如何模拟打开文件并在路径不存在时引发？

Question

我在测试读取文件第一行的函数时遇到问题，并在文件路径不存在时引发

Exception

。

当前代码：

from unittest.mock import patch, mock_open
from pytest import raises
from os.path import exists

def read_from_file(file_path):
    if not exists(file_path):
        raise Exception("File does not exists!")
    with open(file_path, "r") as f:
        return f.read().splitlines()[0]

@patch("builtins.open", new_callable=mock_open, read_data="Correct string\nWrong string\nWrong string")
@patch("os.path.exists", return_value=True)
def test_read_file_and_returns_the_correct_string_with_multiple_lines(mock_os, mock_file):
    result = read_from_file("xyz")
    mock_file.assert_called_once_with("xyz", "r")
    assert result == "Correct string"

@patch("builtins.open", new_callable=mock_open, read_data="Correct string")
@patch("os.path.exists", return_value=False)
def test_throws_exception_when_file_doesnt_exist(mock_os, mock_file):
    with raises(Exception):
        read_from_file("xyz")

装饰器

@patch("os.path.exists", return_value=True)

和

@patch("os.path.exists", return_value=False)

在这两个测试中似乎都没有效果。

如何模拟文件的存在？

Answer 1

您面临的问题与

@patch

装饰器的顺序有关。装饰器是从最内层到最外层应用的。因此，在您的测试中，

mock_file

参数对应于

builtins.open

，而

mock_os

参数对应于

os.path.exists

。

当您模拟多个对象时，模拟的对象将按照补丁装饰器出现的顺序传递给测试函数。在您的情况下，您应该切换函数签名中模拟参数的顺序以匹配装饰器。

from unittest.mock import patch, mock_open
from pytest import raises
from os.path import exists

def read_from_file(file_path):
    if not exists(file_path):
        raise Exception("File does not exists!")
    with open(file_path, "r") as f:
        return f.read().splitlines()[0]

@patch("os.path.exists", return_value=True)
@patch("builtins.open", new_callable=mock_open, read_data="Correct string\nWrong string\nWrong string")
def test_read_file_and_returns_the_correct_string_with_multiple_lines(mock_file, mock_os):
    result = read_from_file("xyz")
    mock_file.assert_called_once_with("xyz", "r")
    assert result == "Correct string"

@patch("os.path.exists", return_value=False)
@patch("builtins.open", new_callable=mock_open, read_data="Correct string")
def test_throws_exception_when_file_doesnt_exist(mock_file, mock_os):
    with raises(Exception):
        read_from_file("xyz")