Archived:Persistent Storage in PySymbian

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Article

Created: hurenkam (15 Aug 2008)

Last edited: hamishwillee (25 Sep 2012)

The class below implements a wrapper around a e32dbm file in PySymbian which can be used as persistent storage:

import e32dbm
 
class PersistentStorage:
    def __init__(self,filename):
        try:
            # Try to open an existing file
            self.data = e32dbm.open(filename,"wf")
        except:
            # Failed: Assume the file needs to be created
            self.data = e32dbm.open(filename,"nf")
 
    def SetValue(self,key,value):
        if str(value) == value:
            # if value is a string, it needs special treatment
            self.config[key] = "u\"%s\"" % value
        else:
            # otherwise simply convert it to a string
            self.config[key] = str(value)
 
    def GetValue(self,key):
        try:
            return eval(self.config[key])
        except:
            # Assume item doesn't exist (yet), so return None
            return None
 
    def Close(self):
        self.data.close()

How to use this code

The following code shows how to use this class to change a persistent setting:

import appuifw
import persistentstorage
 
def Main():
 
    # Create the store
    storage = PersistentStorage("e:\\config")
 
    # Retrieve position
    position = storage.GetValue("position")
 
    # Set a default if it didn't exist
    if position == None:
        position = (51.47,5.48)
 
    # Query new values, show current as default
    latitude = appuifw.query(u"Latitude:","float",position[0])
    longitude = appuifw.query(u"Longitude:","float",position[1])
 
    # If new values were given, store them
    if latitude != None and longitude != None:
        storage.SetValue("position",(latitude,longitude))
 
    storage.Close()
 
Main()

Caveats

The PersistentStorage.SetValue() method wraps a string between quotes to have eval() parse it properly. However, this doesn't work right if you use the backslash character in a string. Basically, you need to double every backslash character to have eval() parse it correctly. To extend the code to do this automatically is left as an exercise for the reader.