Title: BlockingIOError.characters_written represents number of bytes not characters
Type: behavior Stage:
Components: IO Versions:
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: msakai, pitrou, terry.reedy
Priority: normal Keywords:

Created on 2020-02-25 02:47 by msakai, last changed 2020-02-29 00:32 by terry.reedy.

Messages (2)
msg362611 - (view) Author: Masahiro Sakai (msakai) Date: 2020-02-25 02:47
According to , 'characters_written' is "An integer containing the number of characters written to the stream before it blocked". But I observed that it represents number of *bytes* not *characters* in the following program.

import os
import threading
import time

r, w = os.pipe()
os.set_blocking(w, False)
f_r = os.fdopen(r, mode="rb")
f_w = os.fdopen(w, mode="w", encoding="utf-8")

msg = "\u03b1\u03b2\u03b3\u3042\u3044\u3046\u3048\u304a" * (1024 * 16)
    print(msg, file=f_w, flush=True)
except BlockingIOError as e:
    print(f"BlockingIOError.characters_written == {e.characters_written}")
    written = e.characters_written

def close():
    os.set_blocking(w, True)

b =

print(f"{written} characters correspond to {len(msg[:written].encode('utf-8'))} bytes in UTF-8")
print(f"{len(b)} bytes read")

BlockingIOError.characters_written == 81920
81920 characters correspond to 215040 bytes in UTF-8
81920 bytes read

I think it is confusing behavior.
If this is intended behavior, then it should be documented as such and I think 'bytes_written' is more appropriate name.
msg362949 - (view) Author: Terry J. Reedy (terry.reedy) * (Python committer) Date: 2020-02-29 00:32
Antoine, Although the text may have preceded your OSError reorganization, you were the last to touch this entry.  Is is correct, or does it need change?

Revision: f55011f8b63d3b046c1ec580312bc52ca47d721b
Author: Antoine Pitrou <>
Date: 10/12/2011 12:57:23 PM
Update doc for BlockingIOError and its alias in the io module
Date User Action Args
2020-02-29 00:32:25terry.reedysetnosy: + terry.reedy, pitrou
messages: + msg362949
2020-02-25 04:33:02msakaisettype: behavior
2020-02-25 02:47:59msakaicreate