Intro to Python

Intro to Python
by Daniel Greenfeld

Intro to Python
a.k.a.
21 cool things you can do with Python

Tons of content
• Please hold your questions until the end
• Latest slides will be made available
• Special thanks to:
• Raymond Hettinger
• David Beazley
• Audrey Roy
• The Python community

Daniel Greenfeld
• pydanny
• twitter.com/pydanny
• github.com/pydanny
• pydanny.blogspot.com
• pydanny-event-notes.rtfd.org
IMG goes here

http://www.ﬂickr.com/photos/pydanny/4442245488/

Daniel Greenfeld
• Python/Django Developer
• Cartwheel Web
• Makers of
• Makers of Open Comparison
• Los Angeles
IMG goes here • Capoeira
• Fiancee is Audrey Roy
http://www.ﬂickr.com/photos/pydanny/4442245488/

Who uses Python?
All the cool kids do!

What is Python?
• Over 20 years old
• Dynamic, strongly typed scripting language
• Multi-paradigm programming language
• Object Oriented
• Functional
• Procedural
• Reﬂective

What is Python?
• Free and open source
• Fast enough
• Check out PyPy
• Batteries included language

What is Python?

http://en.wikipedia.org/wiki/File:Flyingcircus_2.jpg

It’s named after Monty Python

Python is similar to...
• Perl
• Ruby
• Lisp
• Java

Python is different than...
• Perl
• Ruby
• Lisp
• Java

Whitespace!
""" whitespace.py """
from random import randrange

def numberizer():
# Generate a random number from 1 to 10.
return randrange(1, 11)

number = numberizer()
if number > 5:
print("This number is big!")

class RandomNumberHolder(object):
# Create and hold 20 random numbers using numberizer

def __init__(self):
self.numbers = [numberizer(x) for x in range(20)]

random_numbers = RandomNumberHolder()

Whitespace!

def numberizer():

Whitespace!

if number > 5:

Whitespace!

# Create and hold 20 random numbers
# using numberizer

def __init__(self):

self.numbers = [numberizer(x) for x...


Philosophy of
Core Developers
• Conservative growth
• Aim for a simple implementation
• “We read Knuth so you don’t have to”

Zen of Python
>>> import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

Culture of Documentation
• Django Web Framework
• http://djangoproject.com
• Document everything and document well
• Make your documentation accessible
• http://readthedocs.org
• http://python-requests.org
• http://bit.ly/oc_ref (Open Comparison reference)

For learning and simple scripting...

Use what is on your
system by default.
If you are running Linux or
BSD you already have Python

$
$ python
Python 2.7.1+ (r271:86832, Apr 11 2011,
18:13:53)
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or
"license" for more information.
>>>
>>> 3 + 4
7
>>> a = 5 * 10
>>> a
50
>>> def add(a, b):
... return a + b
...
>>> add(3,4)
7
>>> add('Py','thon')
'Python'

Don’t have Python yet?

Download 3.2
Unless you are working with a tool
with a speciﬁc Python dependency
(e.g. Django requires Python 2.7)

21 cool things you
can do with Python

#1 Run it anywhere
Linux Windows
FreeBSD Mac OS X JVM
OpenBSD Solaris .NET
NetBSD HP-UX Android OS
BSD OS/2
http://en.wikipedia.org/wiki/CPython#Supported_platforms

#2 Learn it fast
Python is easy to learn but powerful.

Experienced developers get up to speed in days.

Lots of tutorials in the area taught by PyLadies.

http://learnpythonthehardway.org/

#3 Introspect
a.k.a Introducing the String type
>>> foo = 'bar'
>>> spam = 'eggs'
>>> fun = 'spam and EGGS '
dir() is a Python built-in function
>>> dir(fun)
['__add__', '__class__', '__contains__', '__delattr__', '__doc__',
'__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__',
'__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__',
'__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__',
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__',
http://en.wikipedia.org/wiki/File:Flyingcircus_2.jpg
'__rmul__', '__setattr__', '__sizeof__', '__str__','__subclasshook__',
'_formatter_field_name_split', '_formatter_parser', 'capitalize',
'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs',
'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower',
'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip',
'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition',
'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip',
'swapcase', 'title', 'translate', 'upper','zfill']

#3 Introspect
>>> fun
'spam and EGGS '
>>> fun.strip()
'spam and EGGS' type() returns the type of object
>>> spam.title()
'Spam And Eggs '
>>> fun.capitalize()
'Spam and eggs '
>>> fun.index('a') Line comments start with ‘# ‘
2
>>> type(fun)
<type 'str'>
>>> len(fun) # built-in that gives length of object
16
>>> fun[0:5] # String slicing help() is a
'spam '
>>> help(fun) Python built-in
no Python documentation found for 'spam and EGGS '
>>> help(str)
str is the Python
string type object

#4 Things with Strings
>>> scale = 'Southern California Linux Expo'
>>> scale[0]
'S'
>>> scale[0:8]
'Southern'
>>> scale[:-5]
'Southern California Linux'
Strings are immutable
>>> scale[0:8] = 'Northern'
Traceback (most recent call last):
File "<input>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> scale.replace('Southern California','SoCal')
'SoCal Linux Expo'
>>> scale
'Southern California Linux Expo'
>>> scale = scale.replace('Southern California','SoCal')
>>> scale
'SoCal Linux Expo'
>>> scale.startswith('Windows')
False
>>> scale.endswith('Windows')
False
>>> scale.startswith('SoCal')
True
>>> 'Windows' in scale
False
>>> 'Linux' in scale
True

#5 String formatting
>>> a = "Daniel"
>>> b = "Adam"
>>> c = "Greenfeld"
>>> a + b + c
'DanielAdamGreenfeld'
>>> "{0} {1} {2}".format(a, b, c)
'Daniel Adam Greenfeld'
>>> "{first} {middle} {last}".format(first=a, middle=b, last=c)
>>> lst = [a,b,c]
>>> lst
['Daniel', 'Adam', 'Greenfeld']
>>> name =" ".join(lst)
>>> name

#6 Basic Operations
>>> x, y, z = 5, 10, 15
>>> 5 < 10
True
>>> 5 > 10
False
Python has advanced math
>>> True == False features that comes with
False
>>> (5 == x) or (10 == x) the standard library.
True
>>> (5 == x) and (10 == x)
False
>>> x + y - z
0
>>> 10 * 5 For scientiﬁc needs,
50
>>> 10 / 5 numpy is available.
2
>>> 10 + 5
15
>>> 10 ** 2
100

#7 Lists
>>> my_list = [1, 2, 3]
>>> my_list.append(4)
>>> my_list Lists are mutable
[1, 2, 3, 4]
>>> my_list.insert(2, 'dog')
>>> my_list
Tuples are not mutable
[1, 2, 'dog', 3, 4]
>>> my_list.extend([5, 6])
>>> my_list
[1, 2, 'dog', 3, 4, 5, 6]
>>> my_list.append([7, 8])
>>> my_list
[1, 2, 'dog', 3, 4, 5, 6, [7, 8]]
>>> my_list.pop(2)
'dog'
>>> my_list
[1, 2, 3, 4, 5, 6, [7, 8]]
>>> my_list.reverse()
>>> my_list
[[7, 8], 6, 5, 4, 3, 2, 1]

#8 Lists + Functional Programming
>>> def divisible_by_2(x):
... return x % 2 == 0
...
>>> Filter constructs a list from
>>> def cube(x):
... return x ** 3
those elements of an iterable
... for which the speciﬁed function
>>>
>>> numbers = [1, 2, 3, 4, 6, 31]
returns True.
>>>
>>> filter(divisible_by_2, numbers)
[2, 4, 6]
>>>
>>> map(cube, numbers)
[1, 8, 27, 64, 216, 29791] Map applies the speciﬁed
function to every item of the
iterable and returns the results.

#9 List Comprehensions
Remember
""" whitespace.py """
from random import randrange this
def numberizer(): from the
beginning?
if number > 5:
List Comprehension!

# Create and hold 20 random numbers using numberizer

def __init__(self):
self.numbers = [numberizer(x) for x in range(20)]


#9 List Comprehensions
>>> items = [x for x in range(20)]
>>> items
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
>>> [x for x in range(20) if x % 2]
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

List Comprehensions are
wonderful syntactical sugar.
>>> # Fizzbuzz solved using Python's List Comprehension
>>> lst = [(x, 'Fizz', 'Buzz', 'FizzBuzz')
... [(not x % 3) | (not x % 5) << 1] for x in range(20)]

Backslash can be used to
break up long statements.
Please use sparingly!

#10 Generators
>>> def countdown(n):
... print("Counting down from {0}".format(n))
... while n > 0:
... yield n A generator evaluates only when the
... n -= 1 iterable is at that iteration. This is really
powerful, especially when working with
>>> x = countdown(10) large iterables.
>>> x
<generator object at 0x58490>
>>> x.next()
Counting down from 10
10 A billion iterations for
>>> x.next()
9 a generator is NOTHING.
>>> x.next()
8
>>> x.next()
7

http://dabeaz.com/generators/Generators.pdf

#11 Generator Expressions
>>> items = (str(x) for x in xrange(10000)) Generator expressions are
>>> items
<generator object <genexpr> at 0x100721460> shorthand for generators. Just
like list comprehensions, but
with () instead of [].


#11 Generator Expressions
Problem: count the bytes saved to huge apache access log.
wwwlog = open("access-log")
total = 0 Open the whole ﬁle, then
for line in wwwlog:
bytestr = line.rsplit(None,1)[1] iterate through the results.
if bytestr != '-':
total += int(bytestr)
Lots of memory usage!
print "Total", total

# generator expressions way
Generator wwwlog = open("access-log")
bytecolumn = (line.rsplit(None,1)[1] for line in wwwlog)
way bytes = (int(x) for x in bytecolumn if x != '-')
print "Total", sum(bytes)


#12 Sets
>>> lst = [1,1,1,1,1,2,2,2,3,3,3,3,3,3]
>>> s = set(lst)
>>> s
set([1,2,3])

Counting unique words in the Gettysburg Address
>>> address = """Four score and seven years ago our fathers brought..."""
>>> for r in [',','.','-']:
... address = address.replace(r,'')
>>>
>>>
words = address.split(' ')
len(words)
All items in a set need to
278
>>> unique_words = set(words)
be of the same type.
>>> len(unique_words)
143

>>> data = { #13 Dictionaries
'name':'Daniel Greenfeld',
'nickname':'pydanny',
'states_lived':['CA','KS','MD','NJ','VA','AD'],
'fiancee':'Audrey Roy'
}
>>> data['name']
'Daniel Greenfeld'
>>> data['nickname'] = 'audreyr' Mutable Key/Value objects
>>> data['nickname']
'audreyr'
>>> data['nickname'] = 'pydanny'
>>> data.keys()
['fiancee', 'nickname', 'name', 'states_lived']
>>> data.get('fiancee')
'Audrey Roy'
>>> data.get('fiance')
None
>>> data.pop('fiancee')
'Audrey Roy'
>>> data
{'nickname': 'pydanny', 'name': 'Daniel Greenfeld', 'states_lived': ['CA',
'KS', 'MD', 'NJ', 'VA']}
>>> data['fiancee'] = 'Audrey Roy'
>>> data
{'fiancee': 'Audrey Roy', 'nickname': 'pydanny', 'name': 'Daniel
Greenfeld', 'states_lived': ['CA', 'KS', 'MD', 'NJ', 'VA', 'AD']}

#14 Object-Oriented Programming
class Animal(object):
def __init__(self, name):
self.name = name
def talk(self):
raise NotImplementedError("Subclass must implement abstract method")

class Cat(Animal):
def talk(self):
return 'Meow!'

class Dog(Animal): Missy: Meow!
def talk(self):
return 'Woof! Woof!' Mr. Mistoffelees: Meow!
animals = [Cat('Missy'), Lassie: Woof! Woof!
Cat('Mr. Mistoffelees'),
Dog('Lassie')]

for animal in animals:
print animal.name + ': ' + animal.talk()

Barely scratching the surface!
http://en.wikipedia.org/wiki/Polymorphism_in_object-oriented_programming#Examples

#15 Isolate Environments
$ curl http://bit.ly/get-pip | python
$ pip install virtualenv
$ virtualenv my_env
$ source my_env/bin/activate
(my_env) $
Pro Tip: easy_install is legacy. Use pip.
(my_env) $ pip install django==1.3.1
(my_env) $ pip install requests==0.9.1
(my_env) $ pip install mongoengine==0.5.2
Warning!
(my_env) $ pip install celery==2.4.6 Only installs
(my_env) $ pip freeze
celery==2.4.6 Python drivers!
django==1.3.1
mongoengine==0.5.2 Not MongoDB
requests==0.9.1
(my_env) $ pip freeze > requirements.txt or RabbitMQ
...
(another_env) $ pip install -r requirements.txt

#16 Colorize Code
How Github and Bitbucket do it
$ pip install pygments
from pygments import highlight
from pygments.lexers import get_lexer_by_name
from pygments.formatters import HtmlFormatter

if __name__ == '__main__':
# get this file
pygments_demo.py
code = open("pygments_demo.py", "rw").read()

# figure out the lexer
lexer = get_lexer_by_name("python", stripall=True)

# construct the formatter
formatter = HtmlFormatter(linenos=False, cssclass="source")

# style and formatting
css = HtmlFormatter().get_style_defs('.source')
highlighted_code = highlight(code, lexer, formatter)
page = """
$ python pygments_demo.py > text.html
<html>
<head><style>{css}</style></head>
<body>{highlighted_code}</body>
</html>
""".format(css=css, highlighted_code=highlighted_code)
print(page)

Output of the program

from pygments import highlight
from pygments.lexers import get_lexer_by_name
from pygments.formatters import HtmlFormatter text.html
if __name__ == '__main__':
# get this file
code = open("pygments_demo.py", "rw").read()

# figure out the lexer
lexer = get_lexer_by_name("python", stripall=True)

# construct the formatter
formatter = HtmlFormatter(linenos=False, cssclass="source")

# style and formatting
css = HtmlFormatter().get_style_defs('.source')
highlighted_code = highlight(code, lexer, formatter)
page = """
<html>
<head><style>{css}</style></head>
<body>{highlighted_code}</body>
</html>
""".format(css=css, highlighted_code=highlighted_code)
print(page)

#17 Work with Relational Databases
(my_env)$ pip install django
from datetime import datetime Internationalization!
from django.contrib.auth.models import User
from django.db import models
from django.utils.translation import ugettext_lazy as _

class Post(models.Model):

author = models.ForeignKey(User)
title = models.CharField(_('Title'), max_length=100)
content = models.TextField(_("Content"))
pub_date = models.DateTimeField(_("Publication date"))

class Comment(models.Model):
post = models.ForeignKey(Post)
name = models.CharField(_('Title'), max_length=100)
content = models.TextField(_("Content"))

#18 Work with NoSQL
(my_env)$ pip install pymongo
>>> import pymongo
>>> connection = pymongo.Connection("localhost", 27017)
>>> db = connection.test
>>> db.name
u'test'
>>> db.my_collection
Collection(Database(Connection('localhost', 27017), u'test'),
u'my_collection')
>>> db.my_collection.save({"x": 10})
ObjectId('4aba15ebe23f6b53b0000000')
ObjectId('4aba160ee23f6b543e000000')
ObjectId('4aba160ee23f6b543e000002')
>>> db.my_collection.find_one()
{u'x': 10, u'_id': ObjectId('4aba15ebe23f6b53b0000000')}
>>> db.my_collection.create_index("x")
u'x_1'
>>> [item["x"] for item in db.my_collection.find().limit(2).skip(1)]
[8, 11]

#19 Message Queues
(my_env)$ pip install celery==2.4.6
(my_env)$ pip install requests==0.9.2
import logging products/tasks.py
import requests
from celery import task

from products.models import Product
Decorators wrap a
logger = logging.getLogger('products.tasks')
function or method
@task
def check_all_images():
with a function.
for product in Product.objects.all():
response = request.get(product.medium_image.url)
if response.status_code != 200:
msg = "Product {0} missing image".format(product.id)
logging.warning(msg)
>>> from products.tasks import confirm_all_images
>>> result = confirm_all_images.delay()
>>> result.ready()
False
>>> result.ready()
True

#20 Work with JSON
>>> import json
>>> data = {
'name':'Daniel Greenfeld',
'nickname':'pydanny',
'states_lived':['CA','KS','MD','NJ','VA','AD'],
'fiancee':'Audrey Roy'
}
>>> type(data)
<type 'dict'>
>>> payload = json.dumps(data)
>>> payload
'{"fiancee": "Audrey Roy", "nickname": "pydanny", "name": "Daniel
Greenfeld", "states_lived": ["CA", "KS", "MD", "NJ", "VA", "AD"]}'
>>> type(payload)
<type 'str'>
>>> restored = json.loads(payload)
>>> type(restored)
<type 'dict'>
>>> restored
{u'fiancee': u'Audrey Roy', u'nickname': u'pydanny', u'name': u'Daniel
Greenfeld', u'states_lived': [u'CA', u'KS', u'MD', u'NJ', u'VA', u'AD'
]}

#21 Serve the Web
$ pip install ﬂask==0.8
# webapp.py
from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
return "Hello World!"

if __name__ == "__main__":
app.run()

ﬂask.pocoo.org

#21 Serve the Web

Lots more frameworks!

#21 Serve the Web

http://science.nasa.gov/

#21 Serve the Web

http://djangopackages.com
http://opencomparison.org

#21 Serve the Web

http://consumernotebook.com

#21 Serve the Web

http://grove.io

Hosted IRC

Finis
• Learn more at the following booths:
• Python
• Django
• Pyladies

Intro to Python

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