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Unit 1, Lesson 3
Program Execution Process
AOITIntroduction 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 mousePython 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 mousePython 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 runningand 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 tablePython parser and
.py file
Computer processor and
disk drive
2 User: Run > Run Module
Hardware ComponentsSoftware Components
Computer processor, disk drive, and mousePython 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 mousePython 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
5. Execute program Python executor and .py file
Computer processor and
screen
5 Build execution table Python 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 mousePython 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 shell Computer 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 ComponentsSoftware Components
Computer processor, disk drive, and mousePython 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
6 Execute program Python executor and .py file
Computer processor and screen
7 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 shell Computer 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 mousePython 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 shell Computer processor, screen, and mouse
Yes
No
