bytes not being printed as expected

[Skaperen]

i ran into an issue printing a byte string to a file object opened in binary mode and minimized it to this little 4 line script that runs into the same problem:

Output:
lt2a/forums /home/forums 1> cat -n write_bytes.py
     1	#!/usr/bin/env python3
     2	from sys import stdout
     3	foo = open(stdout.fileno(),'wb')
     4	print(b'abcdef xyz foobar',file=foo)
lt2a/forums /home/forums 2> cat write_bytes.py
#!/usr/bin/env python3
from sys import stdout
foo = open(stdout.fileno(),'wb')
print(b'abcdef xyz foobar',file=foo)
lt2a/forums /home/forums 3> python3 write_bytes.py
Traceback (most recent call last):
  File "write_bytes.py", line 4, in <module>
    print(b'abcdef xyz foobar',file=foo)
TypeError: a bytes-like object is required, not 'str'
lt2a/forums /home/forums 4>

can someone explain what is going wrong here?

the source of the issue is a program that is reading from a pipe created by subprocess.Popen() which gets bytes because it opened the pipe in binary. rather than decoding those bytes for every line i was copying, i just re-opened stdout in binary, as this example shows. but it seems to be confused when i print bytes. is print() decoding them back to strings?

[stranac]

Right, you can't use print with binary files. This is mentioned in the docs:

Since printed arguments are converted to text strings, print() cannot be used with binary mode file objects. For these, use file.write(...) instead.

So just use sys.stdout.buffer.write() and you'll be fine.
I never really understood the point of using print() with a file anyway...

[Skaperen]

i was just trying to copy, to stdout, what a child process printed to its stdout, piped to the parent. ok, so read() and write().