Log overriden Scrapy settings when Scrapy starts.

This is useful for debugging, to quickly find out which settings where
used for a specific spider run. dict settings are omitted for brevity.
DEBUG level is used for consistency with "Enabled
extensions/middlewares" lines (which share a similar purpose).

scrapy/crawler.py

@@ -8,6 +8,7 @@ from scrapy.extension import ExtensionManager
 from scrapy.signalmanager import SignalManager
 from scrapy.utils.ossignal import install_shutdown_handlers, signal_names
 from scrapy.utils.misc import load_object
+from scrapy.settings import overridden_settings
 from scrapy import log, signals
 
 
@@ -33,6 +34,8 @@ class Crawler(object):
         if self.configured:
             return
         self.configured = True
+        d = dict(overridden_settings(self.settings))
+        log.msg(format="Overridden settings: %(settings)r", settings=d, level=log.DEBUG)
         lf_cls = load_object(self.settings['LOG_FORMATTER'])
         self.logformatter = lf_cls.from_crawler(self)
         self.extensions = ExtensionManager.from_crawler(self)

scrapy/settings/__init__.py

@@ -57,3 +57,17 @@ class CrawlerSettings(Settings):
 
     def __str__(self):
         return "<CrawlerSettings module=%r>" % self.settings_module
+
+
+def iter_default_settings():
+    """Return the default settings as an iterator of (name, value) tuples"""
+    for name in dir(default_settings):
+        if name.isupper():
+            yield name, getattr(default_settings, name)
+
+def overridden_settings(settings):
+    """Return a dict of the settings that have been overridden"""
+    for name, defvalue in iter_default_settings():
+        value = settings[name]
+        if not isinstance(defvalue, dict) and value != defvalue:
+            yield name, value