Wrong type interpretation in `path.write`

[ahsimb]

Here is the offending piece of code

The write method is supposed to take data in one of three forms:

• A byte-stream-like object, e.g. returned by open()
• ByteString
• An Iterable of ByteStrings.

The code is faulty in two ways.

1. The test not isinstance(data, IOBase) and isinstance(data, Iterable) does not rule out a stream-like object. For example tempfile.NamedTemporaryFile passes the test.
2. By iterating over the "chunks" in the test the iterator can be exhausted, so the next line - b"".join(tmp_file) - will return an empty ByteString.

The bug is first of all in the interface. The Iterable option was unnecessary. Since it is already out there, a reasonable compromise is changing the type from Iterable to Sequence. Then change the test to

if isinstance(tmp_file, Sequence):
   # optionally
   if not all(isinstance(chunk, ByteString) for chunk in tmp_file):
      raise ValueError(...)
   ...

It should not effect the user code, since an Iterable, if it's actually an Iterable and not a Sequence, wouldn't work anyway.