Question

I need to loop until I hit the end of a file-like object, but I’m not finding an “obvious way to do it”, which makes me suspect I’m overlooking something, well, obvious. 🙂

I have a stream (in this case, it’s a StringIO object, but I’m curious about the general case as well) which stores an unknown number of records in “<length><data>” format, e.g.:

data = StringIO("\x07\x00\x00\x00foobar\x00\x04\x00\x00\x00baz\x00")

Now, the only clear way I can imagine to read this is using (what I think of as) an initialized loop, which seems a little un-Pythonic:

len_name = data.read(4)

while len_name != "":
    len_name = struct.unpack("<I", len_name)[0]
    names.append(data.read(len_name))

    len_name = data.read(4)

In a C-like language, I’d just stick the read(4) in the while‘s test clause, but of course that won’t work for Python. Any thoughts on a better way to accomplish this?

Answer 1

You can combine iteration through iter() with a sentinel:

for block in iter(lambda: file_obj.read(4), ""):
  use(block)