Question

For this example, I have a dictionary, that when I call on it, “Ember Attack” is displayed.

#import shelve
class Pokemon():
"""Each pokemon's attributes"""

def __init__(self):
    self.id=[]
    self.var1=[]
    self.var2=[]
    self.var3=[]
    self.var4=[]
    self.var5=[]

def __str__(self):
     showList=['id','var1', 'var2', 'var3', 'var4', 'var5']

#dict1=shelve.open("shelve.dat")
dict1={}
dict1["Charmander"]=Pokemon()
dict1["Charmander"].var1="Ember Attack"
#dict1.sync()
print dict1["Charmander"].var1
#dict1.close()

However when I start using shelves instead of the dictionary, I get a blank when I call on var1.

import shelve

class Pokemon():
"""Each patient's attributes"""

def __init__(self):
    self.id=[]
    self.var1=[]
    self.var2=[]
    self.var3=[]
    self.var4=[]
    self.var5=[]

def __str__(self):
    showList=['id','var1', 'var2', 'var3', 'var4', 'var5']

dict1=shelve.open("shelve.dat")
#dict1={}

dict1["Charmander"]=Pokemon()
dict1["Charmander"].var1="Ember Attack"

dict1.sync()

print dict1["Charmander"].var1

dict1.close()

The only difference is that I made dict1 a shelve dictionary instead of a regular dictionary. It probably has to do with memory scope or something. Anyway, can someone help me revise my code so that it will work with shelves? Thanks!

Answer 1

dict1=shelve.open("shelve.dat", writeback=True)

you can also specify the protocol which should improve performance

dict1=shelve.open("shelve.dat", protocol=2, writeback=True)

Because of Python semantics, a shelf
cannot know when a mutable
persistent-dictionary entry is
modified. By default modified objects
are written only when assigned to the
shelf (see Example). If the optional
writeback parameter is set to True,
all entries accessed are also cached
in memory, and written back on sync()
and close(); this can make it handier
to mutate mutable entries in the
persistent dictionary, but, if many
entries are accessed, it can consume
vast amounts of memory for the cache,
and it can make the close operation
very slow since all accessed entries
are written back (there is no way to
determine which accessed entries are
mutable, nor which ones were actually
mutated).