Python language data types

Introduction to the
Python Language
Part 1. Python data types

Install Python
• Find the most recent distribution for your
computer at:
http://www.python.org/download/releases/
• Download and execute Python MSI installer
for your platform
– For version 2.7 or higher
– Note: You may need to be logged in as
administrator to your computer to run the
installation!

Modes of Interaction with Python
• You can interact with the Python interpreter through two built-in
modes:
– Basic (command line) – not recommended!
– IDLE (Integrated development environment) - recommended for this class
• Command line mode:
– You can find and double click the Python.exe executable (e.g., python27)
in the downloaded folder.
– Make sure to right click it, and then create a shortcut to it, and pin it to
the taskbar for convenience!
– Click the icon on your taskbar to start the command line
You will get the >>> Python prompt
Type ‘exit’ at the command prompt to get out of the console!
>>> exit # or ctrl z
– Note: you can use the Home, Pg up, Pg dn, and End keys, to scroll
through previous entries, and repeat them by pressing the Enter key!

IDLE
• This is the integrated development environment for
Python
• Involves interactive interpreter with editing and
debugging capabilities (it prompts you for the names of
functions, etc., and color codes code)!
• You can find and double click the Pythonw.exe
executable (e.g., python27) in the downloaded folder
• Pin it to the taskbar.

Python IDLE Shell Window
• The IDLE (Python shell window) is more convenient than the
basic python command line
• Provides automatic indentation and highlights the code
• You can use the Home, Pg up, Pg dn, and End keys to search
the buffer in your session!

Learn Basic parts of Python
• Like other languages, Python has many data types
– These can be manipulated with operators, functions, and
methods
• You can also make your own types (classes) and instantiate and
manipulate them with operators, functions, and your own
methods
• Python also has conditional and iterative control, e.g.:
– if-elif-else
– while loop
– for loop

Python’s suggested naming style
• Function name is lower case
>>> def add (): # a function to add numbers; returns the result;
# note that function calls end with the : character
• Variable names are written in lower case, and are case sensitive!
inputFieldName
>>> fc # a variable to hold a feature class as in: fc=“Roads.shp”
Note: the two variables Road and road are different variables!
• Class name is UpperCamelCase
Class StrikeSlipFault, or class Lake
• Constants are written in all caps, e.g. PI, SPREADING_RATE
• Indentation: 4 spaces per level, do not use tabs!
– IDLE does it for you! So don’t worry about it.

Data
types
10/3 = 3, but 10/3.0 = 3.33333
The // operator is for integer division (quotient without remainder).
10//3 = 3 and 4//1.5 = 2, and 13//4 = 3

Python has several data types:
• Numbers, lists, strings, tuples, tuples, dictionaries, etc.
• Numbers:
– Integers
• 5, -8, 43, -234, 99933
– Floats
• 6.8, 24e12, -4e-2 #2e3 = 2000, scientific notation
– Complex numbers:
• 4+3j, 8.0+4.6j
– Booleans
• True, False

Manipulation of numbers
Addition (+)
>>> x=8+2
>>> print x # prints 10
Subtraction (-)
>>> x = 2
>>> x = x-2
>>> print x # prints 0
>>> (2+3j) – (5-4j) # prints (-3+7j)
Multiplication (*)
>>> 8*2 # prints 16
>>> 8*2.0 # prints 16.0

Division (/) # normal division
>>> 7/2 3
>>> 7/2.0 3.5
>>> 7//2 3 # integer results with truncation
Multiplication (*)
>>> 2*3.0 6.0
Exponentiation (**)
>>> 2**3.0 8.0
Modulus (%) # prints the remainder of the division
>>> 5%2 1
>>> 5.0%2 1.0
>>> 5.0%2.5 0.0 >>> 5%5 0

Precedence
>>> 5-2*3/4 4 # same as 5-((2*3)/4)
>>> 50/2+2**3-8/2 29 # (50/2)+(2**3)-(8/2)
# For equal precedence go from left to right!
# first power, then left division, then add them,
# then right division, then subtraction
>>> 24-(12-4)/2*2+3 19
# first do the parenthesis, then divide by 2,
# then multiply the result by 2, then
# subtract from 24, then add to 3

Built-in functions
>>> round (7.78) # Built-in function returns: 8
>>> int (230.5) # Built-in converts to integer, returns: 230
>>> float (230) # Built-in converts to float, returns: 230.0
>>> int (3e3) # returns: 3000
>>> type (151.0) # returns: <type 'float'>
>>> type (151) # returns: <type 'int'>
>>> type ('Python') # returns: <type 'str'>

Calling Python’s library functions in modules
>>> x = math.floor (-13.4) # leads to the following error:
Traceback (most recent call last): File "<interactive input>", line
1, in <module>NameError: name 'math' is not defined
>>> import math # imports the math module
# now we can use math’s functions
>>> x = math.floor (-13.4) # call math’s floor () function
>>> print x
-14.0
>>>

Python library module functions
• Trigonometric, factorial, pi, sqrt, ceil, degrees, exp, floor, etc., are in
the math module library
>>> Import math
>>> math.pi 3.14 # call a math library constant
>>> math.ceil (3.14) 4 # call a module function ()
>>> math.sqrt (64) 8
>>> math.log10 (1000000) 6.0
>>> math.e 2.718281828459045
>>> import random # import the random module
>>> x = random.uniform (1, 5) # returns a float between 1 and 5
>>> x 2.8153025790665516
>>> x = random.randint (1, 5) # returns an integer between 1 and 5
>>> x 1

Python’s built-in numeric functions
abs (), float (), hex (), oct (), long (), max (),
min (), pow (), round ()
• There are many more functions for
trigonometry, etc.
• ‘none’ is a special data type for empty value
– e.g., when a function does not return a value
– It is used as a placeholder. Similar to null.

Built-in numeric functions - examples
>>> range (6, 18, 3) # returns: [6, 9, 12, 15]
>>> range (10) # returns: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> abs (-17) # returns: 17
>>> max (12, 89) # returns: 89
>>> round (12.7) # returns: 13.0
>>> hex (10) # returns: '0xa‘
>>> oct (10) # returns: '012'

Getting input from user
>>> age = int (input("What is your age? "))
# displays a dialog for user to enter data
# user enters a number (e.g., 45)
>>> print age # prints the entered age (45)
45

Assignment to a variable
• To assign a value to a variable, you put the variable on the left
side of an equal sign (=) and put (or retrieve) the value to the
right, for example:
>>> result = pow (4, 2)
>>> print result # prints 16
• Let’s assign the number of zip codes in a GIS layer by calling
the GetCount tool’s management () function in arcpy and
passing it the name of the “zipcodes” layer as argument.
>>> count =arcpy.GetCount_management (“zipcodes’)
# This will return a number and assigns it to the count variable,
# which can then be used in another call , for example:
>>> print count

arcpy package for ArcGIS
• Geoprocessing in ArcGIS is done through the arcpy package
>>> import arcpy
>>> arcpy.env.workspace = “C:/Data” # sets the current workspace
# env is a class in arcpy, and workspace is a property.
>>> from arcpy import env # this does not import the entire arcpy
>>> env.workspace = “C:/Data”

Strings
• Strings are immutable sequence of characters (text)
• These are inserted either in “ ” or ‘ ’.
• A double quote (“ ”) can contain single quote (‘ ’), e.g.,
“ ’dog’, ‘cat’ ” in a string
• and vice versa, e.g., ‘ ”dog”, “cat” ’ in a string
• They can also be in triple single or double quotes:
‘’’ ‘‘‘ or “”” “””.
– This is used for multi-line commenting

Escape characters
’ Single
quote
” Double
quote
Backslash bBackspace n newline
r Carriage
return
t tab
We have to escape special characters such as or “
>>> x = "Location of the file is: C:GeologyGeoinformatics“
>>> print x
Location of the file is: C:GeologyGeoinformatics
>>> print ("tabc") # t is tab
abc
>>> x="tHello Python World!“
>>> print x
Hello Python World!
>>> tictactoe = ("XtOtXnXtXtXnOtOtO")
>>> print tictactoe
X O X
X X X
O O O

Strings
# The split () function breaks a string into pieces completely or at a specific character.
>>> x = "Chattahoochie“
>>> x.split ("oo")
['Chattah', 'chie']

Strings …
The find() function returns the index of the first occurrence of a character.
The replace (a, b) function takes two arguments . Replaces a with b.

Formatting strings with the
string modulus operator %

String operators + and *
>>> x = "Mississippi“
>>> y = "River“
>>> z = x + ' ' + y # concatenates the two strings, puts space in between
>>> print z
Mississippi River
>>> T = 32 # note: 32 is a number (not a string).
>>> print ("Today is freezing! The temperature is: " + str (T) + ' ' +
"degrees Fahrenheit") # we cast the number into string by str (T)
Today is freezing! The temperature is: 32 degrees Fahrenheit
>>> 3*'A‘ # * repeats the character
‘AAA‘

String’s join () methods
• The join () method puts spaces or other characters between
strings
>>> "-".join (['to', 'be', 'or', 'not', 'to', 'be'])
'to-be-or-not-to-be‘ # put hyphen between words
>>> "-".join ("To be or not to be")
‘T-o- -b-e- -o-r- -n-o-t- -t-o- -b-e‘
>>> "-".join ("Tobeornottobe")
‘T-o-b-e-o-r-n-o-t-t-o-b-e'
>>> " ".join(['to', 'be', 'or', 'not', 'to', 'be'])
'to be or not to be‘ # puts a blank between them

>>> x = "Diamond is the hardest mineral"
>>> print x Diamond is the hardest mineral
>>> X[0] ’D’ # prints the first character of the string
>>> x.split() ['Diamond', 'is', 'the', 'hardest', 'mineral']
# The following is a multi-line comment in triple quote:
>>> """ Minerals are naturally occurring, inorganic, solid
... substances with crystalline structure and composition """
>>> min = 'Minerals are naturally occurring, inorganic, solidn substances
with crystalline structure and composition‘
>>> print min
Minerals are naturally occurring, inorganic, solid
substances with crystalline structure and composition
>>> pi = 3.14
>>> print "The value for pi is:", pi The value for pi is: 3.14 # or
>>> print "The value for pi is: ", math.pi # assume math is imported
The value for pi is: 3.14159265359

>>> x = " Map is NOT territory ! “
>>> x.strip()
'Map is NOT territory!‘ # strips white space
>>> x.lstrip ()
'Map is NOT territory ! ‘ # strips white space from left
>>> x.rstrip ()
' Map is NOT territory !‘ # strips white space from right
>>> email = 'www.gsu.edu/~geohab‘
>>> email.strip (“~geohab”) # strips the passed string
‘www.gsu.edu/'
The strip () method

String searching
>>> word = "Uniformitarianism“
>>> word.find ('i') #find the index of the first occurrence of ‘I’
2
>>> word.find ('i', 6) #find ‘i’ after index 6
7
>>> word.rfind('m') # find index for ‘m’. moves from the right end
16
>>> word.count('i') # find how many times ‘i’ is there in the word
4
>>> word.startswith ('U')
True
>>> word.endswith('P')
False

Strings are immutable
• Cannot change a string, but can modify and return a new string!
>>> x = "Mississippi“
>>> x.replace('ss', 'pp')
'Mippippippi‘ # this is a new string
>>> x.upper() # original value of x is still unchanged
MISSISSIPPI
>>> x.lower()
mississippi # original is still unchanged
>>> bookTitle = "knowledge engineering for beginners"
>>> bookTitle.title () 'Knowledge Engineering For Beginners‘
>>> file = 'Rivers.shp‘
>>> file.replace ('.shp', "") 'Rivers‘
>>> len ('Diamond') 7

Casting strings to numbers
>>> float ("12345.34") # it works; numeric string is ok
12345.34
>>> int ('23456') # it work!
23456
>>> int ('123.45') # error: decimal does not work
>>> float ("xyz") # error: letter character to number

Methods
>>> course = "Geoinformatics: Geol 4123“
>>> print course.lower ()
geoinformatics: geol 4123
>>> print course.upper () GEOINFORMATICS: GEOL 4123
>>> 'GEO' in course False
>>> 'Geo' in course True
>>> course = "Geoinformatics Geol 4123“
>>> course.find ('Geo') # returns the index
0 # index of the first item
>>> course.find('GEO')
-1 # not found

Lists
• List is an ordered collection of objects
• List is modifiable (mutable).
• They can have elements of different types
• Elements are comma separated in a []
>>> rivers = [Missouri, Fox, Mississippi] # assigns to the rivers list
>>> x = ['apple', 3, [4.0, 5.0]] # multi-type list
>>> fileExtension = ["jpg", "txt", "doc", "bmp”, "tif"]
>>> print fileExtension
['jpg', 'txt', 'doc', 'bmp', 'tif']

List
>>> x [:-2] -> [‘hydroxide’, ‘phosphate’, ‘carbonate’]
# up to, but not including index -2, i.e., gets rid of the last two
>>> x [2:] -> [‘carbonate’, ‘oxide’, ‘silicate’] # values from position 2 to the end (inclusive)

List indices
• Lists start counting from 0 on the left side (using + numbers) and -
1 from the right side
>>> x = ['River', 'Lake', 'Rock', 'Water', 'Air']
>>> X[2] ‘Rock’
>>> X[-5] ‘River’
>>> x[:3] ['River', 'Lake', 'Rock']
>>> X[0:3] ['River', 'Lake', 'Rock'] # includes 0 but excludes 3
>>> x[3:10] ['Water', 'Air'] # ignores if the items don’t exist
>>> x[3:] ['Water', 'Air'] # index 3 and higher
X = [ ‘River ‘Lake’ ‘Rock’ ‘Water’ ‘Air’ ]
+ index 0 1 2 3 4
- Index -5 -4 -3 -2 -1

>>> lang = ['P', ‘Y', 'T', 'H', 'O', 'N']
>>> lang[3] 'H‘
>>> lang[-1] 'N‘
>>> lang[-6] 'P‘
>>> lang[-8]
Traceback (most recent call last): File "<interactive input>",
line 1, in <module>IndexError: list index out of range
>>> lang[:3] ['P', ‘Y', 'T']
>>> lang [-3:]['H', 'O', 'N']
>>>
P Y T H O N
0 1 2 3 4 5 6
-6 -5 -4 -3 -2 -1

Lists have mixed types
The len () functions returns the number of
elements in a list
>>> x = [1, 2, 3, [3, 4]]
>>> len (x)
4

More functions for lists
>>> x = ['apple', 3, [4.0, 5.0]]
>>> len(x) 3 # returns the number of elements
>>> cities = ["Atlanta", "Athens", "Macon", "Marietta"]
>>> cities.sort (reverse = True)
>>> print cities ['Marietta', 'Macon', 'Atlanta', 'Athens']
>>> cities.sort ()
>>> print cities ['Athens', 'Atlanta', 'Macon', 'Marietta']
>>> del cities [0]
>>> print cities ['Atlanta', 'Macon', 'Marietta']
>>> 'Smyrna' in cities False
>>> cities.append ('Smyrna')
>>> print cities ['Atlanta', 'Macon', 'Marietta', 'Smyrna']

>>> print cities ['Atlanta', 'Macon', 'Marietta‘, ‘Smyrna’]
>>> cities.insert (1, 'Dalton')
>>> print cities ['Atlanta', 'Dalton', 'Macon', 'Marietta', 'Smyrna']
>>> cities.remove('Atlanta')
>>> print cities ['Dalton', 'Macon', 'Marietta', 'Smyrna']
>>> cities.pop(2) # removes item at index 2, returns the item
'Marietta‘
>>> print cities ['Dalton', 'Macon', 'Smyrna']

Lists can change
>>> x = ['River', 'Lake', 'Rock']
>>> x[1] = 'Horse‘
>>> x[0:]
['River', 'Horse', 'Rock']
>>> y = x[:] # copies all x’s elements and assigns to y
>>> y[0] = 'Timber‘ # substitute
>>> y[2] = 'Lumber‘ # substitute
>>> y[0:] #show y from the beginning
['Timber', 'Horse', 'Lumber']

Modifying lists …
>>> x=[1,2,3] # let’s append a new list to the end of this list
>>> x[len(x):] = [4, 5, 6, 7] # find the length; append after last index
>>> print x [1, 2, 3, 4, 5, 6, 7]
>>> x[ :0] = [-2, -1, 0] # append this list to the front of the original list
>>> print x [-2, -1, 0, 1, 2, 3, 4, 5, 6, 7]
>>> x[1:-1] = []
# removes elements between the 2nd and one before the last index
>>> print x [-2, 7]
>>> x.append("new element") # adds a new element to the list
>>> print x [-2, 7, 'new element']

Modifying lists…
>>> x=[1,2,3,4]
>>> y=[5,6]
>>> x.append(y) # adds y as an element (in this case a list) to the end of x
>>> print x [1, 2, 3, 4, [5, 6]]
>>> x=[1,2,3,4]
>>> y = [5,6]
>>> x.extend(y) # adds the items to the end of an existing list
>>> print x [1, 2, 3, 4, 5, 6]
>>> x.insert(2, 'Hello')
# inserts an element after a given index; always needs two arguments
>>> print x [1, 2, 'Hello', 3, 4, 5, 6]
>>> x.insert(0, 'new') # insert at the beginning (at index 0) an new item
>>> print x ['new', 1, 2, 'Hello', 3, 4, 5, 6]

The + and * operators and lists
>>> X = [1, 2, 3, 4, [5, 6]]
>>> y = x + [7,8] #concatenation with the + charecter
>>> print y [1, 2, 3, 4, [5, 6], 7, 8]
>>> x = ['Wow']
>>> x = x + ['!']
>>> print x ['Wow', '!']
>>> x.append(‘!')
>>> print x ['Wow', '!', ‘!']
>>> x = ['Wow']
>>> x.extend('!')
>>> print x ['Wow', '!']

…
>>> X = ['Mercury','Venus','Earth','Mars','Jupiter','Saturn','Uranus','Neptune',
'Pluto']
>>> del x[8] # removes Pluto from the Solar System list!
>>> print x
['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']
>>> X = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus',
'Neptune']
>>> x.append('Pluto') # add Pluto to the end
>>> print x
['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune',
'Pluto']
>>> x.remove('Pluto') # removes ‘Pluto’ at its first occurrence
>>> print x
['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']

Sorting lists
>>> x = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn',
'Uranus', 'Neptune']
>>> x.sort()
>>> print x
['Earth', 'Jupiter', 'Mars', 'Mercury', 'Neptune', 'Saturn', 'Uranus',
'Venus']
>>> x=[2, 0, 1,'Sun', "Moon"]
>>> x.sort()
>>> print x
[0, 1, 2, 'Moon', 'Sun']

The in operator
>>> x = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus',
'Neptune']
>>> 'Pluto' in x
False
>>> 'Earth' in x
True

The min, max, index, and count functions
>>> x = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus',
'Neptune']
>>> min (x) ‘Earth‘
>>> max (x) ‘Venus‘
>>> x = [8, 0, -3, -1, 45]
>>> min (x) -3
>>> max (x) 45
>>> x = [8, 0, -3, -1, 45]
>>> x.index(-3) 2 #returns the index for ‘-3’
>>> x.index(45) 4 #returns the index for ‘45’
>>> x = [2, 5, 3, 3, 4, 3, 6]
>>> x.count(3) 3

List in ArcGIS
>>> import arcpy
>>> from arcpy import env
>>> env.workspace = “C:/Data/study.gdb”
>>> fcs = arcpy.ListFeatureClasses ()
>>> fcs = sort ()
>>> print fcs
>>> fcs.sort (reverse = True)
>>> print fcs

Sets: unordered collection of objects
Sets are unordered collection of objects and can be changed
Duplicates are removed, even when you add them
The ‘in’ keyword checks for membership in the set

Tuples – ()
• Are similar to lists but are immutable (like strings)
– can only be created but not changed!
– Used as keys for dictionaries
– Instead of x=[] for lists, we use x=() to create tuples
• Many of the functions and operators that work with lists
also work for tuples, e.g.:
len(), min(), max(), and the +, and * operands.

Tuples …
>>> x = () # create an empty tuple
>>> print x # prints ()
>>> (Mercury, Venus, Earth, Mars) = (1, 2, 3, 4)
>>> Earth
3
>>> list ((Mercury, Venus, Earth, Mars))
# cast by the list () function
# the casting function list () converts a tuple to a list
[1, 2, 3, 4]
>>>

Dictionaries – {}
• Called dictionary because they allow mapping from arbitrary objects (e.g.,
words in a dictionary) to other sets of arbitrary objects.
• Values in a dictionary are accessed via keys that are not just numbers.
They can be strings and other Python objects. Keys point to values.
– Compare this to indices in lists
• Both lists and dictionaries store elements of different types
• Values in a list are implicitly ordered by their indices.
• Those in dictionaries are unordered.
>>> x = {} # create an empty dictionary
>>> x[0] = 'Mercury‘ # assigns a value to a dictionary variable as if it is a list
# this cannot be done to a list if x is not defined! Y[0] = ‘1’ does not work

>>> x = {} # declare x as a dictionary
>>> x["Venus"] = 2 # indexing with non numbers (in this case string)
>>> y = {}
>>> y['Earth'] = 3
>>> x['Venus'] * y ['Earth'] # this cannot be done with lists (need int indices)
6
>>> phone = {} # declare phone as a dictionary
# dictionaries are great for indexing lists by names (lists cannot do it)
>>> phone ['Babaie'] = 1234567899
>>> print phone['Babaie']
1234567899

>>> Eng_to_Fr ['river'] = 'fleuve‘
>>> print 'River in French is', Eng_to_Fr ['river']
River in French is fleuve
>>> Eng_to_Fr = {'blue': 'bleu', 'green': 'vert'}
>>> print Eng_to_Fr {'blue': 'bleu', 'green': 'vert'}
>>> 'blue' in Eng_to_Fr True
>>> print Eng_to_Fr.get ('green') vert # uses the get ()
>>> Eng_to_Fr.copy () # copy () copies the dictionary
{'blue': 'bleu', 'green': 'vert'}

>>> x = {1: 'solid', 2: 'liquid', 3: 'gas'}
>>> y = {4: 'plasma'}
>>> y.update(x) # update y with x
>>> y {1: 'solid', 2: 'liquid', 3: 'gas', 4: 'plasma'}
>>> x {1: 'solid', 2: 'liquid', 3: 'gas'} # unchanged
>>> x.update(y) # update x with y
>>> x {1: 'solid', 2: 'liquid', 3: 'gas', 4: 'plasma'}

>>> statelookup = {"Houston": "Texas", "Atlanta": "Georgia",
"Denver": "Colorado"}
>>> statelookup["Atlanta"] 'Georgia‘
>>> zoning = {} # new dictionary
>>> zoning["COM"] = "Commercial“ # uses list format
>>> zoning ["IND"] = "Industry“
>>> zoning ["RES"] = "Residential“
>>> print zoning
{'IND': 'Industry', 'RES': 'Residential', 'COM': 'Commercial'}
>>> del zoning ["COM"]
>>> print zoning {'IND': 'Industry', 'RES': 'Residential'}
>>> zoning.keys() ['IND', 'RES']

• In Python (an OOP language) everything is an
object.
• All objects have id, name, and type
>>> River = "Chattahoochie“
>>> type (River)
<type 'str‘> # this is printed by Python
>>> id (River)
46695832 # this is its address in memory
>>>
Python objects

Objects have methods
• Objects can do things through methods
• Methods are coupled to specific objects of a class
• object.method (arguments)
>>> course = "Geoinformatics“
>>> course.count ('i')
2
• Most of the functions which we have seen so
far are method.

Python language data types

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