Python "with"语句 : __exit__() ignored

Question

考虑到这个小的 Python 类，每当我使用 Ctrl+C 停止脚本时，__exit__ 函数都会在引发异常之前运行:

import time


class MyClass(object):
    def __init__(self):
        pass

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        print('------ cleanup ------')


with MyClass():
    time.sleep(100)

运行:

$ python3 test.py 
^C------ cleanup ------
Traceback (most recent call last):
  File "test.py", line 15, in <module>
    time.sleep(100)
KeyboardInterrupt

在对 Chrome WebDriver 进行子类化的一段类似代码中，为什么我的清理函数被忽略了？

import selenium
from selenium.webdriver.common.by import By


class WebDriver(selenium.webdriver.Chrome):
    def __init__(self, url, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.url = url

    def __enter__(self):
        self.get(self.url)
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        print('------ cleanup ------')


with WebDriver('https://google.com') as driver:
    driver.find_element(By.ID, 'lst-ib').send_keys('Search')

运行:

$ python3 test2.py 
^CTraceback (most recent call last):
  File "test2.py", line 19, in <module>
    with WebDriver('https://google.com') as driver:
  File "test2.py", line 9, in __init__
    super().__init__(*args, **kwargs)
  File "/vagrant/app/lib/python3.5/site-packages/selenium/webdriver/chrome/webdriver.py", line 62, in __init__
    self.service.start()
  File "/vagrant/app/lib/python3.5/site-packages/selenium/webdriver/common/service.py", line 90, in start
    time.sleep(1)
KeyboardInterrupt

用 try: ... finally: 语句强制执行，不过:

try:
    with WebDriver('https://google.com') as driver:
        driver.find_element(By.ID, 'lst-ib').send_keys('Search')
finally:
    print('------ cleanup ------')

运行:

^C------ cleanup ------
Traceback (most recent call last):
  File "test2.py", line 20, in <module>
    with WebDriver('https://google.com') as driver:
  File "test2.py", line 9, in __init__
    super().__init__(*args, **kwargs)
  File "/vagrant/app/lib/python3.5/site-packages/selenium/webdriver/chrome/webdriver.py", line 62, in __init__
    self.service.start()
  File "/vagrant/app/lib/python3.5/site-packages/selenium/webdriver/common/service.py", line 90, in start
    time.sleep(1)
KeyboardInterrupt

Answer 1

请注意，回溯显示您仍在 WebDriver 对象的 __init__() 中 - 换句话说，with 语句尚未执行，Python 仍在评估其参数。我不确定确切的规则，但我很确定如果 __enter__() 尚未被调用，则 __exit__() 将永远不会被调用。