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Introduction to Python
LinuxWorld - New York City - January 2002
Guido van Rossum
Director of PythonLabs at Zope Corporation
guido@python.org
guido@zope.com

�ݺ�ߣ 2 ?2001, 2002 Guido van Rossum
Why Python?
? Have your cake and eat it, too:
Productivity and readable code
? VHLLs will gain on system languages
(John Ousterhout)
? "Life's better without braces"
(Bruce Eckel)

Tutorial Outline
? interactive "shell"
? basic types: numbers, strings
? container types: lists, dictionaries, tuples
? variables
? control structures
? functions & procedures
? classes & instances
? modules & packages
? exceptions
? files & standard library
? what's new in Python 2.0 and beyond

Try It Out!
? If you brought a laptop into the classroom,
feel free to play along
? Download Python from www.python.org
? Any version will do for this class
�C By and large they are all mutually compatible
�C Recommended version: 2.1.1 or 2.2
�C Oldest version still in widespread use: 1.5.2
�C Avoid 1.6/1.6.1 if you can
�C When using 2.0 or 2.1, upgrade to 2.0.1 / 2.1.1
�C 2.1.2 is coming soon!
? Use IDLE if you can

Interactive ��Shell��
? Great for learning the language
? Great for experimenting with the library
? Great for testing your own modules
? Two variations: IDLE (GUI),
python (command line)
? Type statements or expressions at prompt:
>>> print "Hello, world"
Hello, world
>>> x = 12**2
>>> x/2
72
>>> # this is a comment

Numbers
? The usual suspects
? 12, 3.14, 0xFF, 0377, (-1+2)*3/4**5, abs(x), 0<x<=5
? C-style shifting & masking
? 1<<16, x&0xff, x|1, ~x, x^y
? Integer division truncates :-(
? 1/2 -> 0 # 1./2. -> 0.5, float(1)/2 -> 0.5
? Will be fixed in the future
? Long (arbitrary precision), complex
? 2L**100 -> 1267650600228229401496703205376L
�C In Python 2.2 and beyond, 2**100 does the same thing
? 1j**2 -> (-1+0j)

Strings
? "hello"+"world" "helloworld" # concatenation
? "hello"*3 "hellohellohello" # repetition
? "hello"[0] "h" # indexing
? "hello"[-1] "o" # (from end)
? "hello"[1:4] "ell" # slicing
? len("hello") 5 # size
? "hello" < "jello" 1 # comparison
? "e" in "hello" 1 # search
? "escapes: n etc, 033 etc, if etc"
? 'single quotes' """triple quotes""" r"raw strings"

Lists
? Flexible arrays, not Lisp-like linked lists
? a = [99, "bottles of beer", ["on", "the", "wall"]]
? Same operators as for strings
? a+b, a*3, a[0], a[-1], a[1:], len(a)
? Item and slice assignment
? a[0] = 98
? a[1:2] = ["bottles", "of", "beer"]
-> [98, "bottles", "of", "beer", ["on", "the", "wall"]]
? del a[-1] # -> [98, "bottles", "of", "beer"]

More List Operations
>>> a = range(5) # [0,1,2,3,4]
>>> a.append(5) # [0,1,2,3,4,5]
>>> a.pop() # [0,1,2,3,4]
5
>>> a.insert(0, 42) # [42,0,1,2,3,4]
>>> a.pop(0) # [0,1,2,3,4]
5.5
>>> a.reverse() # [4,3,2,1,0]
>>> a.sort() # [0,1,2,3,4]

Dictionaries
? Hash tables, "associative arrays"
? d = {"duck": "eend", "water": "water"}
? Lookup:
? d["duck"] -> "eend"
? d["back"] # raises KeyError exception
? Delete, insert, overwrite:
? del d["water"] # {"duck": "eend", "back": "rug"}
? d["back"] = "rug" # {"duck": "eend", "back": "rug"}
? d["duck"] = "duik" # {"duck": "duik", "back": "rug"}

More Dictionary Ops
? Keys, values, items:
? d.keys() -> ["duck", "back"]
? d.values() -> ["duik", "rug"]
? d.items() -> [("duck","duik"), ("back","rug")]
? Presence check:
? d.has_key("duck") -> 1; d.has_key("spam") -> 0
? Values of any type; keys almost any
? {"name":"Guido", "age":43, ("hello","world"):1,
42:"yes", "flag": ["red","white","blue"]}

Dictionary Details
? Keys must be immutable:
�C numbers, strings, tuples of immutables
? these cannot be changed after creation
�C reason is hashing (fast lookup technique)
�C not lists or other dictionaries
? these types of objects can be changed "in place"
�C no restrictions on values
? Keys will be listed in arbitrary order
�C again, because of hashing

Tuples
? key = (lastname, firstname)
? point = x, y, z # parentheses optional
? x, y, z = point # unpack
? lastname = key[0]
? singleton = (1,) # trailing comma!!!
? empty = () # parentheses!
? tuples vs. lists; tuples immutable

Variables
? No need to declare
? Need to assign (initialize)
? use of uninitialized variable raises exception
? Not typed
if friendly: greeting = "hello world"
else: greeting = 12**2
print greeting
? Everything is a "variable":
? Even functions, classes, modules

Reference Semantics
? Assignment manipulates references
? x = y does not make a copy of y
? x = y makes x reference the object y references
? Very useful; but beware!
? Example:
>>> a = [1, 2, 3]
>>> b = a
>>> a.append(4)
>>> print b
[1, 2, 3, 4]

a
1 2 3
b
a
1 2 3
b
4
a = [1, 2, 3]
a.append(4)
b = a
a 1 2 3
Changing a Shared List

a
1
b
a
1
b
a = 1
a = a+1
b = a
a 1
2
Changing an Integer
old reference deleted
by assignment (a=...)
new int object created
by add operator (1+1)

Control Structures
if condition:
statements
[elif condition:
statements] ...
else:
statements
while condition:
statements
for var in sequence:
statements
break
continue

Grouping Indentation
In Python:
for i in range(20):
if i%3 == 0:
print i
if i%5 == 0:
print "Bingo!"
print "---"
In C:
for (i = 0; i < 20; i++)
{
if (i%3 == 0) {
printf("%dn", i);
if (i%5 == 0) {
printf("Bingo!n"); }
}
printf("---n");
}
0
Bingo!
---
---
---
3
---
---
---
6
---
---
---
9
---
---
---
12
---
---
---
15
Bingo!
---
---
---
18
---
---

Functions, Procedures
def name(arg1, arg2, ...):
"""documentation""" # optional doc string
statements
return # from procedure
return expression # from function

Example Function
def gcd(a, b):
"greatest common divisor"
while a != 0:
a, b = b%a, a # parallel assignment
return b
>>> gcd.__doc__
'greatest common divisor'
>>> gcd(12, 20)
4

Classes
class name:
"documentation"
statements
-or-
class name(base1, base2, ...):
...
Most, statements are method definitions:
def name(self, arg1, arg2, ...):
...
May also be class variable assignments

Example Class
class Stack:
"A well-known data structure��"
def __init__(self): # constructor
self.items = []
def push(self, x):
self.items.append(x) # the sky is the limit
def pop(self):
x = self.items[-1] # what happens if it��s
empty?
del self.items[-1]
return x
def empty(self):
return len(self.items) == 0 # Boolean result

Using Classes
? To create an instance, simply call the class object:
x = Stack() # no 'new' operator!
? To use methods of the instance, call using dot
notation:
x.empty() # -> 1
x.push(1) # [1]
x.empty() # -> 0
x.push("hello") # [1, "hello"]
x.pop() # -> "hello" # [1]
? To inspect instance variables, use dot notation:
x.items # -> [1]

Subclassing
class FancyStack(Stack):
"stack with added ability to inspect inferior stack items"
def peek(self, n):
"peek(0) returns top; peek(-1) returns item below that; etc."
size = len(self.items)
assert 0 <= n < size # test precondition
return self.items[size-1-n]

Subclassing (2)
class LimitedStack(FancyStack):
"fancy stack with limit on stack size"
def __init__(self, limit):
self.limit = limit
FancyStack.__init__(self) # base class
constructor
def push(self, x):
assert len(self.items) < self.limit
FancyStack.push(self, x) # "super" method call

Class / Instance Variables
class Connection:
verbose = 0 # class variable
def __init__(self, host):
self.host = host # instance variable
def debug(self, v):
self.verbose = v # make instance variable!
def connect(self):
if self.verbose: # class or instance variable?
print "connecting to", self.host

Instance Variable Rules
? On use via instance (self.x), search order:
�C (1) instance, (2) class, (3) base classes
�C this also works for method lookup
? On assignment via instance (self.x = ...):
�C always makes an instance variable
? Class variables "default" for instance
variables
? But...!
�C mutable class variable: one copy shared by all
�C mutable instance variable: each instance its own

Modules
? Collection of stuff in foo.py file
�C functions, classes, variables
? Importing modules:
�C import re; print re.match("[a-z]+", s)
�C from re import match; print match("[a-z]+", s)
? Import with rename:
�C import re as regex
�C from re import match as m
�C Before Python 2.0:
? import re; regex = re; del re

Packages
? Collection of modules in directory
? Must have __init__.py file
? May contain subpackages
? Import syntax:
�C from P.Q.M import foo; print foo()
�C from P.Q import M; print M.foo()
�C import P.Q.M; print P.Q.M.foo()
�C import P.Q.M as M; print M.foo() # new

Catching Exceptions
def foo(x):
return 1/x
def bar(x):
try:
print foo(x)
except ZeroDivisionError, message:
print "Can��t divide by zero:", message
bar(0)

Try-finally: Cleanup
f = open(file)
try:
process_file(f)
finally:
f.close() # always executed
print "OK" # executed on success only

Raising Exceptions
? raise IndexError
? raise IndexError("k out of range")
? raise IndexError, "k out of range"
? try:
something
except: # catch everything
print "Oops"
raise # reraise

More on Exceptions
? User-defined exceptions
�C subclass Exception or any other standard exception
? Old Python: exceptions can be strings
�C WATCH OUT: compared by object identity, not ==
? Last caught exception info:
�C sys.exc_info() == (exc_type, exc_value, exc_traceback)
? Last uncaught exception (traceback printed):
�C sys.last_type, sys.last_value, sys.last_traceback
? Printing exceptions: traceback module

File Objects
? f = open(filename[, mode[, buffersize])
�C mode can be "r", "w", "a" (like C stdio); default "r"
�C append "b" for text translation mode
�C append "+" for read/write open
�C buffersize: 0=unbuffered; 1=line-buffered; buffered
? methods:
�C read([nbytes]), readline(), readlines()
�C write(string), writelines(list)
�C seek(pos[, how]), tell()
�C flush(), close()
�C fileno()

Standard Library
? Core:
�C os, sys, string, getopt, StringIO, struct, pickle,
...
? Regular expressions:
�C re module; Perl-5 style patterns and matching
rules
? Internet:
�C socket, rfc822, httplib, htmllib, ftplib, smtplib, ...
? Miscellaneous:
�C pdb (debugger), profile+pstats
�C Tkinter (Tcl/Tk interface), audio, *dbm, ...

Python 2.0: What's New
? Augmented assignment: x += y
? List comprehensions:
[s.strip() for s in f.readlines()]
? Extended print: print >>sys.stderr, "Hello!"
? Extended import: import foo as bar
? Unicode strings: u"u1234"
? New re implementation (faster, Unicode)
? Collection of cyclic garbage
? XML, distutils

? From __future__ import nested_scopes
�C def make_adder(n):
def adder(x): return x+n
return adder
�C add2 = make_adder(2)
�C add2(10) == 12
? Rich comparisons
�C Overload <, <=, ==, !=, >=, > separately
? Warnings framework
�C Prepare for the future

? Iterators and Generators
? from __future__ import generators
def inorder(tree):
if tree:
for x in inorder(tree.left): yield x
yield tree.label
for x in inorder(tree.right): yield x
? Type/class unification
�C class mydict(dict): ��
? Fix division operator so 1/2 == 0.5; 1//2
== 0
�C Requires __future__ statement in Python 2.x
�C Change will be permanent in Python 3.0

URLs
? http://www.python.org
�C official site
? http://starship.python.net
�C Community
? http://www.python.org/psa/bookstore/
�C (alias for http://www.amk.ca/bookstore/)
�C Python Bookstore

Further Reading
? Learning Python: Lutz, Ascher (O'Reilly '98)
? Python Essential Reference: Beazley (New Riders '99)
? Programming Python, 2nd Ed.: Lutz (O'Reilly '01)
? Core Python Programming: Chun (Prentice-Hall '00)
? The Quick Python Book: Harms, McDonald (Manning '99)
? The Standard Python Library: Lundh (O'Reilly '01)
? Python and Tkinter Programming: Grayson (Manning '00)
? Python Programming on Win32:
Hammond, Robinson (O'Reilly '00)
? Learn to Program Using Python: Gauld (Addison-W. '00)
? And many more titles...

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