Prohibit ending SubTextIO past length of base io

src/multicsv/exceptions.py

@@ -25,7 +25,7 @@ class BaseMustBeReadable(SubTextIOErrror, ValueError):
     pass
 
 
-class StartsBeyondBaseContent(SubTextIOErrror, ValueError):
+class EndsBeyondBaseContent(SubTextIOErrror, ValueError):
     pass
 
 

src/multicsv/subtextio.py

@@ -4,7 +4,7 @@
 from .exceptions import OpOnClosedError, \
     InvalidWhenceError, InvalidSubtextCoordinates, \
     BaseMustBeReadable, BaseMustBeSeakable, \
-    StartsBeyondBaseContent, BaseIOClosed
+    EndsBeyondBaseContent, BaseIOClosed
 
 
 class SubTextIO(TextIO):
@@ -142,18 +142,16 @@ def _load(self) -> None:
             self._base_io.seek(0, os.SEEK_END)
             base_last_position = self._base_io.tell()
 
-            if self.start > base_last_position:
-                raise StartsBeyondBaseContent(
-                    "Start position is greater than base TextIO length.")
+            if self.end > base_last_position:
+                raise EndsBeyondBaseContent(
+                    "End position is greater than base TextIO length.")
 
             if self.end > self.start:
                 self._base_io.seek(self.end)
                 base_final_position = self._base_io.tell()
                 self.is_at_end = base_final_position == base_last_position
-
-                if self.end <= base_last_position:
-                    self._base_io.seek(self.start)
-                    self._buffer = self._base_io.read(self.end - self.start)
+                self._base_io.seek(self.start)
+                self._buffer = self._base_io.read(self.end - self.start)
             else:
                 base_final_position = self.start
                 self._base_io.seek(0, os.SEEK_END)

tests/test_subtextio.py

@@ -4,7 +4,7 @@
 import pytest
 import os
 from multicsv.subtextio import SubTextIO
-from multicsv.exceptions import OpOnClosedError, InvalidWhenceError, InvalidSubtextCoordinates, StartsBeyondBaseContent
+from multicsv.exceptions import OpOnClosedError, InvalidWhenceError, InvalidSubtextCoordinates, EndsBeyondBaseContent
 
 
 @pytest.fixture
@@ -394,7 +394,10 @@ def test_mode(mode):
     import tempfile
 
     with tempfile.NamedTemporaryFile(mode=mode) as tmp:
-        if tmp.readable():
+        if tmp.writable():
+            tmp.truncate(30)
+
+        if tmp.readable() and tmp.writable():
             sub_text = SubTextIO(tmp, start=0, end=21)
             assert sub_text.mode == mode
 
@@ -407,8 +410,8 @@ def test_invalid_range(base_textio):
         SubTextIO(base_textio, start=15, end=10)
 
 def test_invalid_range_past_initial(base_textio):
-    with pytest.raises(StartsBeyondBaseContent):
-        SubTextIO(base_textio, start=30, end=40)
+    with pytest.raises(EndsBeyondBaseContent):
+        SubTextIO(base_textio, start=5, end=40)
 
 def test_no_readable_requirement():
     import tempfile