使用unittest测试argparse-退出错误

Question

要去关格雷格·哈斯在这个问题的答案，我试图让一个单元测试来检查当我通过它的一些ARGS不存在在该argparse是给相应的错误choices。但是，unittest使用以下try/except语句会产生误报。

另外，当我仅使用一条with assertRaises语句进行测试时，会argparse强制系统退出，并且程序不再执行任何其他测试。

我希望能够对此进行测试，但是鉴于argparse错误退出，也许这是多余的？

#!/usr/bin/env python3

import argparse
import unittest

class sweep_test_case(unittest.TestCase):
    """Tests that the merParse class works correctly"""

    def setUp(self):
        self.parser=argparse.ArgumentParser()
        self.parser.add_argument(
            "-c", "--color",
            type=str,
            choices=["yellow", "blue"],
            required=True)

    def test_required_unknown_TE(self):
        """Try to perform sweep on something that isn't an option.
        Should return an attribute error if it fails.
        This test incorrectly shows that the test passed, even though that must
        not be true."""
        args = ["--color", "NADA"]
        try:
            self.assertRaises(argparse.ArgumentError, self.parser.parse_args(args))
        except SystemExit:
            print("should give a false positive pass")

    def test_required_unknown(self):
        """Try to perform sweep on something that isn't an option.
        Should return an attribute error if it fails.
        This test incorrectly shows that the test passed, even though that must
        not be true."""
        args = ["--color", "NADA"]
        with self.assertRaises(argparse.ArgumentError):
            self.parser.parse_args(args)

if __name__ == '__main__':
    unittest.main()

错误：

Usage: temp.py [-h] -c {yellow,blue}
temp.py: error: argument -c/--color: invalid choice: 'NADA' (choose from 'yellow', 'blue')
E
usage: temp.py [-h] -c {yellow,blue}
temp.py: error: argument -c/--color: invalid choice: 'NADA' (choose from 'yellow', 'blue')
should give a false positive pass
.
======================================================================
ERROR: test_required_unknown (__main__.sweep_test_case)
Try to perform sweep on something that isn't an option.
----------------------------------------------------------------------
Traceback (most recent call last): #(I deleted some lines)
  File "/Users/darrin/anaconda/lib/python3.5/argparse.py", line 2310, in _check_value
    raise ArgumentError(action, msg % args)
argparse.ArgumentError: argument -c/--color: invalid choice: 'NADA' (choose from 'yellow', 'blue')

During handling of the above exception, another exception occurred:

Traceback (most recent call last): #(I deleted some lines)
  File "/anaconda/lib/python3.5/argparse.py", line 2372, in exit
    _sys.exit(status)
SystemExit: 2

Answer 1

这里的窍门是抓住SystemExit而不是ArgumentError。这是您改写的测试以捕获SystemExit：

#!/usr/bin/env python3

import argparse
import unittest

class SweepTestCase(unittest.TestCase):
    """Tests that the merParse class works correctly"""

    def setUp(self):
        self.parser=argparse.ArgumentParser()
        self.parser.add_argument(
            "-c", "--color",
            type=str,
            choices=["yellow", "blue"],
            required=True)

    def test_required_unknown(self):
        """ Try to perform sweep on something that isn't an option. """
        args = ["--color", "NADA"]
        with self.assertRaises(SystemExit):
            self.parser.parse_args(args)

if __name__ == '__main__':
    unittest.main()

现在可以正常运行，并且测试通过：

$ python scratch.py
usage: scratch.py [-h] -c {yellow,blue}
scratch.py: error: argument -c/--color: invalid choice: 'NADA' (choose from 'yellow', 'blue')
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK

但是，您会看到正在打印用法消息，因此您的测试输出有些混乱。检查使用情况消息是否包含“无效选择”也可能很好。

您可以通过打补丁来做到这一点sys.stderr：

#!/usr/bin/env python3

import argparse
import unittest
from io import StringIO
from unittest.mock import patch


class SweepTestCase(unittest.TestCase):
    """Tests that the merParse class works correctly"""

    def setUp(self):
        self.parser=argparse.ArgumentParser()
        self.parser.add_argument(
            "-c", "--color",
            type=str,
            choices=["yellow", "blue"],
            required=True)

    @patch('sys.stderr', new_callable=StringIO)
    def test_required_unknown(self, mock_stderr):
        """ Try to perform sweep on something that isn't an option. """
        args = ["--color", "NADA"]
        with self.assertRaises(SystemExit):
            self.parser.parse_args(args)
        self.assertRegexpMatches(mock_stderr.getvalue(), r"invalid choice")


if __name__ == '__main__':
    unittest.main()

现在，您仅看到常规测试报告：

$ python scratch.py
.
----------------------------------------------------------------------
Ran 1 test in 0.002s

OK

对于pytest用户，这是不检查消息的等效项。

import argparse

import pytest


def test_required_unknown():
    """ Try to perform sweep on something that isn't an option. """
    parser=argparse.ArgumentParser()
    parser.add_argument(
        "-c", "--color",
        type=str,
        choices=["yellow", "blue"],
        required=True)
    args = ["--color", "NADA"]

    with pytest.raises(SystemExit):
        parser.parse_args(args)

Pytest默认情况下会捕获stdout / stderr，因此它不会污染测试报告。

$ pytest scratch.py
================================== test session starts ===================================
platform linux -- Python 3.6.7, pytest-3.5.0, py-1.7.0, pluggy-0.6.0
rootdir: /home/don/.PyCharm2018.3/config/scratches, inifile:
collected 1 item                                                                         

scratch.py .                                                                       [100%]

================================ 1 passed in 0.01 seconds ================================

您还可以使用pytest检查stdout / stderr内容：

import argparse

import pytest


def test_required_unknown(capsys):
    """ Try to perform sweep on something that isn't an option. """
    parser=argparse.ArgumentParser()
    parser.add_argument(
        "-c", "--color",
        type=str,
        choices=["yellow", "blue"],
        required=True)
    args = ["--color", "NADA"]

    with pytest.raises(SystemExit):
        parser.parse_args(args)

    stderr = capsys.readouterr().err
    assert 'invalid choice' in stderr

和往常一样，我发现pytest更易于使用，但是您可以使它在任何一个中都能工作。

Answer 2

虽然解析器在解析特定参数期间可能会引发 ArgumentError，但它通常会被捕获并传递给parser.error和parse.exit。结果是打印用法以及错误消息，然后sys.exit(2).

所以这asssertRaises不是测试此类错误的好方法argparse。该模块的单元测试文件test/test_argparse.py有一个复杂的方法来解决这个问题，涉及子类化ArgumentParser，重新定义其error方法，以及重定向输出。

parser.parse_known_args（由 调用parse_args）结尾为：

    try:
        namespace, args = self._parse_known_args(args, namespace)
        if hasattr(namespace, _UNRECOGNIZED_ARGS_ATTR):
            args.extend(getattr(namespace, _UNRECOGNIZED_ARGS_ATTR))
            delattr(namespace, _UNRECOGNIZED_ARGS_ATTR)
        return namespace, args
    except ArgumentError:
        err = _sys.exc_info()[1]
        self.error(str(err))

=================

这个测试怎么样（我借用了一些想法test_argparse.py：

import argparse
import unittest

class ErrorRaisingArgumentParser(argparse.ArgumentParser):
    def error(self, message):
        #print(message)
        raise ValueError(message)  # reraise an error

class sweep_test_case(unittest.TestCase):
    """Tests that the Parse class works correctly"""

    def setUp(self):
        self.parser=ErrorRaisingArgumentParser()
        self.parser.add_argument(
            "-c", "--color",
            type=str,
            choices=["yellow", "blue"],
            required=True)

    def test_required_unknown(self):
        """Try to perform sweep on something that isn't an option.
        Should pass"""
        args = ["--color", "NADA"]
        with self.assertRaises(ValueError) as cm:
            self.parser.parse_args(args)
        print('msg:',cm.exception)
        self.assertIn('invalid choice', str(cm.exception))

if __name__ == '__main__':
    unittest.main()

跑步：

1931:~/mypy$ python3 stack39028204.py 
msg: argument -c/--color: invalid choice: 'NADA' (choose from 'yellow', 'blue')
.
----------------------------------------------------------------------
Ran 1 test in 0.002s

OK