CIS 192: Lecture 11 Databases (SQLite3)

Transcription

1 CIS 192: Lecture 11 Databases (SQLite3) Lili Dworkin University of Pennsylvania

2 In-Class Quiz app = Flask( main def home():... app.run() 1. type(app.run) 2. type(app.route( / )) Hint: what is the type of a decorator?

3 In-Class Quiz app = Flask( main def home():... app.run() 1. type(app.run) instancemethod 2. type(app.route( / )) Hint: what is the type of a decorator? function

4 In-Class Quiz If we decorate a function with app.route( /client/<name> ) which of the following paths will map to the function? 1. /client/name 2. /client 3. /client/sarah

5 In-Class Quiz If we decorate a function with app.route( /client/<name> ) which of the following paths will map to the function? 1. /client/name 2. /client 3. /client/sarah

6 In-Class Quiz If someone appends?version=2.0&type=json to a url, how can we get the value of the parameters version and type in the function corresponding to that endpoint?

7 In-Class Quiz If someone appends?version=2.0&type=json to a url, how can we get the value of the parameters version and type in the function corresponding to that endpoint? request.args[ version ]

8 In-Class Quiz What keyword argument and value should we pass to app.route if we want the corresponding endpoint to accept POST requests methods=['post']) def post():...

9 More Flask Practice Let s write a little app that provides a slimmed-down version of the Google Places API.

10 Modules Python code is organized into modules We ve already used these a bunch import requests from bs4 import BeautifulSoup We ve also created them every Python file is a module! That s why we can do from hw4 import *, etc

11 Modules What does Python do to load a module? 1. Creates an empty module object 2. Adds module object to sys.modules dictionary 3. Loads the module code (compiles if necessary) 4. Excecutes the module code

12 Modules Modules are just objects with attributes and methods! >>> import math >>> math <module 'math' from '/System/Library/Frameworks/ Python.framework/...'> >>> math. name 'math' >>> math.sqrt(9) 3.0

13 Modules We can organize modules into directories/folders You just need to add an init.py file to the directory The directory is known as a package We now need to import them by their path

14 Modules Say we have two files and we want to group them together into a package: put them both in a folder with a blank init.py file. prompt$ ls myproject init.py file1.py file2.py prompt$ cat init.py prompt$ cat myproject/file1.py def hello(): print 'hello' prompt$ cat myproject/file2.py def bye(): print 'bye'

15 Modules >>> import myproject.file1 >>> myproject.file1.hello() hello >>> from myproject import file1 >>> file1.hello() hello >>> from myproject.file1 import hello >>> hello() hello

16 Modules >>> import Sound.Effects.echo >>> Sound.Effects.echo.function() >>> from Sound.Effects import echo >>> echo.function() >>> from Sound.Effects.echo import function >>> function()

17 Data Persistence Most of the programming we ve done so far is stateless - no need to persist data beyond a single execution We ve used text files (CSV and JSON) to store data But databases are a more powerful option

18 SQLite3 Python has built in support for sqlite3: Download sqlite3 yourself: Use Homebrew on a Mac: and brew install sqlite3

19 Relational Databases Relational databases contain tables Each table has columns Each column has a type

20 DB Connections import sqlite3 conn = sqlite3.connect('data.db') c = conn.cursor() # execute queries conn.commit() conn.close()

21 DB Connections Remember with statements? import sqlite3 conn = sqlite3.connect('data.db') with conn: c = conn.cursor() # execute queries conn.close()

22 with Statement Digression with object: # do stuff What s happening? 1. Try to do stuff 2. No matter what happens, call object s exit method When object was a file, call close() When object is a db connection, call commit()

23 Executing Queries We use cursors to execute queries: >>> conn = sqlite3.connect('data.db') >>> c = conn.cursor() >>> c.execute('select sqlite version()') >>> data = c.fetchone() >>> data (u'3.7.13',)

24 Creating Tables Create a table with three columns: c.execute(''' create table friends (id integer primary key, name text, age int) ''') c.execute(''' create table if not exists friends (id integer primary key, name text, age int) ''') The integer primary key field type autoincrements.

25 Dropping Tables c.execute("drop table friends") c.execute("drop table if exists friends")

26 Inserting Rows c.execute(''' insert into friends (name, age) values ('Susan', 25) ''')

27 Inserting Rows c.execute(''' insert into friends (name) values ('Sally') ''') sqlite3.programmingerror: Incorrect number of bindings supplied. The current statement uses 1, and there are 5 supplied. What happened here?!

28 Remember Singleton Tuples? >>> type(('susan', 25)) <type 'tuple'> >>> type(('susan')) <type 'str'> >>> type(('susan',)) <type 'tuple'>

29 Inserting Rows c.execute(''' insert into friends (name) values ('Sally', ) ''') All better. Note that the age field will be None.

30 Inserting Rows What if we have variables for each field value? c.execute(''' insert into friends (name) values ('%s', )''' % friend_name) This is a bad idea...

31 Little Bobby Tables

32 Inserting Rows c.execute(''' insert into friends (name, age) values (?,?)''', (friend_name, friend_age))

33 Inserting Rows two_friends = [("Amy", 19), ("Bill", 23)] c.executemany(''' insert into friends (name, age) values (?,?)''', two_friends)

34 Selecting Results >>> results = c.execute("select * from friends") >>> results.fetchall() [(1, u'susan', 25), (2, u'john', 22), (3, u'sally', None), (4, u'amy', 19), (5, u'bill', 23)]

35 Selecting Results Another option: >>> for friend in results: print friend (1, u'susan', 25) (2, u'john', 22) (3, u'sally', None) (4, u'amy', 19) (5, u'bill', 23) results is an iterator!

36 Filtering Results >>> name = "John" >>> results = c.execute(''' select * from friends where name =?''', (name,)) >>> results.fetchall() [(2, u'john', 22)]

37 Filtering Results >>> age = 22 >>> results = c.execute(''' select * from friends where age >?''', (age,)) >>> results.fetchall() [(1, u'susan', 25), (5, u'bill', 23)]

38 Ordering Results >>> results = c.execute(''' select * from friends order by age''') >>> results.fetchall() [(3, u'sally', None), (4, u'amy', 19), (2, u'john', 22), (5, u'bill', 23), (1, u'susan', 25)]

39 Updating Rows >>> newname = "John Smith" >>> id = 2 >>> c.execute(''' update friends set name=? where id=?''', (newname, id)) >>> results = c.execute("select * from friends") >>> results.fetchall() [(1, u'susan', 25), (2, u'john Smith', 22), (3, u'sally', None), (4, u'amy', 19), (5, u'bill', 23)

40 Deleting Rows >>> age = 20 >>> c.execute(''' delete from friends where age <?''', (age,)) >>> results = c.execute("select * from friends") >>> results.fetchall() [(1, u'susan', 25), (2, u'john Smith', 22), (3, u'sally', None), (5, u'bill', 23)]

41 SQLite3 with Flask from flask import Flask DATABASE = 'data.db' app = Flask( name def get_entries(): conn = sqlite3.connect(database) c = conn.cursor() results = c.execute(...) conn.close() return jsonify({"results": results})

42 Schema Files To initialize data.db, we write a schema file that tells sqlite3 what tables, columns, and types we want: drop table if exists entries; create table entries ( id integer primary key, title text, text text ); prompt$ sqlite3 data.db < schema.sql