How do I take integer keys in shelve?

Question

I want to store an integer key in shelve. But when I try to store integer key in shelve it give me an error

Traceback (most recent call last):
  File "./write.py", line 12, in 
    data[id] = {"Id": id, "Name": name}
  File "/usr/lib/python2.5/shelve.py", line 124, in __setitem__
    self.dict[key] = f.getvalue()
  File "/usr/lib/python2.5/bsddb/__init__.py", line 230, in __setitem__
    _DeadlockWrap(wrapF)  # self.db[key] = value
  File "/usr/lib/python2.5/bsddb/dbutils.py", line 62, in DeadlockWrap
    return function(*_args, **_kwargs)
  File "/usr/lib/python2.5/bsddb/__init__.py", line 229, in wrapF
    self.db[key] = value
TypeError: Integer keys only allowed for Recno and Queue DB's

My Code :

#!/usr/bin/python

import shelve

data = shelve.open("data.txt")

ans = 'y'
while ans == "y":
    id = input("Enter Id : ")
    name = raw_input("Enter name : ")

    data[id] = {"Id": id, "Name": name}

    ans = raw_input("Do you want to continue (y/n) ? : ")

data.close()

Is something wrong in my program or shelve does not supports integer keys at all ?

---

Edit 1 :

In program I am trying to store a dictionary of Id and Name inside another dictionary with Id as a key. And then trying to store it in a file.

Do I need to use Recno or Queue DB's along with shelve? I am a beginner and things are confusing.

Let me know if I am not clear with my question.

Thanks.

Answer 1

The shelve module uses an underlying database package (such as dbm, gdbm or bsddb) .

A "shelf" is a persistent, dictionary-like object. The difference with "dbm" databases is that the values (not the keys!) in a shelf can be essentially arbitrary Python objects -- anything that the pickle module can handle. This includes most class instances, recursive data types, and objects containing lots of shared sub-objects. The keys are ordinary strings. The examples section gives you the proof.

This should work. Here's what I do in my code -

import shelve

#Create shelve
s = shelve.open('test_shelf.db')
try:
    s['key1'] = { 'int': 10, 'float':9.5, 'string':'Sample data' }
finally:
    s.close()

#Access shelve
s = shelve.open('test_shelf.db')
try:
    existing = s['key1']
finally:
    s.close()
print existing

UPDATE: You could try pickle module. It is not a key-value database but you can always build your data structure as a key-value pairs and then send it to pickle -

If you have an object x, and a file object f that's been opened for writing, the simplest way to pickle the object takes only one line of code

pickle.dump(x, f)

To unpickle the object again, if f is a file object which has been opened for reading:

x = pickle.load(f)

I hear cPickle is a lot faster than pickle. You can try this if you have lot of data to store.

Answer 2

In your example the keys in your database will always be integers, so it should work fine to convert them to strings,

data[str(id)] = {"Id": id, "Name": name}

My test code

def shelve_some_data(filename):
    db = shelve.open(filename, flag="c")
    try:
        # note key has to be a string
        db[str(1)]    = "1 integer key that's been stringified" 
        db[str(2)]    = "2 integer key that's been stringified" 
        db[str(3)]    = "3 integer key that's been stringified" 
        db[str(10)]   = "10 integer key that's been stringified" 
    finally:
        db.close()

def whats_in(filename):
    db = shelve.open(filename, flag="r")
    for k in db:
        print("%s : %s" % (k, db[k]))
    return

filename = "spam.db"
shelve_some_data(filename)
whats_in(filename)

And the output; it works like a dict so it's not sorted.

2 : 2 integer key that's been stringified
10 : 10 integer key that's been stringified
1 : 1 integer key that's been stringified
3 : 3 integer key that's been stringified