introduction to python, fundamentals and basics

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

©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
– By and large they are all mutually compatible
– Recommended version: 2.1.1 or 2.2
– Oldest version still in widespread use: 1.5.2
– Avoid 1.6/1.6.1 if you can
– When using 2.0 or 2.1, upgrade to 2.0.1 / 2.1.1
– 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
– 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:
– numbers, strings, tuples of immutables
• these cannot be changed after creation
– reason is hashing (fast lookup technique)
– not lists or other dictionaries
• these types of objects can be changed "in place"
– no restrictions on values
• Keys will be listed in arbitrary order
– 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:
– (1) instance, (2) class, (3) base classes
– this also works for method lookup
• On assignment via instance (self.x = ...):
– always makes an instance variable
• Class variables "default" for instance
variables
• But...!
– mutable class variable: one copy shared by all
– mutable instance variable: each instance its own

Modules
• Collection of stuff in foo.py file
– functions, classes, variables
• Importing modules:
– import re; print re.match("[a-z]+", s)
– from re import match; print match("[a-z]+", s)
• Import with rename:
– import re as regex
– from re import match as m
– 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:
– from P.Q.M import foo; print foo()
– from P.Q import M; print M.foo()
– import P.Q.M; print P.Q.M.foo()
– 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
– subclass Exception or any other standard exception
• Old Python: exceptions can be strings
– WATCH OUT: compared by object identity, not ==
• Last caught exception info:
– sys.exc_info() == (exc_type, exc_value, exc_traceback)
• Last uncaught exception (traceback printed):
– sys.last_type, sys.last_value, sys.last_traceback
• Printing exceptions: traceback module

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

Standard Library
• Core:
– os, sys, string, getopt, StringIO, struct, pickle,
...
• Regular expressions:
– re module; Perl-5 style patterns and matching
rules
• Internet:
– socket, rfc822, httplib, htmllib, ftplib, smtplib, ...
• Miscellaneous:
– pdb (debugger), profile+pstats
– 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
– def make_adder(n):
def adder(x): return x+n
return adder
– add2 = make_adder(2)
– add2(10) == 12
• Rich comparisons
– Overload <, <=, ==, !=, >=, > separately
• Warnings framework
– 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
– class mydict(dict): …
• Fix division operator so 1/2 == 0.5; 1//2
== 0
– Requires __future__ statement in Python 2.x
– Change will be permanent in Python 3.0

URLs
• http://www.python.org
– official site
• http://starship.python.net
– Community
• http://www.python.org/psa/bookstore/
– (alias for http://www.amk.ca/bookstore/)
– 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...

introduction to python, fundamentals and basics

More Related Content

Similar to introduction to python, fundamentals and basics

More from KNaveenKumarECE

Recently uploaded

introduction to python, fundamentals and basics