[docs/item_pipeline] process_item returning Deferred docs

* quote url
* use hash of url as filename

docs/topics/item-pipeline.rst

@@ -178,6 +178,8 @@ and Deferred callback fires, it saves item to a file and adds filename to an ite
 ::
 
     import scrapy
+    import hashlib
+    from urllib.parse import quote
 
 
     class ScreenshotPipeline(object):
@@ -187,8 +189,8 @@ and Deferred callback fires, it saves item to a file and adds filename to an ite
         SPLASH_URL = "http://localhost:8050/render.png?url={}"
 
         def process_item(self, item, spider):
-            item_url = item["url"]
-            screenshot_url = self.SPLASH_URL.format(item_url)
+            encoded_item_url = quote(item["url"])
+            screenshot_url = self.SPLASH_URL.format(encoded_item_url)
             request = scrapy.Request(screenshot_url)
             dfd = spider.crawler.engine.download(request, spider)
             dfd.addBoth(self.return_item, item)
@@ -199,8 +201,10 @@ and Deferred callback fires, it saves item to a file and adds filename to an ite
                 # Error happened, return item.
                 return item
 
-            # Save screenshot to file.
-            filename = "item_file_name.png"
+            # Save screenshot to file, filename will be hash of url.
+            url = item["url"]
+            url_hash = hashlib.md5(url.encode("utf8")).hexdigest()
+            filename = "{}.png".format(url_hash)
             with open(filename, "wb") as f:
                 f.write(response.body)
 
@@ -208,7 +212,6 @@ and Deferred callback fires, it saves item to a file and adds filename to an ite
             item["screenshot_filename"] = filename
             return item
 
-
 .. _Splash: http://splash.readthedocs.io/en/stable/
 .. _Deferred: https://twistedmatrix.com/documents/current/core/howto/defer.html