int]] = None, /, length: Optio
     def __new__(cls: Type[TBits], auto: Optional[Union[BitsType, int]] = None, /, length: Optional[int] = None,
                 offset: Optional[int] = None, pos: Optional[int] = None, **kwargs) -> TBits:
         x = super().__new__(cls)
-        x._filename = ''
         if auto is None and not kwargs:
             # No initialiser so fill with zero bits up to length
             if length is not None:
@@ -242,9 +241,8 @@ def __getitem__(self: TBits, key: Union[slice, int], /) -> Union[TBits, bool]:
         """
         if isinstance(key, numbers.Integral):
             return bool(self._bitstore.getindex(key))
-        x = self._bitstore.getslice_withstep(key)
-        bs = self.__class__()
-        bs._bitstore = x
+        bs = super().__new__(self.__class__)
+        bs._bitstore = self._bitstore.getslice_withstep(key)
         return bs
 
     def __len__(self) -> int:
@@ -282,7 +280,7 @@ def __str__(self) -> str:
 
     def _repr(self, classname: str, length: int, pos: int):
         pos_string = f', pos={pos}' if pos else ''
-        if self._filename:
+        if hasattr(self, '_filename') and self._filename:
             return f"{classname}(filename={self._filename!r}, length={length}{pos_string})"
         else:
             s = self.__str__()

bitstring/bitstore.py

@@ -62,6 +62,8 @@ def setall(self, value: int, /) -> None:
         self._bitarray.setall(value)
 
     def tobytes(self) -> bytes:
+        if self.modified_length is not None:
+            return self._bitarray[:self.modified_length].tobytes()
         return self._bitarray.tobytes()
 
     def slice_to_uint(self, start: Optional[int] = None, end: Optional[int] = None) -> int:

bitstring/bitstream.py

@@ -468,6 +468,15 @@ def fromstring(cls: TBits, s: str, /) -> TBits:
         x._bitstore.immutable = True
         return x
 
+    def __getitem__(self: TBits, key: Union[slice, int], /) -> Union[TBits, bool]:
+        """Return a new bitstring representing a slice of the current bitstring."""
+        if isinstance(key, numbers.Integral):
+            return bool(self._bitstore.getindex(key))
+        bs = super().__new__(self.__class__)
+        bs._bitstore = self._bitstore.getslice_withstep(key)
+        bs._pos = 0
+        return bs
+
     pos = property(_getbitpos, _setbitpos,
                    doc="""The position in the bitstring in bits. Read and write.
                       """)

tests/test_bits.py

@@ -315,6 +315,9 @@ def test_bit_operators(self):
         assert (x & self.c).hex == self.c.hex
         assert self.c ^ self.b[4:20] == Bits(16)
         assert self.a[23:36] | self.c[3:] == self.c[3:]
+        y = x & self.b[4:20]
+        assert y == self.c
+        assert repr(y) == repr(self.c)
 
     def test_addition(self):
         _ = self.d + '0x1'

tests/test_bitstream.py

@@ -1949,8 +1949,6 @@ def test_repr(self):
                   ConstBitStream(filename=filename, length=17),
                   ConstBitStream(filename=filename, length=23, offset=23102)]:
             f2 = eval(f.__repr__())
-            assert f._filename == f2._filename
-
             assert f2.tobytes() == f.tobytes()
         a = BitStream('0b1')
         assert repr(a) == "BitStream('0b1')"