mkaz.com : My Python Notes

My Python Notes

Marcus Kazmierczak, marcus@mkaz.com
Created On: July 29, 2004
Last Updated: August 14, 2005

Introduction
List Data Structures
Dictionary Data Structures
Sets Data Structures
Working with Files
Misc Stuff
Full Examples

Introduction

Here are my notes for programming in Python. Lately, I've found using Python as a more rewarding and friendly language then Perl, which is where I have a lot of experience. Python is nice especially for co-development and maintaining code down-the-line. Python code tends to be a little less cryptic since there is usually only one way of doing things; which is definitely not the Perl way. To be honest though, I can still do anything in Perl faster.

If you are really into these holy wars you can read about the great Python -vs- Perl debate here, here and even here. But as some one who just wants to do some work and stop wasting time. Who cares, pick a language and Just Code Baby™.

List Data Structures

Names: Lists, Arrays, Tuples, (Strings). A tuple is a list that can't be changed; so append and delete do not work on tuples.

Create Empty List:

mylist = []

Create Shopping List:

mylist = ['apples', 'bananas', 'oranges']

Print List Item:

print mylist[0]
>> apples

Append to List:

mylist.append('coffee')

Delete from List:

del mylist['coffee']

Size of List:

size = len(mylist)

Dict Data Structures

Names: Dictionaries, Dicts, Hash, Associative Array

Create Empty Dict:

mydict = {}

Create Address Dict:

mydict = {
    'firstname': 'Marcus',
    'lastname': 'Kazmierczak',
    'e-mail': 'marcus@mkaz.com'}

NOTE: That can all be on one line if you like

Print Dict Item:

print mydict['firstname']
>> Marcus

Delete from Dict:

del mydict['firstname']

Size of Dict:

size = len(mydict)

List Dict Keys:

mydict.keys()
>> ['firstname', 'lastname', 'e-mail']

Sets Data Structures

Python 2.3 introduced sets as another data structure, which have the same mathematical set properties: union, intersection, etc...

import sets

Create Sets:

set1 = sets.Set(['apple','orange','banana'])
set2 = sets.Set(['blue','red','orange','green'])

Union Sets:

set1.union(set2)
>> Set(['blue', 'green', 'apple', 'orange', 'banana', 'red'])

# shortcut operator
set1 | set2

Intersection Sets:

set1.intersection(set2)
>> Set(['orange'])

# shortcut operator
set1 & set2

Difference Sets:

set1.difference(set2)
>> Set(['apple', 'banana'])

set2.difference(set1)
>> Set(['blue', 'green', 'red'])

Sym Difference:

set1.symmetric_difference(set2)
>> Set(['blue', 'green', 'apple', 'red', 'banana'])

Check if Element in Set:

'apple' in set1

Working with Files

Read Lines from File

filename = "files.txt"
f = open(filename, 'r')
lines = f.readlines();
for line in lines:
print line
f.close()

Check if File/Directory Exists, similar to Perl's -e checks
Use the os module, see the full documentation at http://python.org/doc/current/lib/module-os.path.html

import os

file = "/my/file/path/name"

if os.path.exists(file):
    print "%s exists" % file

# here are some other things you can check for
if os.path.isabs(file):
    print "%s is absolute path" % file

if os.path.isdir(file):
    print "%s is directory" % file

if os.path.isfile(file):
    print "%s is file " % file

if os.path.islink(file):
    print "%s is link" % file

if os.path.ismount(file):
    print "%s is a mount point" % file

Grab a list of files in directory, similar to Perl's glob

dir = "/my/dir"

files = os.listdir(dir)
for file in files:
    print file

# or if you only want *.py
for file in os.listdir(dir):
    if (file.endswith(".py")):
        print file

# or if you want all files the start with 'test'
for file in os.listdir(dir):
    if (file.startswith("test")):
        print file

Misc Snippets

Read User Input from STDIN, Prompt User

cont = raw_input("Continue [Y/n]? ")

It's nice to strip out any whitespace and the newline, this can be done in one line:

cont = raw_input("Continue [Y/n]? ").strip()

Read Data in from STDIN

data = sys.stdin.read()

Full Examples

GetOpt Example - Parsing Command Line Arguments: getopt_ex.py.html

Process E-mail Message, pipe STDIN, write file: sproc.py.html

	Learning Python by Mark Lutz; Paperback; Buy New: $23.77 Buy now from Amazon.com

	Python Cookbook Alex Martelli; Paperback; Buy New: $27.17 Buy now from Amazon.com

My Python Notes

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