Python introduction

Introduction to Python Master in Free Software 2009/2010 Joaquim Rocha <jrocha@igalia.com> 23, April 2010

What is it? * Generalpurpose, highlevel programming language * Created by Guido van Rossum Master in Free Software | 20092010 2

What is it? * Named after Monty Python's Flying Circus * Managed by the Python Software Foundation * Licensed under the Python Software Foundation license (Free Software and compatible with GPL) Master in Free Software | 20092010 3

How is it? * Multiparadigm * Features garbagecollection * Focuses on readability and productivity * Strong introspection capabilities Master in Free Software | 20092010 4

How is it? * Batteries included! * Guido is often called Python's Benevolent Dictator For Life (BDFL) for he makes the final calls in Python's matters Master in Free Software | 20092010 5

Brief history * Created in 1991 at the Stichting Mathematisch Centrum (Amsterdam) * Designed as a scripting language for the Amoeba system * January 1994: version 1.0 Master in Free Software | 20092010 6

Brief history * October 2000: version 2.0 * December 2001: version 2.2 (first release under PSF license) * October 2008: version 2.6 * December 2008: version 3.0 Master in Free Software | 20092010 7

Python Software Foundation License * Free Software license (recognized by the Open Source Initiative) * GPL-compatible * http://python.org/psf/license/ * http://www.python.org/download/releases/2.6.2/license/ Master in Free Software | 20092010 8

Companies using Python * Google (and YouTube) * NASA * Philips * Industrial Light & Magic * EVE Online ... Master in Free Software | 20092010 9

Projects using Python * Django * Pitivi * Trac * Plone * Frets on Fire * Miro * OCRFeeder ... Master in Free Software | 20092010 10

Some books on Python * Programming Python (O'Reilly) * Python in a Nutshell (O'Reilly) * Dive Into Python (Apress) * Expert Python Programming (Packt Publishing) Master in Free Software | 20092010 11

How to install it * It's already installed in most mainstream distros * If not, on Debian: aptget install python Master in Free Software | 20092010 12

Python Shell $ python Python 2.6.4 (r264:75706, Dec 7 2009, 18:45:15) [GCC 4.4.1] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import this The Zen of Python, by Tim Peters ... Master in Free Software | 20092010 13

Documentation * Python is really well documented http://docs.python.org * On Debian distros, install python2.6doc to view it with Devhelp Master in Free Software | 20092010 14

Some helpful functions * The help command displays the help on a symbol >>> import os >>> help(os.path) * The dir command displays attributes or available names >>> dir() # displays names in current scope >>> dir(str) # displays attributes of the class and its bases Master in Free Software | 20092010 15

Python packages and modules # This is a module, and would be the execution script myapp_run.py myapp/ # A package __init__.py # A special module myappcode.py # A module lib/ # A subpackage __init__.py util_functions.py # Another module * The __init__.py is often empty but is what allows the package to be imported as a if it was a module. Master in Free Software | 20092010 16

Example of types * In Python everything is an object * str : strings (there is also a string module, that most functions are now available in str) 'This is a string' “This is also a string!” “””And this is a multiline string...””” Master in Free Software | 20092010 17

Example of types * int: integer numbers i = 12 + 4 * float: floating point numbers f = 0.45 Master in Free Software | 20092010 18

Example of types * bool: booleans b = True e = b and c or d * list: list objects list_var = ['foo', 45, False] Master in Free Software | 20092010 19

Example of types * dict: dictionary objects d = {'type': 'person', 'age': 24, 10: 'just to show off an int key'} ... Master in Free Software | 20092010 20

Checking a type >>> type(some_object) # gives the object's type >>> isinstance('foo bar', str) # returns True Master in Free Software | 20092010 21

Castings int('5') # gives the integer 5 str(5) # gives the string '5' bool('') # gives False Master in Free Software | 20092010 22

Whitespace indentation * Python blocks are defined by indentation instead of curly brackets or begin/end statements. * Failing to properly indent blocks raises indentation exceptions Master in Free Software | 20092010 23

Whitespace indentation * If a new block is expected and none is to be defined, the pass keyword can be used if x is None: pass Master in Free Software | 20092010 24

Control flow: if if x < 0: print 'X is negative!' elif x > 0: print 'X is positive!' else: print 'X is zero...' Master in Free Software | 20092010 25

Control flow: for for x in range(10): # this is [0..9] if x % 2 == 0: continue print x ** 2 Master in Free Software | 20092010 26

Control flow: while fruits = ['bananas', 'apples', 'melon', 'grapes', 'oranges'] i = 0 while i < len(fruits): if len(fruits[i]) == 5: break i += 1 Master in Free Software | 20092010 27

Functional programming * Being multiparadigm, Python also has functions common in functional programming * It features lambda, map, filter, reduce, ... numbers = [1, 2, 3, 4] map(lambda x: x ** 2, numbers) # gives [1, 4, 9, 16] filter(lambda x: x % 2 == 0, numbers) # gives [2, 4] reduce(lambda x, y: x + y, numbers) # gives 10 Master in Free Software | 20092010 28

Functional programming * List comprehension numbers = [1, 2, 3, 4, 5, 6, 7, 8] even_numbers = [n for n in numbers if n % 2 == 0] Master in Free Software | 20092010 29

Error handling: try / except * Equivalent to try, catch statements import random n = random.choice(['three', 1]) try: a = 'three ' + n except TypeError: a = 3 + n else: # else body is executed is try's body didn't raise an exception a = 'The answer is ' + a finally: # this is always executed print a Master in Free Software | 20092010 30

Error handling: try / except * Getting exception details try: a = not_defined_var except Exception as e: # the "as" keyword exists since Python 2.6 # for Python 2.5, use “,” instead print type(e) Master in Free Software | 20092010 31

Error handling: try / except * Raising exceptions password = 'qwerty' if password != 'ytrewq': raise Exception('Wrong password!') Master in Free Software | 20092010 32

Imports * Import a module: import datetime d = datetime.date(2010, 4, 22) * Import a single class from a module: from datetime import date d = date(2010, 4, 22) Master in Free Software | 20092010 33

Imports * Import more than one class from a module: from datetime import date, time d = date(2010, 4, 22) t = time(15, 45) * Import a single module from another module: from os import path notes_exist = path.exists('notes.txt') Master in Free Software | 20092010 34

Imports * Import everything from a module: from datetime import * t = time(15, 45) * Rename imported items: from datetime import date from mymodule import date as mydate # otherwise it would have overridden the previous date Master in Free Software | 20092010 35

Functions def cube(number): return number ** 3 # optional arguments def power_from_square(base, exponent = 2): # every argument after an optional argument # must be also optional return base ** exponent Master in Free Software | 20092010 36

Functions # the exponent argument is optional power(2) # returns 4 power(2, 3) # returns 8 # named arguments can be used power(base=8, exponent=4) # and can be even interchanged power(exponent=2, base=4) Master in Free Software | 20092010 37

Object Oriented Programming class Car: # this is a class variable min_doors = 3 # contructor def __init__(self, model): # this is an instance variable self.model = model # this is a private variable self._color = 'white' Master in Free Software | 20092010 38

Object Oriented Programming # private method def _is_black(self): if self._color == 'black' or self._color == '#000': return True return False # public method def set_color(self, color): if color in ['black', 'white', 'red', 'blue']: self._color = color def get_extense_name(self): return 'A very fine %s %s' % (self.model, self._color) Master in Free Software | 20092010 39

Inheritance # this is how we say Car extends the Vehicle class class Car(Vehicle): def __init__(self, model): # we need to instantiate the superclass Vehicle.__init__(self, tyres = 4) self.model = model … Master in Free Software | 20092010 40

Executing code * Here is how is Python's main: if __name__ == '__main__': a = 1 print a * Execute it with: $ python print_one.py Master in Free Software | 20092010 41

Properties * Properties abstract actions on an attribute, as get, set, del or doc # When you set/get person.name, it will do a customized set/get class Person: def __init__(self, name): self.name = name def _set_name(self, name): self._name = str(name) def _get_name(self): return 'My name is ' + self._name name = property(_get_name, _set_name) Master in Free Software | 20092010 42

Distributing your application * Distutils allows to easily packages your application, install it, etc. * Our app structure: SupApp/ setup.py supaapp data/ supaapp.png src/ supaApp/ supaapp.py lib/ util.py Master in Free Software | 20092010 43

Distributing your application * The magic script from distutils.core import setup setup(name = 'SupaApp', version = '0.5beta' description = 'A supadupa app!' author = 'Cookie Monster' author_email = 'cookiesmonster@sesamestree.fun' license = 'GPL v3', packages = ['SupaApp', 'SupaApp.lib'], package_dir = {'': 'src'}, scripts = ['supaapp'], data_files = [('share/icons/hicolor/scalable/apps', ['data/supaapp.png'])] ) Master in Free Software | 20092010 44

Distributing your application * The MANIFEST.in generates the MANIFEST file which describes what's to be included in the package: include supaapp recursiveinclude data * Master in Free Software | 20092010 45

Distributing your application * How to generate a source package $ python setup.py sdist * How to install an app $ python setup.py install Master in Free Software | 20092010 46

Docstrings * Are nowadays written in ReStructuredText * Documentation of a module: """This is a nice module. It shows you how to document modules """ class Person: """This class represents a person. A person can have a name and ... """ def __init__(self, name): "Instantiates a Person object with the given name" self.name = name Master in Free Software | 20092010 47

Some useful modules import sys # Available packages/modules must be under the folders # sys.path has print sys.path # shows the arguments given after the python command print sys.argv # get png image names import os png_names = [os.path.splitext(f)[0] for f in os.listdir('/home/user/images') if f.endswith('png')] Master in Free Software | 20092010 48

Some useful modules * ElementTree is an easy to use XML library from xml.etree import ElementTree as ET tree = ET.parse("index.html") print tree.findall('body/p') Master in Free Software | 20092010 49

Decorators * Decorators are functions that control other functions import sys import os def verbose(function): def wrapper(*args, **kwargs): print 'Executing function: ', function.__name__ print 'Arguments: ', args print 'Keyword Arguments: %sn' % kwargs return function(*args, **kwargs) return wrapper Master in Free Software | 20092010 50

Decorators @verbose # this is how you apply a decorator def list_dir(directory): for f in os.listdir(directory): print f if __name__ == '__main__': # list the directory we specify as the first program's argument list_dir(sys.argv[1]) Master in Free Software | 20092010 51

Decorators * Testing the decorator $ python decorator_example.py /home/user/Documents Executing function: list_dir Arguments: ('/home/user/Documents/',) Keyword Arguments: {} notes.txt python_presentation.odp book_review.odt Master in Free Software | 20092010 52

PDB: The Python Debugger * Calling pdb.set_trace function will stop the execution and let you use commands similar to GDB def weird_function(): import pdb; pdb.set_trace() # code that is to be analyzed... Master in Free Software | 20092010 53

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Python introduction

by Joaquim Rocha on Apr 24, 2010

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