[Grok-dev] Automatically execute a procedure on saving/loading from Sql database with MeGrok and SQLAlchemy

Hector Blanco white.lists at gmail.com
Mon Oct 25 13:20:14 EDT 2010

[ I hate when the identation gets messed up... Gonna try again ]

Yuuup...  Type decorators was a right solution:


Yuuup... It works like a charm!

I have created a simple class that gets a dictionary and "serializes"
its values. The underlying database is MySql...

Just in case my code may help someone (or if someone has any
suggestions...) here it goes:


from sqlalchemy import types
import logging
log = logging.getLogger(__name__)

class ZepSimpleDict(types.TypeDecorator):
	impl = types.String
	size = -1
	__separatorChar = chr(0x1D)
	__boolPrefix = "b_"
	__intPrefix = "i_"
	__floatPrefix = "f_"
	__nullPrefix = "n_"
	__specialPrefixes = set([__boolPrefix, __intPrefix, __floatPrefix,
	__nullValues = set(["null", "None"])

	def __init__(self, length = 1024):
		self.size = int(length)
		super(ZepSimpleDict, self).__init__(self.size)

	def __toString(self, value):
		retval = None
		if isinstance(value, bool):
			retval = self.__boolPrefix + str(value)
		elif isinstance(value, float):
			retval = self.__floatPrefix + str(value)
		elif isinstance(value, int):
			retval = self.__intPrefix + str(value)
		elif (value is None) or (value in self.__nullValues):
			retval = self.__nullPrefix + str(None)
			retval = str(value)
		return retval

	def __fromString(self, value):
		retval = None
		prefix = None
		actualValue = None
		if len(value) > 2:
			prefix = value[0:2]
			if (prefix in self.__specialPrefixes):
				actualValue = value[2:]
				if prefix == self.__boolPrefix:
					if actualValue == "True":
						retval = True
					elif actualValue == "False":
						retval = False
						retval = value
				elif prefix == self.__floatPrefix:
						retval = float(actualValue)
					except ValueError:
						retval = value
				elif prefix == self.__intPrefix:
						retval = int(actualValue)
					except ValueError:
						retval = value
				elif prefix == self.__nullPrefix:
					if actualValue == str(None):
						retval = None
						retval = value
				retval = value
			retval = value
		return retval

	def process_bind_param(self, value, dialect):
		value_tmp = None
		flattenedValue = list()
		retval = None

		if isinstance(value, dict):
			value_tmp = dict()
			for key, val in value.iteritems():
				value_tmp[self.__toString(key)] = self.__toString(val)
			value_tmp = None

		if (value_tmp is not None):
			for key, val in value_tmp.iteritems():
			retval = self.__separatorChar.join(flattenedValue)
			retval = None
		return retval

	def process_result_value(self, value, dialect):
		retval = dict()
		value_tmp = value.split(self.__separatorChar)
		if (len(value_tmp) > 0):
			if (len(value_tmp) % 2 != 0):
				log.warn("process_result_value > Processing an string with odd
number of elements. This should not have happened.")
			for i in range(0, len(value_tmp), 2):
				retval[self.__fromString(value_tmp[i])] = self.__fromString(value_tmp[i+1])
		return retval

In my previous message, I said:
In my brain, I have "dreamed" about something like an special type of
sqlalchemy.Column in which you can specify something like "on_save =
execute this()" and "on_load = execute that()"...

This does exactly that! :)

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