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
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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)
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How is it?
* Multi­paradigm
* Features garbage­collection
* Focuses on readability and productivity
* Strong introspection capabilities
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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
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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
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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
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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/
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Companies using Python
* Google (and YouTube)
* NASA
* Philips
* Industrial Light & Magic
* EVE Online
...
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Projects using Python
* Django
* Pitivi
* Trac
* Plone
* Frets on Fire
* Miro
* OCRFeeder
...
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Some books on Python
* Programming Python (O'Reilly)
* Python in a Nutshell (O'Reilly)
* Dive Into Python (Apress)
* Expert Python Programming (Packt Publishing)
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How to install it
* It's already installed in most mainstream
distros
* If not, on Debian:
apt­get install python
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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
...
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Documentation
* Python is really well documented
http://docs.python.org
* On Debian distros, install python2.6­doc to
view it with Devhelp
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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
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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.
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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...”””
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Example of types
* int: integer numbers
i = 12 + 4
* float: floating point numbers
f = 0.45
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Example of types
* bool: booleans
b = True
e = b and c or d
* list: list objects
list_var = ['foo', 45, False]
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Example of types
* dict: dictionary objects
d = {'type': 'person', 'age': 24,
10: 'just to show off an int key'}
...
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Checking a type
>>> type(some_object) # gives the object's type
>>> isinstance('foo bar', str) # returns True
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Castings
int('5') # gives the integer 5
str(5) # gives the string '5'
bool('') # gives False
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Whitespace indentation
* Python blocks are defined by indentation
instead of curly brackets or begin/end
statements.
* Failing to properly indent blocks raises
indentation exceptions
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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
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Control flow: if
if x < 0:
print 'X is negative!'
elif x > 0:
print 'X is positive!'
else:
print 'X is zero...'
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Control flow: for
for x in range(10): # this is [0..9]
if x % 2 == 0:
continue
print x ** 2
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Control flow: while
fruits = ['bananas', 'apples', 'melon', 'grapes',
'oranges']
i = 0
while i < len(fruits):
if len(fruits[i]) == 5:
break
i += 1
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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
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Functional programming
* List comprehension
numbers = [1, 2, 3, 4, 5, 6, 7, 8]
even_numbers = [n for n in numbers
if n % 2 == 0]
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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
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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)
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Error handling: try / except
* Raising exceptions
password = 'qwerty'
if password != 'ytrewq':
raise Exception('Wrong password!')
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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)
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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')
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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
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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
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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)
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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'
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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)
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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
…
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Executing code
* Here is how is Python's main:
if __name__ == '__main__':
a = 1
print a
* Execute it with:
$ python print_one.py
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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)
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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
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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'])]
)
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Distributing your application
* The MANIFEST.in generates the MANIFEST file
which describes what's to be included in the
package:
include supaapp
recursive­include data *
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Distributing your application
* How to generate a source package
$ python setup.py sdist
* How to install an app
$ python setup.py install
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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
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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')]
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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')
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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
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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])
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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
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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...
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