Unit 1, Lesson 3 Program Execution Process AOIT Introduction to Programming Copyright © 2009–2012 National Academy Foundation. All rights reserved.

When Python executes circle.py, it draws a red circle from turtle import * title("circle program") setup() turtle.up() turtle.goto(0,20) turtle.down() turtle.color("red") turtle.width(2) turtle.begin_fill() turtle.circle(50) turtle.end_fill() done()

Program execution begins with Start Before you begin to run a program, think about the results you expect. What does circle.py do? What is the output? Start Hardware ComponentsSoftware Components Program execution is ready to start when IDLE is open and you have opened circle.py. Each step of the program execution process includes software and hardware components.

In step 1, the user selects Run Module 1 Start 1. User: Run > Run Module Hardware ComponentsSoftware Components Computer processor, disk drive, and mouse Python shell and.py file In which IDLE window is the Run menu? Using the mouse, the user selects Run > Run Module. What do you know about the software and hardware components used in this step?

In step 2, the parser deconstructs the program 1 Start 2 User: Run > Run Module Hardware ComponentsSoftware Components Computer processor, disk drive, and mouse Python shell and.py file 2. Read in and parse program Python parser and.py file Computer processor and disk drive What do you suppose “deconstruct” means? The parser reads in the Circle program and deconstructs it into its primitive components.

In step 3, the parser displays an error, if any 1 Start 2 User: Run > Run Module Hardware ComponentsSoftware Components Computer processor, disk drive, and mouse Python shell and.py file 3 Read in and parse program Python parser and.py file Computer processor and disk drive 3. If syntax error Send error message and exit Yes No If the parser detects an error, it stops running and returns an error message to the screen. If there is no error, the parser keeps running. The pop-up error message always says the same thing. What is it?

In step 4, the parser builds an execution table Start 4. Build execution table Python parser and.py file Computer processor and disk drive 2 User: Run > Run Module Hardware ComponentsSoftware Components Computer processor, disk drive, and mouse Python shell and.py file 3 Read in and parse program Python parser and.py file Computer processor and disk drive If Syntax Error 4 Send error message and exit Yes No The parser builds an execution table. This table contains a description of each statement in the program. Why might the parser need to build an execution table? Why couldn’t it always read the program top to bottom, left to right, like a book?

In step 5, the executor runs the program 1 Start 2 User: Run > Run Module Hardware ComponentsSoftware Components Computer processor, disk drive, and mouse Python shell and.py file 3 Read in and parse programPython parser and.py file Computer processor and disk drive If Syntax Error 4 Send error message and exit 5. Execute programPython executor and.py file Computer processor and screen 5 Build execution tablePython parser and.py file Computer processor and disk drive Yes No A software component called the Python executor runs the program by calling internal machine instruction functions one after the other. For circle.py, this means the machine instruction functions actually draw the circle. What are some functions you have seen in Python programs?

In step 6, the executor returns control to the shell Start 2 User: Run > Run Module Hardware ComponentsSoftware Components Computer processor, disk drive, and mouse Python shell &.py file 3 Read in and parse program Python parser and.py file Computer processor and disk drive If Syntax Error 4 Send error message and exit 6 Execute program Python executor and.py file Computer processor and screen 6. End program and wait for drawing window to be closed 5 Build execution table Python parser and.py file Computer processor and disk drive Python shellComputer processor, screen, and mouse Yes No When the executor reaches the last statement, it returns control to the shell. The shell waits for the drawing window to be closed. What is the last statement of the Circle program?

The end point is always Start End 2 User: Run > Run Module Hardware Components Software Components Computer processor, disk drive, and mouse Python shell and.py file 3 Read in and parse programPython parser and.py file Computer processor and disk drive If Syntax Error 4 Send error message and exit 6 Execute programPython executor and.py file Computer processor and screen 7 End program and wait for drawing window to be closed 5 Build execution tablePython parser and.py file Computer processor and disk drive Python shellComputer processor, screen, and mouse Yes No When the program ends, the drawing window closes. What is the drawing window? End

The program execution process draws the circle Start End 1. User: Run > Run Module Hardware ComponentsSoftware Components Computer processor, disk drive, and mouse Python shell and.py file 2. Read in and parse program Python parser and.py file Computer processor and disk drive If syntax error 3. Send error message and exit 5. Execute program Python executor and.py file Computer processor and screen 6. End program and wait for drawing window to be closed 4. Build execution table Python parser and.py file Computer processor and disk drive Python shellComputer processor, screen, and mouse Yes No