Introduction
to
Python

Aim of this talk
Show you that Python is not a noddy language

Get you interested in learning Python
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Before we start
Who am I?
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Tim Penhey
Otago University Comp Sci 1991­1994
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Intermittent contractor for 12 years
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Started working for Canonical over 2 years ago
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First started with Python 8 years ago after

reading “The Cathedral & the Bazaar”
Python has been my primary development

language for around three years now
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Quick Question
What languages are you tought as an

undergraduate now?
When I was here we did: Pascal; Modula­2;
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LISP; Prolog; Assembly; C; Haskell; ML; Ada;
and Objective C (kinda)
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Python
Not named after this...
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Python
... but this ...
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Python
... by this man ...
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Guido van Rossum
Python's BDFL

http://www.python.org/~guido/
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Blog http://neopythonic.blogspot.com/
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Now works for Google
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Python History
Implementation started Dec 1989
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Feb 1991 released to alt.sources
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Jan 1994 1.0.0 released
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Oct 2000 2.0 released
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Oct 2008 2.6 released
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Dec 2008 3.0 released
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2.7 and 3.1 in development
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Python has...
very clear, readable syntax
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strong introspection capabilities
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intuitive object orientation
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natural expression of procedural code
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full modularity, supporting hierarchical
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packages
exception­based error handling
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Python has...
very high level dynamic data types
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an extensive standard libraries and third party
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modules for virtually every task
extensions and modules easily written in C,

C++ (or Java for Jython, or .NET languages for
IronPython)
the ability to be embedded within applications
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as a scripting interface
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Python plays well with others
Python can integrate with COM, .NET, and

CORBA objects
Jython is Python for the JVM and can interact
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fully with Java classes
IronPython is Python for .NET
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Well supported in the Internet Communication
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Engine (ICE ­ http://zeroc.com)
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Python runs everywhere
All major operating systems
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Windows
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Linux/Unix
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Mac
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And some lesser ones
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OS/2
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Amiga
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Nokia Series 60 cell phones
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Python is Open
Implemented under an open source license
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Freely usable and distributable, even for
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commercial use.
Python Enhancement Proposals – PEP
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propose new features
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collecting community input
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documenting decisions
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My Python Favourites #1
The Zen of Python
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PEP 20
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Long time Pythoneer Tim Peters succinctly
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channels the BDFL's guiding principles for Python's
design into 20 aphorisms, only 19 of which have
been written down.
aphorism – A tersely phrased statement of a

truth or opinion
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The Zen of Python
Beautiful is better than ugly.
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The Zen of Python
Explicit is better than implicit.
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The Zen of Python
Simple is better than complex.
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The Zen of Python
Complex is better than
complicated.
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The Zen of Python
Flat is better than nested.
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The Zen of Python
Sparse is better than dense.
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The Zen of Python
Readability counts.
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The Zen of Python
Special cases aren't special
enough to break the rules.
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The Zen of Python
Although practicality beats
purity.
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The Zen of Python
Errors should never pass
silently.
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The Zen of Python
Unless explicitly silenced.
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The Zen of Python
In the face of ambiguity, refuse
the temptation to guess.
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The Zen of Python
There should be one — and
preferably only one — obvious
way to do it.
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The Zen of Python
Although that way may not be
obvious at first unless you're
Dutch.
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The Zen of Python
Now is better than never.
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The Zen of Python
Although never is often better
than right now.
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The Zen of Python
If the implementation is hard to
explain, it's a bad idea.
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The Zen of Python
If the implementation is easy to
explain, it may be a good idea.
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The Zen of Python
Namespaces are one honking
great idea — let's do more of
those!
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Hello World
Python 2.6
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print “Hello World”
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Pyton 3.0
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print(“Hello World”)
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My Python Favourites #2
The interactive interpreter
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Datatypes
All the usual suspects
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Strings (Unicode)
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int
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bool
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float (only one real type)
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complex
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files
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Unusual Suspects
long — automatic promotion from int if needed
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>>> x = 1024
>>> x ** 50
327339060789614187001318969682759915221
664204604306478948329136809613379640467
455488327009232590415715088668412756007
1009217256545885393053328527589376L
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Built­in datastructures
Tuples – Fixed Length
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(1, 2, 3, “Hello”, False)
Lists
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[1, 2, 4, “Hello”, False]
Dictionaries
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{42: “The answer”, “key”: “value”}
Sets
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set([“list”, “of”, “values”])
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Functions
Python uses whitespace to determine blocks of
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code (please don't use tabs)
def greet(person):
if person == “Tim”:
print “Hello Master”
else:
print “Hello %s” % person
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Parameter Passing
Order is important unless using the name
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def foo(name, age, address)
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foo('Tim', address='Home', age=36)
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Default arguments are supported
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def greet(name='World')
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Variable length args acceptable as a list or dict
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def foo(*args, **kwargs)
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Classes
class MyClass:
\"\"\"This is a docstring.\"\"\"
name = \"Eric\"
def say(self):
return \"My name is %s\" % self.name
instance = MyClass()
print instance.say()
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Modules
Any python file is considered a module
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Modules are loaded from the PYTHONPATH
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Nested modules are supported by using
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directories.
~/src/lazr/enum/__init__.py
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If PYTHONPATH includes ~/src
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import lazr.enum
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Exceptions
Also used for flow control – StopIteration
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Exceptions are classes, and custom exceptions
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are easy to write to store extra state information
raise SomeException(params)
try:
# Do stuff
except Exception, e:
# Do something else
finally:
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# Occurs after try and except block

Duck Typing
If it walks like a duck and quacks like a duck, I
would call it a duck – James Whitcomb Riley
There is no function or method overriding
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Methods can be checked using getattr
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Consider zope.interface
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Batteries Included
The Python standard library is very extensive
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regular expressions, codecs
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date and time, collections, theads and mutexs
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OS and shell level functions (mv, rm, ls)
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Support for SQLite and Berkley databases
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zlib, gzip, bz2, tarfile, csv, xml, md5, sha
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logging, subprocess, email, json
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httplib, imaplib, nntplib, smtplib
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and much, much more
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Metaprogramming
Descriptors
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Decorators
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Meta­classes
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My Python Favourites #3
The Python debugger
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import pdb;
pdb.set_trace()
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Other Domains
Asynchronous Network Programming
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Twisted framework ­ http://twistedmatrix.com
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Scientific and Numeric
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Bioinformatics ­ biopython
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Linear algebra, signal processing – SciPy
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Fast compact multidimensional arrays – NumPy
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Desktop GUIs
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wxWidgets, GTK+, Qt
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What is Python bad at?
Anything that requires a lot of mathmatical

computations
Anything that wants to use threads across
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cores or CPUs
Real­time systems
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Work arounds
Write extension libraries in C or C++
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Use multiple processes instead of multiple

threads
Use a different language
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NZ Python Users Group
http://nzpug.org
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Regional meetings, DunPUG
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Mailing list using google groups
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Planning KiwiPyCon
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2 day event over a weekend in Christchurch
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7­8 November 2009 (that's this year!)
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Questions?
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