PYTHON INTRODUCTION31 May 2012

Colin Sturm

OVERVIEW

Brief history

Why use Python?

Syntax, command line and scripting

Good programming techniques

Real examples

HISTORY AND BACKGROUND OF PYTHON

Guido van Rossum from Holland Developed Python 1.0 in the late 1980s. Officially

released December 1989. Python 2.0 released in 2000 Python 3.0 released in 2008

Name originates from Monty Python Open source and free to use. Any style of programming can be used:

E.g. object-oriented, structured, functional

WHY USE PYTHON? Open source and free. Scripting language for fast turn around. No compiling which makes debugging easier

and faster. Designed to be readable. Cross platform functionality. Works natively on

Windows, Mac, and Linux Python does a lot of the work for you

No need to declare variables, clean memory, bookkeep

Many companies have built their programs with Python: Altair, Ensight, Dropbox, Civilization IV, Battlefield

2, Gmail backend, etc.

EXAMPLE: CREATE FAST COMPARISONS

EXAMPLE: COMPARE DATA WITH GRAPHS

EXAMPLE: LOAD VIDEO FOR COMPARISON

EXAMPLE: EXTRACT DATA QUICKLY

Need to download specific files spread over several directories

Save the data locally after located remotely

GET STARTED Open Python terminal or

command prompt: Either search for Python (command

line) from the start menu or Start a command prompt using cmd

(better option because the working environment will be in the directory that python is started)

If typing python doesn’t work at the command prompt then add the path as shown in the image: Control Panel -> system -> Advanced

system setting -> Environment variables -> edit Path under system variables -> add at the end of the line ;C:\Python27 (assuming you are using Python 2.7 and it is located here). Save and reopen a command prompt

PYTHON PROMPT The prompt will look

similar to the image. Commands can be

directly typed at the prompt and executed by pressing enter.

This environment can be used to directly test pieces of code.

To exit the environment in Windows OS use: Ctrl-Z then enter

SYNTAX: TEXT AND MATH

text = “Hello World!” print text print text[1] print text[-1] print text[:3] print len(text) print text*3 print text + text

Mathematics x=2; y=7 print x**2 + x*y + y**2 – x/y y % x this is the modulus

10 % 6 evaluates to 4

SYNTAX: LISTS AND DICTIONARIES Lists

people = [‘Matlock’, ‘Bussum’, ‘McDuff’, ‘Sheth’, “Sturm”]

children = [2, 5, 0, 1, 0] various = [‘States’, 50, [“Pi”, 3.1415926] ] print sorted(people, reverse=True) various.append(False) various.pop() various.insert(42, 1)

Dictionaries family = {‘Matlock’: 4, ‘Bussum’: 7} family[‘Matlock’] for key in family: print key for key, value in family.items(): print key, value

SYNTAX: LOOPS AND STATEMENTS

When evaluating a loop or if,then statement Python requires indentation to designate start and stop of the loop or statement. Typically 4 spaces are used. if 2 == 2: print “this statement is true” print “a second line”

Code can also be written on a single line if 2 in children: print “yes”

For multiple lines of code on the same line separate using ; if 2 in children: print “yes”; children.append(10) Note: spacing is not required for single line code

SYNTAX: LOOPS AND STATEMENTS The statement and loop example were written as

text and could be run from a file. If the same code were written in the python environment command prompt you would end the statement with a return on a blank line.

Note: in the screenshot four spaces were used as the indent. The same script would work if any number of spaces were used for indent as long as the number is consistent from line to line.

SYNTAX: LOOPS AND STATEMENTS For loops can be of a range or a list. Technically a

range is a generated list of numbers. for i in range(10): print i for i in children: print i

While loop: While 2 == 2: print something

While condition uses the same syntax. While Do is implemented using while true … break while True: print “True”; break

Opening files for input or manipulation infile = open(“filename”,’r’)

print infile.read() for line in infile: print line

infile.close()

SYNTAX: LOOPS AND STATEMENTS

Variables do not have to be declared directly but can de useful to be explicit var = int(3.14) var = float(3.14)

User input is easily done at the prompt. user_text = input(“Enter a number: “)

To be safer use raw_input which is not evaluated directly and is saved as a string user_text = raw_input(“Enter anything: “)

Code can be commented out or comments added using # at the beginning of the comment anywhere on the line.

FINDING HELP

http://docs.python.org/ Internet search / Text books In the python environment:

import math dir(math)

Lists all of the possible functions in the math class help(math)

Details what each function in math does help(math.function_name)

Details about function_name found in the math class Ex: help(math.pow)

pow(x, y) Return x**y (x to the power of y).

PROGRAMMING TECHNIQUES PEP8 – python style guide ALWAYS Make Comments

Sometimes it’s annoying to add comments but comments will always help later on. Even code that seems straight forward can become confusing on what is exactly done as time passes.

Add a header to programs – a quick definition of the program ################ ## This program turns on the flux capacitor ## Written by Colin Sturm on 31/May/2012 ################

Plenty of space, avoid dense code blocks Useful variable names

Use start_time in place of st Makes search and replace in code much easier

Consistent formatting Constantly test while coding Avoid hard coding constants, make everything a variable Keep related code close together

CREATING A SCRIPT Open a new file in your favorite text editor.

Preferably a text editor that has syntax highlighting (IDLE comes with Python).

Enter the code you want to execute: ########################################## ## Find the sum of the digits for the number 2^100 ## Written by Colin Sturm on 31/May/2012 ########################################## number = str(2**100) sum_of_digits = 0 for i in number: # sum_of_digits = sum_of_digits + int(i) sum_of_digits += int(i)OR for i in range(len(number)): # sum_of_digits = sum_of_digits + int(number[i]) sum_of_digits += int(number[i])

print sum_of_digits Save the file as desired with extension py. (numsum.py)

EXECUTING A SCRIPT At the command prompt:

python numsum.py In the python environment ran from the desired

directory: import numsum or execfile(‘numsum.py’) In the environment you can type in variables from your

script to view the stored values: number = '1267650600228229401496703205376'

To handle directories in the python environment: import os os.getcwd()

'/home/user‘ os.chdir("/tmp/") os.getcwd()

'/tmp'

EXAMPLE: USING FOR LOOPS

Problem 1 (from projecteuler.net): If we list all the natural numbers below 10 that

are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

Find the sum of all the multiples of 3 or 5 below 1000.

SOLUTION

######################################

#Find the sum of all the multiples of 3 or 5 below 1000.

######################################

count = 0 for i in range(1000): if i % 3 == 0 or i % 5 == 0:count += i print count #answer: 233168

EXAMPLE: MORE LOOPS

Problem 2 (from projecteuler.net): Each new term in the Fibonacci sequence is

generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

SOLUTION i = 1 j = 1 count = 0 while True: if j % 2 == 0: count += j temp = j j = i+j i = temp if j >= 4000000:break print count # answer: 4613732

EXAMPLE: READING TEXT DATA

Problem 3 (from Colin Sturm): Data is in the form:

Tue 05/29/2012 09:34:02.93 01 How many occurrences of 13 in the last column can be

found? How many entries for Wed?Tue 05/29/2012 09:34:02.93 01

Tue 05/29/2012 12:18:00.49 11

Tue 05/29/2012 12:33:14.18 12

Tue 05/29/2012 13:11:45.67 13

Tue 05/29/2012 13:11:48.43 03

Tue 05/29/2012 13:59:49.59 04

Tue 05/29/2012 15:24:33.24 13

Tue 05/29/2012 16:17:20.54 13

Tue 05/29/2012 17:15:25.84 13

Tue 05/29/2012 17:50:15.45 13

Tue 05/29/2012 18:17:35.12 02

Wed 05/30/2012 09:23:38.48 01

data.txt

Wed 05/30/2012 12:52:34.38 03

Wed 05/30/2012 14:05:55.88 04

Wed 05/30/2012 14:38:29.20 13

Wed 05/30/2012 16:16:52.53 13

Wed 05/30/2012 17:11:50.93 13

Wed 05/30/2012 18:03:58.72 13

Wed 05/30/2012 18:11:13.24 02

Thu 05/31/2012 08:27:15.67 01

Thu 05/31/2012 08:27:15.67 05

Thu 05/31/2012 09:06:58.87 06

Thu 05/31/2012 09:28:33.20 05

Thu 05/31/2012 10:16:13.58 06

Thu 05/31/2012 10:25:31.49 13

Thu 05/31/2012 11:01:20.94 11

Thu 05/31/2012 11:18:26.31 12

Thu 05/31/2012 11:56:39.87 13

Thu 05/31/2012 11:59:08.26 03

Thu 05/31/2012 13:07:45.51 04

Thu 05/31/2012 13:21:40.40 13

Thu 05/31/2012 15:04:22.63 13

Thu 05/31/2012 15:52:45.47 13

Thu 05/31/2012 17:11:12.89 13

Thu 05/31/2012 18:09:18.71 02

SOLUTION count = 0 daycount = 0 infile = open('data.txt','r') for line in infile: line = line.split() if line[3] == '13': count += 1 if line[0] == 'Wed': daycount += 1 print str(count)+’ occurrences of 13 and ’ +

str(daycount) + ‘entries on Wed’ # answer: 15 occurrences of 13 and 8 entries on

Wed

EXAMPLE: CREATING TEXT DATA

Problem: Your simulation program needs to read in a

simple text file with increasing times listed on each line.

You have a pipe with length 1000 m and an inlet velocity of 100 m/s. The pipe is divided into 70 sections and you need the particle injection times to have evenly spaced particles of 4 impulses per section over the span of 5 seconds.

SOLUTION per_section = 4 length = 1000 elements_per_length = 70 inlet_velocity = 100 total_time = 5

import math

emission_time = length / elements_per_length / inlet_velocity / per_section

total_count = int(math.ceil(total_time / emission_time)) print “Continual time emission in seconds: " +

str(emission_time) f = open("emission_times",'w') for i in xrange(total_count+1): f.write(str(round(i*emission_time,3))+"\n") f.close()