practical python programmingccn/teaching/ppp2013_tasks.pdf · practical python programming spring...

Practical Python Programming Spring 2013

[email protected] of 17

Practical Python Programming

[email protected]

May 23rd and 24th, 2013

9:00 a.m. to 12:00 noon

BioSci B118

This is an informal introduction to the basic concepts and features of Python - an interpreted,

object-oriented, high-level programming language that is used in a wide variety of application

domains (www.python.org) - but really it is "a programming language that knows how to stay out

of your way when you write your programs. It enables you to implement the functionality you

want without any hassle, and lets you write programs that are clear and readable" (Magnus Lie

Hetland). Python is:

• powerful... and fast

• plays well with others

• runs everywhere

• is friendly... and easy to learn

• is Open

Through hands-on exercises, you will learn basic calculations, work with coordinate data, and

get a taste of just some of the many library modules (primarily in preparation for scripting with

ArcGIS software).

“If you're using a programming language named after a sketch comedy troupe,

you had better have a sense of humor.” www.python.org

http://www.python.org/



Tasks The workshop’s coding samples and scripts are shown below as plain text and screen captures

from IDLE, so you know exactly what you should be typing in yourself. Please see the

companion slides for more information.

Note: This document is named ppp_tasks_s2013.pdf and the companion slides are named

ppp_slides_s2013.pdf. The companion slides have information on the following background

information, programming concepts, and tutorial theory:

1. Who, when, what, where, why, how?

2. Easy environments

3. Programming primer

4. Coding conventions

5. Fussing with the flow

6. Script samples

7. Excellent e-resources

Environments and Primer

Tip: Make note of what happens with each of the lines of code.

Click the START button > PROGRAMS > ARCGIS > PYTHON 2.6 > IDLE (PYTHON GUI)

The following text – except do NOT type the ‘>>>’ prompt – can be typed directly in to the

interactive window (also called the Python Shell window):

>>> import this

>>> help()

>>> import

>>> topics

>>> NUMBERS

>>> strings # note the lower case

>>> quit

>>> help(keywords) # now retype with quotes around 'keywords'

>>> help('print')

>>> print 'what is a blue moon?'

>>> abluemoon = 'second to last full moon in a four-moon season'

>>> print abluemoon

NOTE: We are using the ArcGIS default

installation of the Integrated DeveLopment

Environment because it is readily available

for GIS development. Many other IDEs are

available for your use, but this one makes

interacting with ArcGIS easy peasy.



>>> for once in abluemoon:

print once # we’ll learn more about looping later

Tip: Type Alt-P a couple of times (and try Alt-N)

>>> 1 + 2 + 3

>>> _

>>> _ - 4

>>> a = 1

>>> b, c = 2, 3 # we’ll do more variables later

>>> A + b * c # then type a + b * c

>>> 'stuff'

>>> _

Note: The default result variable ‘_’ is available only in the interactive window.

>>> # calculator expressions

>>> 1 / 2

>>> 1 / 2.0

>>> 1.0 / 2

>>> -3 ** 2

>>> (-3) ** 2

>>> 'p' + 'p' + 'p'

>>> 'p' * 3

>>> 'p' + 3

>>> 'Ha, ' * 5 + 'Ha!'

Note: Try a few calculations on your own.

>>> # commenting and docstrings

>>> # dear interpreter, please ignore all comments, thank you



>>> """ Geography is an important antidote to the "infantile"

habit of thinking the world is a laboratory in which we can

carry out all kinds of experiments, or a huge rubbish heap where

we can get rid of all our trash. """

>>> """ Type something

and type some more

and more

and then some more again

and maybe

put yourself out of your misery

and finish """

>>> print _

>>> # print statements

>>> print 'And now for something completely different!'

>>> print 'And now for something\ncompletely different!'

>>> print 'Green' + 'eggs' + 'and' + 'ham'

>>> print 'Green', 'eggs', 'and', 'ham'

>>> print 'Today is', 24, 'February', 2011

>>> print "This is my pretend long string \

that I just don't want to fit on one line"

>>> print "Let's go!"

>>> print 'Let\'s go!'

>>> print "She said, \"Hello\""

>>> print 'C:\new folder'

>>> print 'C:\temp'

>>> print 'C:\\newfolder', 'C:\\temp'

>>> # sequences (lists and strings)

>>> mylist = [123, 456, 'seven ate nine']

>>> mylist[0]



>>> mylist[1]

>>> mylist[2]

>>> mylist[-1]

>>> 'mylist'[0]

>>> 'mylist'[2]

>>> list('mylist')

>>> 'm' in mylist

>>> 'm' in 'mylist'

Note: An entire day could be devoted to a workshop on lists, strings, other sequences such as

tuples, and all kinds of indexing, formatting, operations, slicing, and methods! The examples

here are modestly minor. An excellent quick reference for many topics is available here:

http://rgruet.free.fr/PQR26/PQR2.6.html.

First file

In the Python Shell (interactive window), click FILE >>> NEW WINDOW

Tip: It’s a good idea to type a few meaningful comments at the top of your script file. Once you

have opened a new file editor script window, type the following, including the blank lines for

readability (note: choose your preference for commenting using the ‘#’ character OR the triple-

quoted string):

# Date: yyyymmdd

# By: [email protected]

# Description: This script is helping me learn Python

OR

"""

Date: yyyymmdd

By: [email protected]

Description: This script is helping me learn Python

"""

Click FILE >>> SAVE as C:\Workspace\ppp\sequences.py (do NOT forget the .py)

http://rgruet.free.fr/PQR26/PQR2.6.html



In the interactive window, type help(‘SEQUENCES’) and give the documentation a

quick look through. Similarly, type help(‘METHODS’) , help(‘string’), and help(‘list’) and

examine those topics. Return your focus to the script window and start typing:

# the following sequence commands and methods use palindromes

pal1 = "never odd or even"

pal2 = 'kayak'

pal3 = 'a man a plan a canal panama'

# fun with strings - indices point between characters

print pal1.capitalize()

print pal1[0]

print pal1[-1]

print pal1[1:4]

print pal1[0:5]

print pal1[6:]

# type the dot, press ctrl-space, and use arrow key to select

method

print pal2.upper()

print pal2[-3:]

length = len(pal2)

print '# of characters in "' + pal2 + '" =' + length

Click RUN >>> RUN MODULE. Click OK to SAVE. Can you figure out what the error

message is? Change the last ‘+’ to a ‘,’ or apply the str() function.

print '# of characters in "' + pal2 + '" =', length



OR

print '# of characters in "' + pal2 + '" =' + str(length)

print pal2.strip('k')

Click RUN >>> RUN MODULE again (or press the F5 key). View the results in the

interactive window, cross-referencing with the code in the script window. Type:

print pal3.swapcase()

print 'last word: ' + pal3[21:len(pal3)]

print pal3.strip(' panama')

print pal3.lstrip(' panama')

print pal3.rstrip(' panama')

print pal3.replace('a', '@')

Click RUN >>> RUN MODULE again (or press the F5 key). Examine.

Now, use the mouse cursor to click and drag to highlight/select all the text from “# fun

with strings…” to the end of the file. Click FORMAT > COMMENT OUT REGION.

Click RUN again to see what happens.

Type some more, alternating between running the code and inspecting the results as

you go along. Note: Wherever you see blank lines would be a good point to RUN.

# fun with lists

list1 = list(pal3)

print list1

print list1.count('a')

list2 = pal3.split()

print list2

list3 = list2



list3.sort()

print list3

print list2[0]

print list2[-2:]

list4 = [10, 20, 30, 40, 50]

print list4

print list4[1]

print len(list4)

list4.append(60)

print list4

# not sequences, but useful functions to know

a = 1

b = 2

c, d = 'three', '100'

print a + b

print str(a) + str(b) + c

print int(d)

print float(d)

goodbye = '\nokay, that\'s pretty good for a first script file!'

print '*' * len(goodbye)

print goodbye



Tip: Remember the useful trick of highlighting lines of your program and then using the

FORMAT menu to COMMENT OUT REGION and UNCOMMENT REGION. The INDENT

REGION and DEDENT REGION will come in handy when you start using loops and other

constructs requiring blocks of code.

Coding conventions

Thoroughly read the Style Guide for Python Code:

http://www.python.org/dev/peps/pep-0008/

Fussing with the flow

Start a new file for each of the scripts below. Give them a meaningful file name with a .py

extension! Make notes (comments in the code will do!) if you think the instructor says something

important during this hands-on demonstration of programming with control and decision.

Extra ‘if’ example:

# test if number is even or odd

number = input("Tell me a number: ")

if number % 2 == 0:

print number, "is even."

elif number % 2 == 1:

print number, "is odd."

else:

print number, "is not an integer."




Reminder on IDLE for Python (as installed with ArcGIS)

1. Click START > PROGRAMS > ARCGIS > PYTHON 2.6 > IDLE (PYTHON GUI)

2. Click HELP > IDLE HELP for a quick recap on the integrated development environment

application that installs with Python; close when finished reading

3. Click FILE > NEW WINDOW to open the file editor where you will type the workshop’s

coding samples and scripts as shown below as plain text and screen captures from IDLE

4. Type a few meaningful comments at the top of your script file, similar to below (choose

your preference for commenting using the ‘#’ character OR the triple-quoted string):

# Date: yyyymmdd

# By: [email protected]

# Description: This script is helping me learn Python

5. Click FILE >>> SAVE as C:\Workspace\ppp\scriptname.py (do NOT forget the .py)

6. Click RUN >>> RUN MODULE (or press the F5 key). Click OK to SAVE. View the results

in the interactive window, cross-referencing with the code in the script window.

7. You are strongly encouraged to access Python’s built-in and online HELP.



Tip: Remember the useful trick of highlighting lines of your program and then using the

FORMAT menu to COMMENT OUT REGION and UNCOMMENT REGION. The INDENT

REGION and DEDENT REGION will come in handy when you start using loops and other

constructs requiring blocks of code.

Script samples

Start a new file for each of the scripts below. Make notes (comments in the code will do!) if you

think the instructor says something important during this hands-on demonstrations of practical

Python programming. Especially note the built-in modules highlighted from the standard library.

Various bits of the Python language are highlighted, along with the thought process of how to

end up with a simple program that does what you want. Regions of code have been commented

out. The instructor will guide you through typing via trail an error, along with tips and tricks, to

help you realize the complete program, plus with occasional variations. The samples alone are

not complete lessons. Tip: Zoom in on the text to see it better (small font optimizes page-fitting).

day_of_year.py

# day of year from date string – pseudo-Julian date - online help is highly recommended

# http://www.doughellmann.com/PyMOTW/time, http://docs.python.org/release/2.6.5/library/time.html

import time

date = '2011-03-19'

(year, month, day) = date.split('-')

year = int(year)

month = int(month)

day = int(day)

t = time.mktime((year, month, day, 0, 0, 0, 0, 0, 0))

print t

print time.gmtime(t)

print time.gmtime(t)[7]

list_rename_files.py

# list files of specific type in an input folder and rename

# http://docs.python.org/release/2.6.5/library/glob.html, http://www.doughellmann.com/PyMOTW/glob

import glob

import os

# variables

input_folder = 'C:\\Workspace\\ppp\\files'

remove_text = '20'

# Get listing of only TIFF file types

file_type = "\\*.tif"

data_folder = input_folder + file_type

data_list = glob.glob(data_folder)

for each in data_list:

print each

new_file = each.replace(remove_text,'')

print new_file

os.rename(each, new_file)

NOTE: The code samples

may not work as is! They are

meant to be used in guided

hands-on instruction.

http://www.doughellmann.com/PyMOTW/time

http://docs.python.org/release/2.6.5/library/time.html

http://docs.python.org/release/2.6.5/library/glob.html

http://www.doughellmann.com/PyMOTW/glob



descriptive_stats.py

# calculate descriptive statistics

##from math import sqrt

# list of values

numbers = [235, 689, 568, 654, 639, 779, 301, 655, 299, 443]

# calculations using built-in functions

minval = min(numbers)

maxval = max(numbers)

n = len(numbers)

summed = sum(numbers)

mean = sum(numbers) / n

### convert to float

##mean = sum(numbers) / float(n)

### standard deviation calculation with math module

### should be 192.02343607

##stdev = 0

##for number in numbers:

## diffsq = (number - mean)**2

## # loop for sum

## stdev += diffsq

### complete calculation

##stdev = sqrt(stdev / (n - 1))

###stdev = sqrt(stdev / float(n - 1))

##

### alternative to for loop is a list comprehension

##diffsq2 = [(number - mean)**2 for number in numbers]

##stdev2 = sqrt(sum(diffsq2) / (n - 1))

##

### another alternative to for loop is map with lambda

### http://diveintopython.org/power_of_introspection/lambda_functions.html

##diffsq3 = map(lambda number: (number - mean)**2, numbers)

##stdev3 = sqrt(sum(diffsq3) / (n - 1))

print 'Minimum:', minval

print 'Maximum:', maxval

print 'Sum:', summed

print 'Mean:', mean

##print 'Standard Deviation', stdev#, stdev2, stdev3

random_coordinates.py

# generate random coordinates and zip to tuples

# http://docs.python.org/release/2.6.5/library/random.html

# http://www.doughellmann.com/PyMOTW/random

# specify extent as top-left and bottom-right

import random

# extent x, y coordinates from TopLeft and BottomRight

TL = [170844, 6659344]

BR = [865133, 5425575]

# variables

number = 100

xmin = TL[0]

xmax = BR[0]

ymin = BR[1]

ymax = TL[1]





http://docs.python.org/release/2.6.5/library/random.html

http://www.doughellmann.com/PyMOTW/random



print xmin, xmax

print ymin, ymax

# initilaize lists

xlist = []

ylist = []

for i in xrange(1,number):

# demo this without random. module prefix

x = randint(xmin, xmax)

y = randint(ymin, ymax)

## x = random.randint(xmin, xmax)

## y = random.randint(ymin, ymax)

xlist.append(x)

ylist.append(y)

print xlist

print ylist

##coords = zip(xlist, ylist)

##

##print coords

##

### loop through tuple and print

##for each in coords:

## print 'X:' + str(each[0])

## print 'Y:' + str(each[1])

##

##print len(coords)

### why aren't there 100?

locations2dd.py

# convert DMS to decimal degrees while reading and writing text files

# http://docs.python.org/release/2.6.5/library/os.html

# http://docs.python.org/release/2.6.5/library/os.path.html

import os

# assign table file to a variable

table = 'C:\\Workspace\\ppp\\locations.txt'

# create new file for writing calculation results to

out_dir = os.path.split(table)[0]

file_name = os.path.split(table)[1]

out_file = out_dir + os.sep + 'dd_' + file_name

new_file = open(out_file, 'w')

print out_file

# open file in read mode

f = open(table, 'r')

# loop through each line

for line in f:

# print each line as stored in the table

print 'Original line:', line

# print each line items as stored in the table

print 'Original line items:', line.split(',')

# print each line replacing newline character

print 'Remove newline:', line.replace('\n','')

# print each line as converted to a list object - comma

columns = line.replace('\n','').split(',')

# assign list items to variables

oid = columns[0]





http://docs.python.org/release/2.6.5/library/os.html

http://docs.python.org/release/2.6.5/library/os.path.html



longitude = columns[1]

latitude = columns[2]

# divide column list item in to new list of three values: D, M, S

long_list = longitude.split(' ')

lat_list = latitude.split(' ')

# test if first value is a number

if long_list[0].isdigit():

# with and without str() for new_file writing demo

longdd = str(float(long_list[0]) + float(long_list[1]) / 60 + float(long_list[2]) / 3600)

latdd = str(float(lat_list[0]) + float(lat_list[1]) / 60 + float(lat_list[2]) / 3600)

else:

# not a number, so no calculations

longdd = long_list[0]

latdd = lat_list[0]

print longdd

print latdd

# concatenate expression to write to new file

new_line = oid + ',' + longdd + ',' + latdd # needs \n

## new_line = oid + ',' + longdd + ',' + latdd + '\n'

print new_line

new_file.write(new_line)

### demo the edit > replace command to change all instances of new_line to out_line

## # concatenate expression to write to new file

## out_line = oid + ',' + longdd + ',' + latdd # needs \n

## out_line = oid + ',' + longdd + ',' + latdd + '\n'

## new_file.write(out_line)

# demo the 'w' versus 'a' mode

# close both files

f.close()

new_file.close()

Debugging, errors, and exceptions

Debugging is what is done to fix mistakes that break your code (or cause it to misbehave). The

two types of errors that may need to be debugged are syntax errors and exceptions.

Syntax errors, also known as parsing errors, repeat the offending line and display a little ‘arrow’

pointing at the earliest point in the line where the error was detected. In the File window, you

can choose RUN > CHECK MODULE (Alt+X) to try and view any of these before actually

running the script.

Exceptions are errors detected during execution and result in various messages. As with all

things, and in Python, it’s Easier to Ask Forgiveness than Permission: use the try/except

statements to attempt to perform an operation. If all goes well, great; if not, ask for forgiveness.

A debugger is a program that lets you step through your code one line at a time as Python

executes them, showing you how each affects your program.

For excellent introductions on debugging in IDLE, using either the GUI or module, see:

http://inventwithpython.com/chapter7.html

http://www.dreamincode.net/forums/topic/210537-python-debugging-part-1/

http://pythonconquerstheuniverse.wordpress.com/2009/09/10/debugging-in-python/

Tip: Other IDEs have more user-friendly debugging tools than IDLE (see the Resources below).

http://inventwithpython.com/chapter7.html

http://www.dreamincode.net/forums/topic/210537-python-debugging-part-1/

http://pythonconquerstheuniverse.wordpress.com/2009/09/10/debugging-in-python/



circle_area.py

# calculate area of a circle from user input radius – try out various debugging/exceptions

from math import pi

radius = 10

##radius = raw_input('Type a number: ')

##radius = float(raw_input('Type a number: '))

### flow control test input data type

##if radius.isdigit():

## print radius

## area = pi * float(radius)**2

area = pi * radius**2

##area = pi * pow(radius,2)

print 'Area of a circle with radius', radius, 'is', area

User-defined functions

A function is a series of statements which returns some value to a caller (it can also be passed

zero or more arguments for its use/execution). Built-in functions are objects that can be used by

all Python code without the need of an import statement. A user-defined function is a block of

organized, reusable code that is used to perform a single related action that you create. They

are similar to procedures, subroutines, and functions in other programming languages, but may

or may not return a value. The generic syntax requires the keyword def to define the function:

def functionname( parameters ):

"""function_docstring"""

function_suite

return [expression]

constant_value_field.py

# preliminary coding for adding a constant value field to a database table

# functions

def is_float(s): # Test if number with decimals

try:

float(s)

return True

except ValueError:

return False

def is_integer(s): # Test if number with no decimals

try:

int(s)

return True

except ValueError:

return False

# get user input

Field_Name = raw_input('Type the new field name: ')

Constant_Value = raw_input('Type a constant value: ')







# test user input

if is_integer(Constant_Value):

Constant_Field_Type = "LONG"

elif is_float(Constant_Value):

Constant_Field_Type = "DOUBLE"

else:

# assign defaults

Constant_Field_Type = "TEXT"

# this is where you would add and calculate the new field

print ‘The new field %s has the data type %s’ % (Field_Name, Constant_Field_Type)

# insert code above to test that the Field_Name

# starts with a letter and only contains alphanumeric characters

For more practice with functions, start at chapter 18 of http://learnpythonthehardway.org.

Really useful resources http://www.python.org/dev/peps/pep-0008/

http://docs.python.org/faq/index.html

http://docs.python.org/release/2.6.5/library

http://docs.python.org/release/2.6.5/library/functions.html

http://docs.python.org/release/2.6.5/reference/compound_stmts.html

http://docs.python.org/release/2.6.5/tutorial/errors.html

http://docs.python.org/release/2.6.5/library/pdb.html?highlight=pdb

http://wiki.python.org/moin/IntegratedDevelopmentEnvironments

Be sure to see the companion slides

for excellent e-resources to help you learn even more!

http://learnpythonthehardway.org/


http://docs.python.org/faq/index.html

http://docs.python.org/release/2.6.5/library

http://docs.python.org/release/2.6.5/library/functions.html

http://docs.python.org/release/2.6.5/reference/compound_stmts.html

http://docs.python.org/release/2.6.5/tutorial/errors.html

http://docs.python.org/release/2.6.5/library/pdb.html?highlight=pdb

http://wiki.python.org/moin/IntegratedDevelopmentEnvironments

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