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Interfaces to most popular SQL databases are available from third-party modules, and out of the box Python comes with the sqlite3 module (and with the SQLite 3 database), so database programming can be started right away SQLite is a lightweight SQL database, lacking many of the features of, say, PostgreSQL, but it is very convenient for prototyping, and may prove suf cient in many cases To make it as easy as possible to switch between database backends, PEP 249 (Python Database API Speci cation v20) provides an API speci cation called DB-API 20 that database interfaces ought to honor the sqlite3 module, for example, complies with the speci cation, but not all the third-party modules do There are two major objects speci ed by the API, the connection object and the cursor object, and the APIs they must support are shown in Tables 121 and 122 In the case of the sqlite3 module, its connection and cursor objects both provide many additional attributes and methods beyond those required by the DB-API 20 speci cation The SQL version of the DVDs program is dvds-sqlpy The program stores directors separately from the DVD data to avoid duplication and offers one more menu option that lets the user list the directors The two tables are shown in Figure 121 The program has slightly fewer than 300 lines, whereas the previous section s dvds-dbmpy program is slightly fewer than 200 lines, with most of the difference due to the fact that we must use SQL queries rather than perform simple dictionary-like operations, and because we must create the database s tables the rst time the program runs
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dvds id title year duration director_id directors id name
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Figure 121 The DVD program s database design
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The main() function is similar to before, only this time we call a custom connect() function to make the connection
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def connect(filename): create = not ospathexists(filename) db = sqlite3connect(filename) if create: cursor = dbcursor()
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Table 121 DB-API 20 Connection Object Methods
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Syntax dbclose() dbcommit() dbcursor() dbrollback()
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Closes the connection to the database (represented by the db object which is obtained by calling a connect() function) Commits any pending transaction to the database; does nothing for databases that don t support transactions Returns a database cursor object through which queries can be executed Rolls back any pending transaction to the state that existed before the transaction began; does nothing for databases that don t support transactions
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cursorexecute("CREATE TABLE directors (" "id INTEGER PRIMARY KEY AUTOINCREMENT UNIQUE NOT NULL, " "name TEXT UNIQUE NOT NULL)") cursorexecute("CREATE TABLE dvds (" "id INTEGER PRIMARY KEY AUTOINCREMENT UNIQUE NOT NULL, " "title TEXT NOT NULL, " "year INTEGER NOT NULL, " "duration INTEGER NOT NULL, " "director_id INTEGER NOT NULL, " "FOREIGN KEY (director_id) REFERENCES directors)") dbcommit() return db
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The sqlite3connect() function returns a database object, having opened the database le it is given and created an empty database le if the le did not exist In view of this, prior to calling sqlite3connect(), we note whether the database is going to be created from scratch, because if it is, we must create the tables that the program relies on All queries are executed through a database cursor, available from the database object s cursor() method Notice that both tables are created with an ID eld that has an AUTOINCREMENT constraint this means that SQLite will automatically populate the IDs with unique numbers, so we can leave these elds to SQLite when inserting new records SQLite supports a limited range of data types essentially just Booleans, numbers, and strings but this can be extended using data adaptors , either the prede ned ones such as those for dates and datetimes, or custom ones that we can use to represent any data types we like The DVDs program does not need this functionality, but if it were required, the sqlite3 module s documentation explains the details The foreign key syntax we have used may not be the same as the syntax for other databases, and in any case it is merely doc-
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