capsys fails to capture the output by stdout

[hellock]

Environment:

• OS: macOS 10.12.3
• Python version: 2.7.13/3.6.0
• pytest version: 3.0.7

I find that the outputs by sys.stdout can be captured, but those by stdout will not.

Example:

# a.py
import sys

def func1():
    sys.stdout.write('a')
    sys.stdout.flush()

# b.py
from sys import stdout

def func2():
    stdout.write('a')
    stdout.flush()

# test.py
def test_func1(capsys):
    func1()
    out, err = capsys.readouterr()
    assert out == 'a'  # ok

def test_func2(capsys):
    func2()
    out, err = capsys.readouterr()
    assert out == 'a'  # AssertionError, out is '' other than 'a'

[RonnyPfannschmidt]

this is expected python behavior, alias by import wont be corrected - we cant search all objects in the active python process for stdout just to be sure