Re: First two bytes of 'stdout' are lost

On 4/11/2024 8:42 AM, Olivier B. via Python-list wrote:

I am trying to use StringIO to capture stdout, in code that looks like this:
 import sys
from io import StringIO
old_stdout = sys.stdout
sys.stdout = mystdout = StringIO()
print( "patate")
mystdout.seek(0)
sys.stdout = old_stdout
print(mystdout.read())
 Well, it is not exactly like this, since this works properly
 This code is actually run from C++ using the C Python API.
This worked quite well, so the code was right at some point. But now,
two things changed:
  - Now using python 3.11.7 instead of 3.7.12
  - Now using only the python limited C API
 And it seems that now, mystdout.read() always misses the first two
characters that have been written to stdout.
 My first ideas was something related to the BOM improperly truncated
at some point, but i am manipulating UTF-8, so the bom would be 3
bytes, not 2.
 I ruled out wrong C++ code to extract the string from the python
variable, since running a python print of the content of mystdout in
the real stdout also misses the two first characters.
 Hopefully someone has a clue on what would have changed in Python for
this to stop working compared to python 3.7?

I've not used the C API, so just for fun I asked ChatGPT about this and it suggested that a flush after writing to StringIO might do it.  It suggested using a custom class for this purpose:
class MyStringIO(StringIO):
     def write(self, s):
         # Override write method to ensure all characters are written correctly
         super().write(s)
         self.flush()
You would use it like this:
sys.stdout = mystdout = MyStringIO()
I haven't tested it but it seems reasonable, although I would have naively expected to lose bytes from the end, not the beginning.