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Pure-Python engine

The pure-Python engine consists of one module, PyDbLite.py. To use it, import the class Base from this module :

    from PyDbLite import Base

Create or open a database

  • create a database instance, passing it a path in the file system : db = Base('test.pdl')
  • for a new database, define the field names : db.create('name','age','size')
    You don't have to define the field types. PyDbLite will accept any value that can be serialized by the cPickle module : strings, Unicode strings, integers, floats, dates and datetimes (instances of the date and datetime classes in the datetime module), user-defined classes, etc
  • db.exists() indicates if the base exists
  • if the base exists, open it : db.open()
  • you can pass a parameter "mode" to the create() method, to specify what you want to do if the base already exists in the file system
    • mode = "open" : db.create('name','age','size',mode="open") opens the database and ignores the field definition
    • mode = "override" : db.create('name','age','size',mode="override") erases the existing base and creates a new one with the field definition
    • if mode is not specified and the base already exists, an IOError is raised

Insert, update, delete a record

  • insert a new record
    • by keywords : db.insert(name='homer',age=23,size=1.84)
      If some fields are missing, they are initialized with the value None
    • by positional arguments : db.insert('homer',23,1.84)
      The arguments must be provided in the same order as in the create() method
  • save the changes on disk : db.commit()
    If you don't commit the changes, the insertion, deletion and update operations will not be saved on disk. As long as changes are not commited, use open() to restore the values as they are currently on disk (this is equivalent to rollback in transactional databases)
  • besides the fields passed to the create() method, an internal field called __id__ is added. It is a integer which is guaranteed to be unique and unchanged for each record in the base, so that it can be used as the record identifier
  • another internal field called __version__ is also managed by the database engine. It is a integer which is set to 0 when the record is created, then incremented by 1 each time the record is updated. This is used to detect concurrency control, for instance in a web application where 2 users select the same record and want to update it at the same time
  • to delete a record : db.delete(record) or, if you know the record identifier : del db[rec_id]
  • to delete a list of records : db.delete(list_of_records)
    list_of_records can be any iterable (list, tuple, set, etc) yielding records
  • to update a record : db.update(record,age=24)

Selection

The instance of Base is a Python iterator ; to iterate on all the records :
for r in db:
    do_something_with(r)

Direct access

A record can be accessed by its identifier :
record = db[rec_id]
returns the record such that record['__id__'] == rec_id

Simple selections

  • db(key1=val1,key2=val2) returns the list of records where the keys take the given values
  • db(key) >= val returns an iterator on all records where the value of the field key is greater than or equal to val. Example :
    for rec in (db("age")>30):
         print rec["name"]
  • such "rich comparison" operations can be combined with & (and) and | (or) :
    for rec in (db("age")>30) & (db("country") == "France"):
        print rec["name"]

List comprehension

  • the selection of records can use Python list comprehension syntax :
    recs = [ r for r in db if 30 > r['age'] >= 18 and r['size'] < 2 ]
    returns the records in the base where the age is between 18 and 30, and size is below 2 meters. The record is a dictionary, where the key is the field name and value is the field value
  • Python generator expression syntax can also be used :
    for r in (r for r in db if r['name'] in ('homer','marge') ):
        do_something_with(r)

    iterates on the records where the name is one of 'homer' or 'marge'

Index

To speed up selections, an index can be created on a field : db.create_index('age')

When an index is created, the database instance has an attribute (here _age : note the heading underscore, to avoid name conflicts with internal names). This attribute is a dictionary-like object, where keys are the values taken by the field, and values are the records whose field values are egal to the key :

records = db._age[23] returns the list of records with age == 23

If no record has this value, lookup by this value returns an empty list

The index supports iteration on the field values, and the keys() method returns all existing values for the field

Other attributes and methods

  • db.add_field('new_field'[,default=v]) : adds a new field to an existing base. default is an optional default value ; set to None if not specified
  • db.drop_field('name') : drops an existing field
  • db.path : the path of the database in the file system
  • db.name : the database name : the basename of the path, stripped of its extension
  • len(db) : number of records in the base
  • db.fields : the list of the fields (does not include the internal fields __id__ and __version__)