open(..., 'w') ... flush() fails to write data to the file

Repro:

with open('test.txt', 'w') as test:
  test.write('hi')
  test.flush()
  r = open('test.txt', 'r')
  print(r.read(1))
  r.close()

Expected: ‘h’ printed Actual: ‘’ printed

This seems to be a regression from IronPython 2.

Side note: what surprises me is that in both 3 and 2 BufferedWriter.flush() does not call .flush() on the underlying _raw/_rawIO, but somehow it works in IronPython 2. I’d appreciate an explanation. Does IP2 create the underlying stream with buffering off?

[slozier]

My example is a counter-example to the statement that BufferedWriter should call the underlying IO object’s flush. If it did call flush, then the sample above would fail. You can manually flush the raw and see the difference:

import io
b = io.BytesIO()
f = io.BufferedWriter(io.BufferedWriter(b))
f.write(b"a")
f.flush()
assert b.getvalue() == b""
f.raw.flush()
assert b.getvalue() == b"a"

[lostmsu]

@slozier I am not sure what do you mean here. BufferedWriter should be calling the underlying IO object’s flush, so if flush is fixed to behave correctly, this would pass.