![Other Python Objects
Lists (mutable sets of strings)
var = [] # create list
var = [‘one’, 2, ‘three’, ‘banana’]
Tuples (immutable sets)
var = (‘one’, 2, ‘three’, ‘banana’)
Dictionaries (associative arrays or ‘hashes’)
var = {} # create dictionary
var = {‘lat’: 40.20547, ‘lon’: -74.76322}
var[‘lat’] = 40.2054
Each has its own set of methods
11](https://image.slidesharecdn.com/presentationintrotopython1462930390181219-220820054611-3aa2cdf9/85/presentation_intro_to_python-11-320.jpg)
![Lists
Think of a list as a stack of cards, on which your
information is written
The information stays in the order you place it in until
you modify that order
Methods return a string or subset of the list or modify
the list to add or remove components
Written as var[index], index refers to order within set
(think card number, starting at 0)
You can step through lists as part of a loop
12](https://image.slidesharecdn.com/presentationintrotopython1462930390181219-220820054611-3aa2cdf9/85/presentation_intro_to_python-12-320.jpg)
![List Methods
Adding to the List
var[n] = object
replaces n with object
var.append(object)
adds object to the end of the list
Removing from the List
var[n] = []
empties contents of card, but preserves order
var.remove(n)
removes card at n
var.pop(n)
removes n and returns its value
13](https://image.slidesharecdn.com/presentationintrotopython1462930390181219-220820054611-3aa2cdf9/85/presentation_intro_to_python-13-320.jpg)
![Dictionaries
Dictionaries are sets of key & value pairs
Allows you to identify values by a descriptive name
instead of order in a list
Keys are unordered unless explicitly sorted
Keys are unique:
var[‘item’] = “apple”
var[‘item’] = “banana”
print var[‘item’] prints just banana
16](https://image.slidesharecdn.com/presentationintrotopython1462930390181219-220820054611-3aa2cdf9/85/presentation_intro_to_python-16-320.jpg)
![Looping with For
For allows you to loop over a block of code a set
number of times
For is great for manipulating lists:
a = ['cat', 'window', 'defenestrate']
for x in a:
print x, len(x)
Results:
cat 3
window 6
defenestrate 12
19](https://image.slidesharecdn.com/presentationintrotopython1462930390181219-220820054611-3aa2cdf9/85/presentation_intro_to_python-19-320.jpg)
![Modules
Modules are accessed using import
import sys, os # imports two modules
Modules can have subsets of functions
os.path is a subset within os
Modules are then addressed by
modulename.function()
sys.argv # list of arguments
filename = os.path.splitext("points.txt")
filename[1] # equals ".txt"
22](https://image.slidesharecdn.com/presentationintrotopython1462930390181219-220820054611-3aa2cdf9/85/presentation_intro_to_python-22-320.jpg)
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- 1. 1
- 2. Introduction to Python Python is a high-level programming language Open source and community driven “Batteries Included” a standard distribution includes many modules Dynamic typed Source can be compiled or run just-in-time Similar to perl, tcl, ruby 2
- 3. Why Python? Unlike AML and Avenue, there is a considerable base of developers already using the language “Tried and true” language that has been in development since 1991 Can interface with the Component Object Model (COM) used by Windows Can interface with Open Source GIS toolsets 3
- 4. Why not Visual Basic? Visual Basic is still the method of configuring and customizing ArcMap If you have a button on the toolbar, it’s VB Python scripts can be placed in ArcToolbox Python can be run from the command line without ArcMap or ArcCatalog being open Using just the GIS Engine, lower overhead Rapid prototyping, ease of authoring, etc. 4
- 5. Python Interfaces IDLE – a cross-platform Python development environment PythonWin – a Windows only interface to Python Python Shell – running 'python' from the Command Line opens this interactive shell For the exercises, we'll use IDLE, but you can try them all and pick a favorite 5
- 6. IDLE – Development Environment IDLE helps you program in Python by: color-coding your program code debugging auto-indent interactive shell 6
- 7. Example Python Hello World print “hello world” Prints hello world to standard out Open IDLE and try it out yourself Follow along using IDLE 7
- 8. More than just printing Python is an object oriented language Practically everything can be treated as an object “hello world” is a string Strings, as objects, have methods that return the result of a function on the string 8
- 9. String Methods Assign a string to a variable In this case “hw” hw.title() hw.upper() hw.isdigit() hw.islower() 9
- 10. String Methods The string held in your variable remains the same The method returns an altered string Changing the variable requires reassignment hw = hw.upper() hw now equals “HELLO WORLD” 10
- 11. Other Python Objects Lists (mutable sets of strings) var = [] # create list var = [‘one’, 2, ‘three’, ‘banana’] Tuples (immutable sets) var = (‘one’, 2, ‘three’, ‘banana’) Dictionaries (associative arrays or ‘hashes’) var = {} # create dictionary var = {‘lat’: 40.20547, ‘lon’: -74.76322} var[‘lat’] = 40.2054 Each has its own set of methods 11
- 12. Lists Think of a list as a stack of cards, on which your information is written The information stays in the order you place it in until you modify that order Methods return a string or subset of the list or modify the list to add or remove components Written as var[index], index refers to order within set (think card number, starting at 0) You can step through lists as part of a loop 12
- 13. List Methods Adding to the List var[n] = object replaces n with object var.append(object) adds object to the end of the list Removing from the List var[n] = [] empties contents of card, but preserves order var.remove(n) removes card at n var.pop(n) removes n and returns its value 13
- 14. Lists in ArcToolbox You will create lists: Layers as inputs Attributes to match Arrays of objects You will work with lists: List of field names List of selected features 14
- 15. Tuples Like a list, tuples are iterable arrays of objects Tuples are immutable – once created, unchangeable To add or remove items, you must redeclare Example uses of tuples County Names Land Use Codes Ordered set of functions 15
- 16. Dictionaries Dictionaries are sets of key & value pairs Allows you to identify values by a descriptive name instead of order in a list Keys are unordered unless explicitly sorted Keys are unique: var[‘item’] = “apple” var[‘item’] = “banana” print var[‘item’] prints just banana 16
- 17. Indentation and Blocks Python uses whitespace and indents to denote blocks of code Lines of code that begin a block end in a colon: Lines within the code block are indented at the same level To end a code block, remove the indentation You'll want blocks of code that run only when certain conditions are met 17
- 18. Conditional Branching if and else if variable == condition: #do something based on v == c else: #do something based on v != c elif allows for additional branching if condition: elif another condition: … else: #none of the above 18
- 19. Looping with For For allows you to loop over a block of code a set number of times For is great for manipulating lists: a = ['cat', 'window', 'defenestrate'] for x in a: print x, len(x) Results: cat 3 window 6 defenestrate 12 19
- 20. Looping with For We could use a for loop to perform geoprocessing tasks on each layer in a list We could get a list of features in a feature class and loop over each, checking attributes Anything in a sequence or list can be used in a For loop Just be sure not to modify the list while looping 20
- 21. Modules Modules are additional pieces of code that further extend Python’s functionality A module typically has a specific function additional math functions, databases, network… Python comes with many useful modules arcgisscripting is the module we will use to load ArcGIS toolbox functions into Python 21
- 22. Modules Modules are accessed using import import sys, os # imports two modules Modules can have subsets of functions os.path is a subset within os Modules are then addressed by modulename.function() sys.argv # list of arguments filename = os.path.splitext("points.txt") filename[1] # equals ".txt" 22
- 23. Files Files are manipulated by creating a file object f = open("points.txt", "r") The file object then has new methods print f.readline() # prints line from file Files can be accessed to read or write f = open("output.txt", "w") f.write("Important Output!") Files are iterable objects, like lists 23
- 24. Error Capture Check for type assignment errors, items not in a list, etc. Try & Except try: a block of code that might have an error except: code to execute if an error occurs in "try" Allows for graceful failure – important in ArcGIS 24
- 25. Let us dig a little further 25
- 26. What Is a Program? Usually, one or more algorithms written in a programming language that can be translated to run on a real machine We sometimes call programs software
- 27. What Is a Programming Language? A programming language is somewhat like a natural language, but with a very limited set of statements and strict syntax rules. Has statements to implement sequential, conditional and iterative processing - algorithms Examples: FORTRAN, COBOL, Lisp, Basic, Pascal, C, C++, Java, C#, Python, …
- 28. Compiler A compiler is a program that converts a program written in a programming language into a program in the native language, called machine language, of the machine that is to execute the program.
- 29. From Algorithms to Hardware (with compiler) Algorithm Program A real computer Translate (by a human being) Translate (by compiler program)
- 30. The Program Development Process (Data Flow) Editor Compiler A real computer Algorithm Program in programming language Program in machine’s language Input Output
- 31. The Program Development Process (Control Flow) Edit Compile Run Syntax errors Input Output Runtime errors
- 32. Three kinds of errors Syntax error : Some statement in the program is not a legal statement in the language. Runtime error : An error occurs while the program is executing, causing the program to terminate (divide by zero, etc.) Logic error : The program executes to completion, but gives incorrect results.
- 33. Interpreter An alternative to a compiler is a program called an interpreter. Rather than convert our program to the language of the computer, the interpreter takes our program one statement at a time and executes a corresponding set of machine instructions.
- 34. Interpreter Edit Interpreter Syntax or runtime errors Input Output
- 35. Python Python is a real-world, production language that is freely available for most computers. http:www.python.org If you want a copy of Python to use with this course, go to http://code.google.com/p/mediacomp-jes/ . We are using JES (Jython Environment for Students) which has a lot of special multimedia functionality. Note: Our textbook covers a limited amount of Python. There are many excellent online tutorials. For example, see http://en.wikibooks.org/wiki/Non-Programmer's_Tutorial_for_Python/Contents
- 36. Python Python uses an interpreter. Not only can we write complete programs, we can work with the interpreter in a statement by statement mode enabling us to experiment quite easily. Python is especially good for our purposes in that it does not have a lot of “overhead” before getting started. It is easy to jump in and experiment with Python in an interactive fashion.
- 37. Language terminology Syntax: The formal rules for legal statements in the language. Semantics: The meaning of the statements - what happens when the statement is executed.
- 38. Three major control constructs of programming (Execution flow of instructions) Sequential: Simply do steps one after the other in order they are listed. Conditional: Decide which statement to do next based on some true/false test. Iterative: A set of statements is repeated over and over until some condition is met.
- 39. Sequential Operations “Atomic” Input Computation Output
- 40. The Big Plan We want to get some experience of programming simple algorithms in a real programming language. This gives us an understanding of how software is written and allows us to test our algorithms to see if they work. We’ll first work with programs where the variables have numbers as values. Later we’ll work with programs dealing with pictures and sound. In lab we’ll work with some simple statements and small programs.
- 41. The Basic Pattern Most of our programs will use the basic pattern of Get some user input Perform some algorithm on the input Provide results as output
- 42. Identifiers Identifiers are names of various program elements in the code that uniquely identify the elements. They are the names of things like variables or functions to be performed. They're specified by the programmer and should have names that indicate their purpose. In Python, identifiers Are made of letters, digits and underscores Must begin with a letter or an underscore Examples: temperature, myPayrate, score2
- 43. Keywords Keywords are reserved words that have special meaning in the Python language. Because they are reserved, they can not be used as identifiers. Examples of keywords are if, while, class, import.
- 44. Variables in Python A variable has A name – identifier A data type - int, float, str, etc. Storage space sufficient for the type.
- 45. Numeric Data Types int This type is for whole numbers, positive or negative. Examples: 23, -1756 float This type is for numbers with possible fraction parts. Examples: 23.0, -14.561
- 46. Integer operators The operations for integers are: + for addition - for subtraction * for multiplication / for integer division: The result of 14/5 is 2 % for remainder: The result of 14 % 5 is 4 *, /, % take precedence over +, - x + y * z will do y*z first Use parentheses to dictate order you want. (x+y) * z will do x+y first.
- 47. Integer Expressions Integer expressions are formed using Integer Constants Integer Variables Integer Operators Parentheses
- 48. Python Assignment Statements In Python, = is called the assignment operator and an assignment statement has the form <variable> = <expression> Here <variable> would be replaced by an actual variable <expression> would be replaced by an expression Python: age = 19
- 49. Python Assignment Statement Syntax: <variable> = <expression> Note that variable is on left Semantics: Compute value of expression Store this as new value of the variable Example: Pay = PayRate * Hours Payrate 10 Hours 40 Pay 400
- 50. Assignment Example Before X 3 Z 12 Y 5 After X 3 Z 11 Y 5 Execute Z = X * 3 + Z / Y
- 51. Python Session
- 52. Python Session
- 53. What about floats? When computing with floats, / will indicate regular division with fractional results. Constants will have a decimal point. 14.0/5.0 will give 2.8 while 14/5 gives 2.
- 54. Comments Often we want to put some documentation in our program. These are comments for explanation, but not executed by the computer. If we have # anywhere on a line, everything following this on the line is a comment – ignored
- 55. Numerical Input To get numerical input from the user, we use an assignment statement of the form <variable> = input(<prompt>) Here <prompt> would be replaced by a prompt for the user inside quotation marks If there is no prompt, the parentheses are still needed Semantics The prompt will be displayed User enters number Value entered is stored as the value of the variable
- 56. Print Statement For output we use statements of the form print <expression> Semantics Value of expression is computed This value is displayed Several expressions can be printed – separate them by commas
- 57. Example - Fahrenheit to Centigrade We want to convert a Fahrenheit temperature to Centigrade. The formula is C = (F -32) x 5/9 We use type float for the temperatures.
- 58. Python Session