Add more validation for buffers

This MR adds validation for buffers given to the TSFrame class, making sure that there cannot be both zero-length buffers and non-zero-length buffers in the same frame.

src/sgnts/base/buffer.py

@@ -585,7 +585,7 @@ class TSFrame(Frame):
     def __post_init__(self):
         super().__post_init__()
         assert len(self.buffers) > 0
-        self.__sanity_check(self.buffers)
+        self._validate_buffers(self.buffers)
         self.is_gap = all([b.is_gap for b in self.buffers])
 
     def __getitem__(self, item):
@@ -604,7 +604,17 @@ class TSFrame(Frame):
     def __len__(self):
         return len(self.buffers)
 
-    def __sanity_check(self, bufs: list[SeriesBuffer]) -> None:
+    def _validate_buffers(self, bufs: list[SeriesBuffer]) -> None:
+        """Validate the buffers attribute.
+
+        Args:
+            bufs:
+                list[SeriesBuffer], the buffers to validate
+        """
+        self._validate_buffers_not_mixed_type(bufs)
+        self._validate_buffers_continuous(bufs)
+
+    def _validate_buffers_continuous(self, bufs: list[SeriesBuffer]) -> None:
         """Sanity check that the buffers don't overlap nor have discontinuities.
 
         Args:
@@ -616,9 +626,19 @@ class TSFrame(Frame):
             slices = [buf.slice for buf in bufs]
             off0 = slices[0].stop
             for sl in slices[1:]:
-                assert off0 == sl.start
+                if not off0 == sl.start:
+                    raise ValueError(f"Discontinuity between {off0} and {sl.start}")
                 off0 = sl.stop
 
+    def _validate_buffers_not_mixed_type(self, bufs: list[SeriesBuffer]) -> None:
+        """Helper method for validating the buffers attribute. A set of buffers
+        is valid if it does not contain both zero-length and non-zero-length buffers.
+        """
+        has_zero_length = any([len(buf) == 0 for buf in bufs])
+        has_non_zero_length = any([len(buf) > 0 for buf in bufs])
+        if (has_zero_length and has_non_zero_length):
+            raise ValueError("Cannot mix zero-length and non-zero-length buffers")
+
     def set_buffers(self, bufs: list[SeriesBuffer]) -> None:
         """Set the buffers attribute to the bufs provided.