How to use Shelve to save objects in Python

Shelve is a powerful Python module for object persistence. When shelving an object, you must specify a key that identifies the object's value. In this way, the shelve file becomes a database of stored values, any of which can be accessed at any time.

Sample code for shelving in Python

To shelve an object, first import the module and then assign the object value as follows:

 import shelve
database = shelve.open(filename.suffix)
object = Object()
database['key'] = object

For example, if you want to preserve the stock database, you can adjust the following code:

import shelve
stockvalues_db = shelve.open('stockvalues.db')
object_ibm = Values.ibm()
stockvalues_db['ibm'] = object_ibm
object_vmw = Values.vmw()
stockvalues_db['vmw'] = object_vmw
object_db = Values.db()
stockvalues_db['db'] = object_db

"stock values.db" is already open, you do not need to open it again. Instead, you can open multiple databases at once, write to each at will, and let Python close them when the program terminates. For example, you could keep a separate name database for each symbol and append the following to the previous code:

 ## assuming shelve is already imported
stocknames_db = shelve.open('stocknames.db')
objectname_ibm = Names.ibm()
stocknames_db['ibm'] = objectname_ibm
objectname_vmw = Names.vmw()
stocknames_db['vmw'] = objectname_vmw
objectname_db = Names.db()
stocknames_db['db'] = objectname_db

Note that any change in the name or suffix of the database file constitutes a different file , thus forming different databases.

The result is the second database file containing the given values. Unlike most files written in a custom format, shelved databases are saved in binary form.

After writing the data to the file, it can be called at any time. If you want to restore the data in a future session, reopen the file. If it's the same session, just call the value; the shelve database file is opened in read-write mode. Here is the basic syntax to achieve this:

 import shelve
database = shelve.open(filename.suffix)
object = database['key']

Therefore, the example from the previous example would appear as:

 import shelve
stockname_file = shelve.open('stocknames.db')
stockname_ibm = stockname_file['ibm']
stockname_db = stockname_file['db']

Shelving Considerations

It is important to note that when closing The database will remain open until the database is opened (or until the program terminates). So if you are writing a program of any size, you need to close the database after using it. Otherwise, the entire database (not just the values ​​you want) sits in memory and consumes computing resources.

To close the shelve file, use the following syntax:

database.close()

If all the code examples above were combined into one program, then we would have two database files open and consuming memory. So, after reading the stock names in the previous example, you can close each database in turn like this:

 stockvalues_db.close()
stocknames_db.close()
stockname_file.close()

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