Selenium源码分析之chromedriver的设置

ide:pycharm；
language:python；
selenium:3.141.0

启动pycharm，并创建名为seleniumHQ的工程，创建一个py文件，输入如下代码：

if __name__ == '__main__':
    from selenium.webdriver.chrome.webdriver import WebDriver
    path="/Users/apple/Seleniumdriver/chromedriver"
    driver = WebDriver(executable_path=path)
    driver.get("https://www.baidu.com")
    driver.close()
    driver.quit()

进入WebDriver类源码

class WebDriver(RemoteWebDriver):
    """
    Controls the ChromeDriver and allows you to drive the browser.

    You will need to download the ChromeDriver executable from
    http://chromedriver.storage.googleapis.com/index.html
    """

    def __init__(self, executable_path="chromedriver", port=0,
                 options=None, service_args=None,
                 desired_capabilities=None, service_log_path=None,
                 chrome_options=None, keep_alive=True):
        """
        Creates a new instance of the chrome driver.

        Starts the service and then creates new instance of chrome driver.

        :Args:
         - executable_path - path to the executable. If the default is used it assumes the executable is in the $PATH
         - port - port you would like the service to run, if left as 0, a free port will be found.
         - options - this takes an instance of ChromeOptions
         - service_args - List of args to pass to the driver service
         - desired_capabilities - Dictionary object with non-browser specific
           capabilities only, such as "proxy" or "loggingPref".
         - service_log_path - Where to log information from the driver.
         - chrome_options - Deprecated argument for options
         - keep_alive - Whether to configure ChromeRemoteConnection to use HTTP keep-alive.
        """
        if chrome_options:
            warnings.warn('use options instead of chrome_options',
                          DeprecationWarning, stacklevel=2)
            options = chrome_options

        if options is None:
            # desired_capabilities stays as passed in
            if desired_capabilities is None:
                desired_capabilities = self.create_options().to_capabilities()
        else:
            if desired_capabilities is None:
                desired_capabilities = options.to_capabilities()
            else:
                desired_capabilities.update(options.to_capabilities())

        self.service = Service(
            executable_path,
            port=port,
            service_args=service_args,
            log_path=service_log_path)
        self.service.start()

        try:
            RemoteWebDriver.__init__(
                self,
                command_executor=ChromeRemoteConnection(
                    remote_server_addr=self.service.service_url,
                    keep_alive=keep_alive),
                desired_capabilities=desired_capabilities)
        except Exception:
            self.quit()
            raise
        self._is_remote = False

其构造函数入参executable_path默认为“chromedriver”，在其init方法中，创建了一个Service类实例service，executable_path作为了Service类init方法的入参。executable_path在WebDriver源码中仅仅出现在该处，说明WebDriver在处理executable_path的过程中，仅仅只起到传递值的作用。实际处理executable_path并不在WebDriver。
进入Service类源码，

class Service(service.Service):
    """
    Object that manages the starting and stopping of the ChromeDriver
    """

    def __init__(self, executable_path, port=0, service_args=None,
                 log_path=None, env=None):
        """
        Creates a new instance of the Service

        :Args:
         - executable_path : Path to the ChromeDriver
         - port : Port the service is running on
         - service_args : List of args to pass to the chromedriver service
         - log_path : Path for the chromedriver service to log to"""

        self.service_args = service_args or []
        if log_path:
            self.service_args.append('--log-path=%s' % log_path)

        service.Service.__init__(self, executable_path, port=port, env=env,
                                 start_error_message="Please see https://sites.google.com/a/chromium.org/chromedriver/home")

其init方法中将executable_path传递给了其父类的init方法，整个Service类init方法中仅将executable_path传递给了父类的init方法，未做其他处理。
进入Service的父类Service源码，

class Service(object):

    def __init__(self, executable, port=0, log_file=DEVNULL, env=None, start_error_message=""):
        self.path = executable

        self.port = port
        if self.port == 0:
            self.port = utils.free_port()

        if not _HAS_NATIVE_DEVNULL and log_file == DEVNULL:
            log_file = open(os.devnull, 'wb')

        self.start_error_message = start_error_message
        self.log_file = log_file
        self.env = env or os.environ

其init方法仅仅只是将executable存储下来，存在了self.path中，到此，executable_path的传递过程就结束了，但是我们仍未知道它被用在什么地方。

既然executable_path被存储在了self.path中，path是Service的一个实例属性，那么它只可能被Service类的方法所使用，回顾先前的代码，WebDriver类创建了Service类的实例service，紧接着就调用了service.start()，进入start方法，

    def start(self):
        """
        Starts the Service.

        :Exceptions:
         - WebDriverException : Raised either when it can't start the service
           or when it can't connect to the service
        """
        try:
            cmd = [self.path]
            cmd.extend(self.command_line_args())
            self.process = subprocess.Popen(cmd, env=self.env,
                                            close_fds=platform.system() != 'Windows',
                                            stdout=self.log_file,
                                            stderr=self.log_file,
                                            stdin=PIPE)
        except TypeError:
            raise
        except OSError as err:
            if err.errno == errno.ENOENT:
                raise WebDriverException(
                    "'%s' executable needs to be in PATH. %s" % (
                        os.path.basename(self.path), self.start_error_message)
                )
            elif err.errno == errno.EACCES:
                raise WebDriverException(
                    "'%s' executable may have wrong permissions. %s" % (
                        os.path.basename(self.path), self.start_error_message)
                )
            else:
                raise
        except Exception as e:
            raise WebDriverException(
                "The executable %s needs to be available in the path. %s\n%s" %
                (os.path.basename(self.path), self.start_error_message, str(e)))
        count = 0
        while True:
            self.assert_process_still_running()
            if self.is_connectable():
                break
            count += 1
            time.sleep(1)
            if count == 30:
                raise WebDriverException("Can not connect to the Service %s" % self.path)

在start方法中，主要都是try...except的实现。
其中将self.path放在了list类实例cmd中，并将cmd作为了参数传给subprocess.Popen函数。而subprocess.Popen函数即相当于在命令行执行了cmd命令。

所以为selenium的WebDriver init方法配置executable_path可以有两种方法：
一：将chromedriver的路径加入到环境变量Path中，而不用给WebDriver的构造方法传递任何参数，因为已经有了默认参数“chromedriver”；
二：不必配置环境变量，直接给WebDriver的init方法传递chromedriver的绝对地址，即WebDriver(executable_path=path)；