Better error message if file objects aren't seekable

As reported by teor, stem.descriptor's parse_file() function cannot accept stdin... https://trac.torproject.org/projects/tor/ticket/23859 The trouble is that not all file objects in python are seekable. I'd *like* to handle address this transparently by buffering the content... try: descriptor_file.tell() except IOError: # file's not seekable, wrapping in a buffer that is descriptor_file = io.BytesIO(descriptor_file.read()) This works great if our stream has content... % cat my_descriptors | python demo.py *But* hangs indefinitely if no EOF is present in the stream. % python demo.py <= hangs Turns out non-blocking, platform independent reading of streams like stdin is pretty tricky... http://eyalarubas.com/python-subproc-nonblock.html As such simply providing callers with a more descriptive exception. If they know their stream won't block *they* can add the above wrapper to provide us with a seekable file object.

Better error message if file objects aren't seekable
8ec17d80 · Damian Johnson · d42e9e19 · 8ec17d80
Commit 8ec17d80 authored 6 years ago by Damian Johnson
--- a/stem/descriptor/__init__.py
+++ b/stem/descriptor/__init__.py
@@ -93,6 +93,13 @@ __all__ = [
  'Descriptor',
 ]

+UNSEEKABLE_MSG = """\
+File object isn't seekable. Try wrapping it with a BytesIO instead...
+
+  content = my_file.read()
+  parsed_descriptors = stem.descriptor.parse_file(io.BytesIO(content))
+"""
+
 KEYWORD_CHAR = 'a-zA-Z0-9-'
 WHITESPACE = ' \t'
 KEYWORD_LINE = re.compile('^([%s]+)(?:[%s]+(.*))?$' % (KEYWORD_CHAR, WHITESPACE))
@@ -218,6 +225,16 @@ def parse_file(descriptor_file, descriptor_type = None, validate = False, docume

    return

+  # Not all files are seekable. If unseekable then advising the user.
+  #
+  # Python 3.x adds an io.seekable() method, but not an option with python 2.x
+  # so using an experimental call to tell() to determine this.
+
+  try:
+    descriptor_file.tell()
+  except IOError:
+    raise IOError(UNSEEKABLE_MSG)
+
  # The tor descriptor specifications do not provide a reliable method for
  # identifying a descriptor file's type and version so we need to guess
  # based on its filename. Metrics descriptors, however, can be identified