argparse: type=FileType("wb"), default=sys.stdout returns _io.TextIOWrapper instead of _io.BufferedWriter

[Chaz6]

Bug report

Bug description:

When setting the default of an argument to sys.stdout and the type is argparse.FileType("wb") then the class should be _io.BufferedWriter not _io.TextIOWrapper when no arguments are provided. When the argument is set to - then the class is correctly set to _io.BufferedWriter as expected.

import argparse
import sys

parser = argparse.ArgumentParser()
parser.add_argument("outfile", nargs="?", type=argparse.FileType("wb"), default=sys.stdout)

args = parser.parse_args("-")
print(type(args.outfile))

# <class '_io.BufferedWriter'>
# Correct

args = parser.parse_args()
print(type(args.outfile))

# <class '_io.TextIOWrapper'>
# Incorrect

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[F18-Ray]

Bug report

Bug description:

When setting the default of an argument to sys.stdout and the type is argparse.FileType("wb") then the class should be _io.BufferedWriter not _io.TextIOWrapper when no arguments are provided. When the argument is set to - then the class is correctly set to _io.BufferedWriter as expected.

import argparse
import sys

parser = argparse.ArgumentParser()
parser.add_argument("outfile", nargs="?", type=argparse.FileType("wb"), default=sys.stdout)

args = parser.parse_args("-")
print(type(args.outfile))

<class '_io.BufferedWriter'>

Correct

args = parser.parse_args()
print(type(args.outfile))

<class '_io.TextIOWrapper'>

Incorrect

CPython versions tested on:

3.14

Operating systems tested on:

Linux

But the type of sys.stdout is actually _io.TextIOWrapper if you don't give it any args, so it's nothing to do with python.

[serhiy-storchaka]

Just use default=sys.stdout.buffer in your code.

argparse does nothing with the default value. It just takes it as given. If you specify default=sys.stdout, it sets the default to sys.stdout. If you specify default=42, it sets the default to 42. If this is not correct, the error is on your side.