Feature: minimal addition to run remote webdriver (#55)

* Add remote Selenium option

* Add info about remote driver to readme

* Import webdriver properly

README.md

@@ -25,6 +25,11 @@ Optionally, set the path to the browser executable:
     SELENIUM_BROWSER_EXECUTABLE_PATH = which('firefox')
     ```
 
+In order to use a remote Selenium driver, specify `SELENIUM_COMMAND_EXECUTOR` instead of `SELENIUM_DRIVER_EXECUTABLE_PATH`:
+    ```python
+    SELENIUM_COMMAND_EXECUTOR = 'http://localhost:4444/wd/hub'
+    ```
+
 2. Add the `SeleniumMiddleware` to the downloader middlewares:
     ```python
     DOWNLOADER_MIDDLEWARES = {

scrapy_selenium/middlewares.py

@@ -13,8 +13,8 @@
 class SeleniumMiddleware:
     """Scrapy middleware handling the requests using selenium"""
 
-    def __init__(self, driver_name, driver_executable_path, driver_arguments,
-        browser_executable_path):
+    def __init__(self, driver_name, driver_executable_path,
+        browser_executable_path, command_executor, driver_arguments):
         """Initialize the selenium webdriver
 
         Parameters
@@ -27,6 +27,8 @@ def __init__(self, driver_name, driver_executable_path, driver_arguments,
             A list of arguments to initialize the driver
         browser_executable_path: str
             The path of the executable binary of the browser
+        command_executor: str
+            Selenium remote server endpoint
         """
 
         webdriver_base_path = f'selenium.webdriver.{driver_name}'
@@ -38,6 +40,7 @@ def __init__(self, driver_name, driver_executable_path, driver_arguments,
         driver_options_klass = getattr(driver_options_module, 'Options')
 
         driver_options = driver_options_klass()
+
         if browser_executable_path:
             driver_options.binary_location = browser_executable_path
         for argument in driver_arguments:
@@ -48,7 +51,19 @@ def __init__(self, driver_name, driver_executable_path, driver_arguments,
             f'{driver_name}_options': driver_options
         }
 
-        self.driver = driver_klass(**driver_kwargs)
+        # locally installed driver
+        if driver_executable_path is not None:
+            driver_kwargs = {
+                'executable_path': driver_executable_path,
+                f'{driver_name}_options': driver_options
+            }
+            self.driver = driver_klass(**driver_kwargs)
+        # remote driver
+        elif command_executor is not None:
+            from selenium import webdriver
+            capabilities = driver_options.to_capabilities()
+            self.driver = webdriver.Remote(command_executor=command_executor,
+                                           desired_capabilities=capabilities)
 
     @classmethod
     def from_crawler(cls, crawler):
@@ -57,18 +72,22 @@ def from_crawler(cls, crawler):
         driver_name = crawler.settings.get('SELENIUM_DRIVER_NAME')
         driver_executable_path = crawler.settings.get('SELENIUM_DRIVER_EXECUTABLE_PATH')
         browser_executable_path = crawler.settings.get('SELENIUM_BROWSER_EXECUTABLE_PATH')
+        command_executor = crawler.settings.get('SELENIUM_COMMAND_EXECUTOR')
         driver_arguments = crawler.settings.get('SELENIUM_DRIVER_ARGUMENTS')
 
-        if not driver_name or not driver_executable_path:
-            raise NotConfigured(
-                'SELENIUM_DRIVER_NAME and SELENIUM_DRIVER_EXECUTABLE_PATH must be set'
-            )
+        if driver_name is None:
+            raise NotConfigured('SELENIUM_DRIVER_NAME must be set')
+
+        if driver_executable_path is None and command_executor is None:
+            raise NotConfigured('Either SELENIUM_DRIVER_EXECUTABLE_PATH '
+                                'or SELENIUM_COMMAND_EXECUTOR must be set')
 
         middleware = cls(
             driver_name=driver_name,
             driver_executable_path=driver_executable_path,
-            driver_arguments=driver_arguments,
-            browser_executable_path=browser_executable_path
+            browser_executable_path=browser_executable_path,
+            command_executor=command_executor,
+            driver_arguments=driver_arguments
         )
 
         crawler.signals.connect(middleware.spider_closed, signals.spider_closed)