Python pytest：捕获标准输出总是失败

我试图捕获pytest（Python版本3.6）中stdout/print语句的输出

这总是失败的：

message = 'The meaning of life is not actually 42\n'

def print_greeting():
    """print 42 to stdout"""

    # write to stdout
    sys.stdout.write(message)           # this fails

    # print to stdout
    print('Message again: ', message)   # this fails too


def test_printgreeting(capsys):
    """assert '42' was printed to stdout"""

    # capture stdout / stderr
    out, err = capsys.readouterr()
    print_greeting()

    # 42 should be in stdout from sys.stdout.write
    assert '42' in out

测试结果：

========================================================= test session starts ==========================================================

collected 1 item

test.py
The meaning of life is not actually 42
F

=============================================================== FAILURES ===============================================================
__________________________________________________________ test_printgreeting __________________________________________________________
test.py:42: in test_printgreeting
    assert '42' in out
E   AssertionError: assert '42' in ''
======================================================= 1 failed in 0.03 seconds =======================================================

---

为什么不捕获此信息？

您需要在打印以下内容后调用

readouterr

：

def test_printgreeting(capsys):
    """assert '42' was printed to stdout"""

    # capture stdout / stderr
    print_greeting()
    out, err = capsys.readouterr()

    # 42 should be in stdout from sys.stdout.write
    assert '42' in out

---

打印后需要调用

readouterr

：

def test_printgreeting(capsys):
    """assert '42' was printed to stdout"""

    # capture stdout / stderr
    print_greeting()
    out, err = capsys.readouterr()

    # 42 should be in stdout from sys.stdout.write
    assert '42' in out

---

天哪。我用头撞了三个小时，非常感谢，我的上帝。我用头撞了三个小时，非常感谢。