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一、设置标准输出标准错误输出的模式

pytest捕获标准输出标准错误输出的模式主要有以下几种

pytest ：和pytest —capture=fd 模式是一样的，默认的就是pytest —capture=fd 模式
pytest -s ：打开实时输出，关闭Capture Log输出，
pytest —capture=sys ：打开实时输出，Captrue Log只捕获sys.out，sys.err
pytest —capture=fd ：关闭实时输出，Captrue Log捕获所有的标准输出
pytest —capture=tee-sys ：pytest -s 和 pytest —capture=sys的结合，即打开实时输出，同时Captrue Log只捕获sys.out，sys.err

test_demo.py代码如下：

import sys
import os
def test_demo():
    print("in test_demo...")
    os.system("dir")
    sys.stdout.write("hello\n")
    assert 1==2

(1)使用pytest命令执行，此时关闭实时输出，Capture Log捕获所有的标准输出

如下，可以看到Capture Log既有sys.out的输出，也有执行系统命令的输出(执行系统命令的输出不是sys.out，而是文件描述符1)

$ pytest
============================= test session starts =============================
platform win32 -- Python 3.9.7, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: D:\src\blog\tests, configfile: pytest.ini
plugins: allure-pytest-2.9.43, caterpillar-pytest-0.0.2, hypothesis-6.31.6, forked-1.3.0, rerunfailures-10.1, xdist-2.3.0
collected 1 item
test_demo.py F                                                           [100%]
================================== FAILURES ===================================
__________________________________ test_demo __________________________________
    def test_demo():
        print("in test_demo...")
        os.system("dir")
        sys.stdout.write("hello\n")
>       assert 1==2
E       assert 1 == 2
test_demo.py:8: AssertionError
---------------------------- Captured stdout call -----------------------------
in test_demo...
 Volume in drive D is work
 Volume Serial Number is 84B0-F6B7
 Directory of D:\src\blog\tests
2021/12/27  09:38    <DIR>          .
2021/12/27  09:38    <DIR>          ..
2021/12/24  17:03    <DIR>          .pytest_cache
2021/12/27  09:32                16 1
2021/12/23  15:31               168 conftest.py
2021/12/22  17:13               300 pytest.ini
2021/12/16  09:21             1,797 red_blue_ball_ticket.py
2021/12/27  09:38               143 test_demo.py
2021/12/24  15:38                 0 __init__.py
2021/12/27  09:38    <DIR>          __pycache__
               6 File(s)          2,424 bytes
               4 Dir(s)  165,265,436,672 bytes free
hello
=========================== short test summary info ===========================
FAILED test_demo.py::test_demo - assert 1 == 2
============================== 1 failed in 0.06s ==============================

(2)使用pytest -s 命令，此时打开实时输出，关闭Capture Log输出

如下，只有实时输出，没有Captured stdout call内容

$ pytest -s
============================= test session starts =============================
platform win32 -- Python 3.9.7, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: D:\src\blog\tests, configfile: pytest.ini
plugins: allure-pytest-2.9.43, caterpillar-pytest-0.0.2, hypothesis-6.31.6, forked-1.3.0, rerunfailures-10.1, xdist-2.3.0
collected 1 item
test_demo.py  Volume in drive D is work
 Volume Serial Number is 84B0-F6B7
 Directory of D:\src\blog\tests
2021/12/27  09:38    <DIR>          .
2021/12/27  09:38    <DIR>          ..
2021/12/24  17:03    <DIR>          .pytest_cache
2021/12/27  09:32                16 1
2021/12/23  15:31               168 conftest.py
2021/12/22  17:13               300 pytest.ini
2021/12/16  09:21             1,797 red_blue_ball_ticket.py
2021/12/27  09:38               143 test_demo.py
2021/12/24  15:38                 0 __init__.py
2021/12/27  09:38    <DIR>          __pycache__
               6 File(s)          2,424 bytes
               4 Dir(s)  165,265,436,672 bytes free
in test_demo...
hello
F
================================== FAILURES ===================================
__________________________________ test_demo __________________________________
    def test_demo():
        print("in test_demo...")
        os.system("dir")
        sys.stdout.write("hello\n")
>       assert 1==2
E       assert 1 == 2
test_demo.py:8: AssertionError
=========================== short test summary info ===========================
FAILED test_demo.py::test_demo - assert 1 == 2
============================== 1 failed in 0.06s ==============================

(3)使用pytest —capture=sys 命令时打开实时输出，Capture Log只输出sys.out的内容

如下，Capture Log中只有sys.out的内容，没有执行系统命令的输出

$ pytest --capture=sys
============================= test session starts =============================
platform win32 -- Python 3.9.7, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: D:\src\blog\tests, configfile: pytest.ini
plugins: allure-pytest-2.9.43, caterpillar-pytest-0.0.2, hypothesis-6.31.6, forked-1.3.0, rerunfailures-10.1, xdist-2.3.0
collected 1 item
test_demo.py  Volume in drive D is work
 Volume Serial Number is 84B0-F6B7
 Directory of D:\src\blog\tests
2021/12/27  09:38    <DIR>          .
2021/12/27  09:38    <DIR>          ..
2021/12/24  17:03    <DIR>          .pytest_cache
2021/12/27  09:32                16 1
2021/12/23  15:31               168 conftest.py
2021/12/22  17:13               300 pytest.ini
2021/12/16  09:21             1,797 red_blue_ball_ticket.py
2021/12/27  09:38               143 test_demo.py
2021/12/24  15:38                 0 __init__.py
2021/12/27  09:38    <DIR>          __pycache__
               6 File(s)          2,424 bytes
               4 Dir(s)  165,265,432,576 bytes free
F                                                           [100%]
================================== FAILURES ===================================
__________________________________ test_demo __________________________________
    def test_demo():
        print("in test_demo...")
        os.system("dir")
        sys.stdout.write("hello\n")
>       assert 1==2
E       assert 1 == 2
test_demo.py:8: AssertionError
---------------------------- Captured stdout call -----------------------------
in test_demo...
hello
=========================== short test summary info ===========================
FAILED test_demo.py::test_demo - assert 1 == 2
============================== 1 failed in 0.06s ==============================

(4)使用 pytest —capture=fd 命令关闭实时输出，Captrue Log捕获所有的标准输出

$ pytest --capture=fd
============================= test session starts =============================
platform win32 -- Python 3.9.7, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: D:\src\blog\tests, configfile: pytest.ini
plugins: allure-pytest-2.9.43, caterpillar-pytest-0.0.2, hypothesis-6.31.6, forked-1.3.0, rerunfailures-10.1, xdist-2.3.0
collected 1 item
test_demo.py F                                                           [100%]
================================== FAILURES ===================================
__________________________________ test_demo __________________________________
    def test_demo():
        print("in test_demo...")
        os.system("dir")
        sys.stdout.write("hello\n")
>       assert 1==2
E       assert 1 == 2
test_demo.py:8: AssertionError
---------------------------- Captured stdout call -----------------------------
in test_demo...
 Volume in drive D is work
 Volume Serial Number is 84B0-F6B7
 Directory of D:\src\blog\tests
2021/12/27  09:38    <DIR>          .
2021/12/27  09:38    <DIR>          ..
2021/12/24  17:03    <DIR>          .pytest_cache
2021/12/27  09:32                16 1
2021/12/23  15:31               168 conftest.py
2021/12/22  17:13               300 pytest.ini
2021/12/16  09:21             1,797 red_blue_ball_ticket.py
2021/12/27  09:38               143 test_demo.py
2021/12/24  15:38                 0 __init__.py
2021/12/27  09:38    <DIR>          __pycache__
               6 File(s)          2,424 bytes
               4 Dir(s)  165,265,432,576 bytes free
hello
=========================== short test summary info ===========================
FAILED test_demo.py::test_demo - assert 1 == 2
============================== 1 failed in 0.06s ==============================

(5)使用pytest —capture=tee-sys命令，相当于pytest -s 和 pytest —capture=sys的结合，即打开实时输出，同时Captrue Log只捕获sys.out，sys.err

$ pytest --capture=tee-sys
============================= test session starts =============================
platform win32 -- Python 3.9.7, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: D:\src\blog\tests, configfile: pytest.ini
plugins: allure-pytest-2.9.43, caterpillar-pytest-0.0.2, hypothesis-6.31.6, forked-1.3.0, rerunfailures-10.1, xdist-2.3.0
collected 1 item
test_demo.py  Volume in drive D is work
 Volume Serial Number is 84B0-F6B7
 Directory of D:\src\blog\tests
2021/12/27  09:38    <DIR>          .
2021/12/27  09:38    <DIR>          ..
2021/12/24  17:03    <DIR>          .pytest_cache
2021/12/27  09:32                16 1
2021/12/23  15:31               168 conftest.py
2021/12/22  17:13               300 pytest.ini
2021/12/16  09:21             1,797 red_blue_ball_ticket.py
2021/12/27  09:38               143 test_demo.py
2021/12/24  15:38                 0 __init__.py
2021/12/27  09:38    <DIR>          __pycache__
               6 File(s)          2,424 bytes
               4 Dir(s)  165,265,428,480 bytes free
in test_demo...
hello
F                                                           [100%]
================================== FAILURES ===================================
__________________________________ test_demo __________________________________
    def test_demo():
        print("in test_demo...")
        os.system("dir")
        sys.stdout.write("hello\n")
>       assert 1==2
E       assert 1 == 2
test_demo.py:8: AssertionError
---------------------------- Captured stdout call -----------------------------
in test_demo...
hello
=========================== short test summary info ===========================
FAILED test_demo.py::test_demo - assert 1 == 2
============================== 1 failed in 0.06s ==============================

二、在测试函数中捕获标准输出标砖错误输出

用于捕获标准输出有如下四个fixture：capsys, capsysbinary, capfd,和capfdbinary，capsys用于获取sys.out中的内容，当sys.out中的内容为二进制数据时使用capsysbinary，同样，capfd用于获取系统级标准输出，即文件描述符1和2，当数据为二进制时使用和capfdbinary

test_demo.py代码如下：

def test_demo(capsys):
    print("hello world")
    captured = capsys.readouterr()
    assert "hello world hahaha" in captured.out

执行结果如下,可以发现这里能将上面打印语句的内容捕获到，即”hello world\n”，这里断言报错主要是通过报错信息查看真是额返回值

$ pytest
============================= test session starts =============================
platform win32 -- Python 3.9.7, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: D:\src\blog\tests, configfile: pytest.ini
plugins: allure-pytest-2.9.43, caterpillar-pytest-0.0.2, hypothesis-6.31.6, forked-1.3.0, rerunfailures-10.1, xdist-2.3.0
collected 1 item
test_demo.py F                                                           [100%]
================================== FAILURES ===================================
__________________________________ test_demo __________________________________
capsys = <_pytest.capture.CaptureFixture object at 0x00000226B8916D00>
    def test_demo(capsys):
        print("hello world")
        captured = capsys.readouterr()
>       assert "hello worldd" in captured.out
E       AssertionError: assert 'hello worldd' in 'hello world\n'
E        +  where 'hello world\n' = CaptureResult(out='hello world\n', err='').out
test_demo.py:5: AssertionError
=========================== short test summary info ===========================
FAILED test_demo.py::test_demo - AssertionError: assert 'hello worldd' in 'he...
============================== 1 failed in 0.05s ==============================

还可以设置对指定的内容关闭capture捕获，如下

test_demo.py代码如下，设置当打印hello python的时候关闭捕获

def test_demo(capsys):
    print("hello world")
    with capsys.disabled():
        print("hello python")
    print("hello C++")
    captured = capsys.readouterr()
    assert "hello world" in captured.out
    assert "hello C++" in captured.out
    assert "hello python" in captured.out

执行结果如下，可以发现，这里确实只捕获了hello world 和hello C++的打印

$ pytest
============================= test session starts =============================
platform win32 -- Python 3.9.7, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: D:\src\blog\tests, configfile: pytest.ini
plugins: allure-pytest-2.9.43, caterpillar-pytest-0.0.2, hypothesis-6.31.6, forked-1.3.0, rerunfailures-10.1, xdist-2.3.0
collected 1 item
test_demo.py hello python
F                                                           [100%]
================================== FAILURES ===================================
__________________________________ test_demo __________________________________
capsys = <_pytest.capture.CaptureFixture object at 0x00000294FFB444C0>
    def test_demo(capsys):
        print("hello world")
        with capsys.disabled():
            print("hello python")
        print("hello C++")
        captured = capsys.readouterr()
        assert "hello world" in captured.out
        assert "hello C++" in captured.out
>       assert "hello python" in captured.out
E       AssertionError: assert 'hello python' in 'hello world\nhello C++\n'
E        +  where 'hello world\nhello C++\n' = CaptureResult(out='hello world\nhello C++\n', err='').out
test_demo.py:10: AssertionError
=========================== short test summary info ===========================
FAILED test_demo.py::test_demo - AssertionError: assert 'hello python' in 'he...
============================== 1 failed in 0.06s ==============================