librecipe: new function in GenericBaseRecipe to get value from previous buildout run

slapos/recipe/librecipe/generic.py

@@ -26,6 +26,7 @@
 # Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 #
 ##############################################################################
+import ConfigParser
 import io
 import logging
 import os
@@ -280,3 +281,18 @@ class GenericBaseRecipe(object):
     except:
       shutil.rmtree(destination)
       raise
+
+  def getValueFromPreviousRun(self, section, parameter):
+    """
+    Returns the value of a parameter from a previous run, if it exists.
+    Otherwise, returns None
+    """
+    if os.path.exists(self.buildout['buildout']['installed']):
+      with open(self.buildout['buildout']['installed']) as config_file:
+        try:
+          parser = ConfigParser.RawConfigParser()
+          parser.readfp(config_file)
+          return parser.get(section, parameter)
+        except:
+          pass
+    return None
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