Resolve differences between seek_until docstring and implementation

CHANGES.rst

@@ -57,6 +57,8 @@
 - Add argument to ``asdftool diff`` that ignores tree nodes that match
   a JMESPath expression. [#956]
 
+- Fix behavior of ``exception`` argument to ``GenericFile.seek_until``. [#980]
+
 2.7.4 (unreleased)
 ------------------
 

asdf/asdf.py

@@ -872,7 +872,7 @@ def _open_asdf(cls, self, fd, uri=None, mode='r',
             # now, but we don't do anything special with it until
             # after the blocks have been read
             tree = yamlutil.load_tree(reader)
-            has_blocks = fd.seek_until(constants.BLOCK_MAGIC, 4, include=True)
+            has_blocks = fd.seek_until(constants.BLOCK_MAGIC, 4, include=True, exception=False)
         elif yaml_token == constants.BLOCK_MAGIC:
             has_blocks = True
         elif yaml_token != b'':

asdf/exceptions.py

@@ -12,3 +12,9 @@ class AsdfConversionWarning(AsdfWarning):
     """
     Warning class used for failures to convert data into custom types.
     """
+
+class DelimiterNotFoundError(ValueError):
+    """
+    Indicates that a delimiter was not found when reading or
+    seeking through a file.
+    """

asdf/generic_io.py

@@ -21,6 +21,7 @@
 import numpy as np
 
 from . import util
+from .exceptions import DelimiterNotFoundError
 from .extern import atomicfile
 from .util import patched_urllib_parse
 
@@ -142,7 +143,7 @@ def read(self, nbytes=None):
 
         if content == b'':
             if self._exception:
-                raise ValueError("{0} not found".format(self._delimiter_name))
+                raise DelimiterNotFoundError("{0} not found".format(self._delimiter_name))
             self._past_end = True
             return content
 
@@ -156,7 +157,7 @@ def read(self, nbytes=None):
             self._past_end = True
         elif nbytes is None and self._exception:
             # Read the whole file and didn't find the delimiter
-            raise ValueError("{0} not found".format(self._delimiter_name))
+            raise DelimiterNotFoundError("{0} not found".format(self._delimiter_name))
         else:
             if nbytes:
                 content = content[:nbytes - len(self._initial_content)]
@@ -442,7 +443,7 @@ def read_until(self, delimiter, readahead_bytes, delimiter_name=None,
 
         Raises
         ------
-        ValueError :
+        DelimiterNotFoundError :
             If the delimiter is not found before the end of the file.
         """
         buff = io.BytesIO()
@@ -491,7 +492,7 @@ def reader_until(self, delimiter, readahead_bytes,
 
         Raises
         ------
-        ValueError :
+        DelimiterNotFoundError :
             If the delimiter is not found before the end of the file.
         """
         raise NotImplementedError()
@@ -530,27 +531,28 @@ def seek_until(self, delimiter, readahead_bytes, delimiter_name=None,
 
         Returns
         -------
-        content : bytes
-            The content from the current position in the file, up to
-            the delimiter.  Includes the delimiter if `include` is
-            ``True``.
+        bool
+            ``True`` if the delimiter was found.
 
         Raises
         ------
-        ValueError :
-            If the delimiter is not found before the end of the file.
+        DelimiterNotFoundError :
+            If ``exception`` is enabled and the delimiter is not found
+            before the end of the file.
         """
         reader = self.reader_until(
             delimiter, readahead_bytes, delimiter_name=delimiter_name,
             include=include, initial_content=initial_content,
-            exception=exception)
-        while True:
-            try:
-                content = reader.read(self.block_size)
-            except ValueError:
+            exception=True)
+        try:
+            while reader.read(self.block_size) != b'':
+                pass
+            return True
+        except DelimiterNotFoundError as e:
+            if exception:
+                raise e
+            else:
                 return False
-            if content == b'':
-                return True
 
     def fast_forward(self, size):
         """

asdf/tests/test_generic_io.py

@@ -9,6 +9,7 @@
 import numpy as np
 
 import asdf
+from asdf import exceptions
 from asdf import util
 from asdf import generic_io
 from asdf.asdf import is_asdf_file
@@ -550,14 +551,14 @@ def test_truncated_reader():
 
     # Simple cases where the delimiter is not found at all
     tr = generic_io._TruncatedReader(fd, b'x', 1)
-    with pytest.raises(ValueError):
+    with pytest.raises(exceptions.DelimiterNotFoundError):
         tr.read()
 
     fd.seek(0)
     tr = generic_io._TruncatedReader(fd, b'x', 1)
     assert tr.read(100) == content[:100]
     assert tr.read(1) == content[100:]
-    with pytest.raises(ValueError):
+    with pytest.raises(exceptions.DelimiterNotFoundError):
         tr.read()
 
     fd.seek(0)