What is the difference between pickle and shelve?
Question: I am learning about object serialization for the first time. I tried reading
and 'googling' for differences in the modules pickle and shelve but I am not
sure I understand it. When to use which one? Pickle can turn every python
object into stream of bytes which can be persisted into a file. Then why do we
need the module shelve? Isn't pickle faster?


Answer: `pickle` is for serializing some object (or objects) as a single bytestream in
a file.

`shelve` builds on top of `pickle` and implements a serialization dictionary
where objects are pickled, but associated with a key (some string), so you can
load your shelved data file and access your pickled objects via keys. This
could be more convenient were you to be serializing many objects.

Here is an example of usage between the two. (should work in latest versions
of Python 2.7 and Python 3.x).

## `pickle` Example

    
    
    import pickle
    
    integers = [1, 2, 3, 4, 5]
    
    with open('pickle-example.p', 'wb') as pfile:
        pickle.dump(integers, pfile)
    

This will dump the `integers` list to a binary file called `pickle-example.p`.

Now try reading the pickled file back.

    
    
    import pickle
    
    with open('pickle-example.p', 'rb') as pfile:
        integers = pickle.load(pfile)
        print integers
    

The above should output `[1, 2, 3, 4, 5]`.

## `shelve` Example

    
    
    import shelve
    
    integers = [1, 2, 3, 4, 5]
    
    # If you're using Python 2.7, import contextlib and use
    # the line:
    # with contextlib.closing(shelve.open('shelf-example', 'c')) as shelf:
    with shelve.open('shelf-example', 'c') as shelf:
        shelf['ints'] = integers
    

Notice how you add objects to the shelf via dictionary-like access.

Read the object back in with code like the following:

    
    
    import shelve
    
    # If you're using Python 2.7, import contextlib and use
    # the line:
    # with contextlib.closing(shelve.open('shelf-example', 'r')) as shelf:
    with shelve.open('shelf-example', 'r') as shelf:
        for key in shelf.keys():
            print(repr(key), repr(shelf[key])))
    

The output will be `'ints', [1, 2, 3, 4, 5]`.

