在 Python 中模拟内置 read() 函数，因此它会抛出异常

Question

我正在尝试测试 class 的错误处理，我需要模拟 read() 引发 MemoryError。这是一个简化的例子。

import mock

def memory_error_read():
    raise MemoryError

def read_file_by_filename(filename):
    try:
        handle = open(filename)
        content = handle.read()
    except MemoryError:
        raise Exception("%s is too big" % self.filename)
    finally:
        handle.close()

    return content

@mock.patch("__builtin__.file.read", memory_error_read)
def testfunc():
    try:
        read_file_by_filename("/etc/passwd")
    except Exception, e:
        return
    print("Should have received exception")

testfunc()

当我运行这样做时，我得到以下回溯。

# ./read_filename.py 
Traceback (most recent call last):
  File "./read_filename.py", line 34, in <module>
    testfunc()
  File "build/bdist.linux-i686/egg/mock.py", line 1214, in patched
  File "build/bdist.linux-i686/egg/mock.py", line 1379, in __exit__
TypeError: can't set attributes of built-in/extension type 'file'

看来我无法修补内置读取功能。有办法欺骗它吗？是否可以如我所愿？

回答

下面是根据 Ben 的建议更新后的代码。

from __future__ import with_statement

...

def test_func():
    with mock.patch("__builtin__.open", new_callable=mock.mock_open) as mo:
        mock_file = mo.return_value
        mock_file.read.side_effect = MemoryError

        try:
            read_file_by_filename("/etc/passwd")
        except Exception, e:
            if "is too big" in str(e):
                return
            else:
                raise

        print("Should have caught an exception")

Answer 1

你看过mock_open了吗？

您应该能够 return 一个模拟对象，它对 read() 有异常引发的副作用。

在 Python 中模拟内置 read() 函数，因此它会抛出异常

Mocking builtin read() function in Python so it throws Exception

python

unit-testing

mocking

python-2.5