Transcript Python programming - Villanova University

Python programming
Introduction to the JES
environment and basics of Python
•Reading: Chapters 1, 2 from “Introduction to Computing
and Programming in Python”
Python
• The programming language we will be using is
called Python
– http://www.python.org
– It’s used by companies like Google, Industrial Light &
Magic, Nextel, and others
• The kind of Python we’re using is called Jython
– It’s Java-based Python
– http://www.jython.org
• We’ll be using a specific tool to make Python
programming easier, called JES (Jython
Environment for Students).
JES - Jython Environment for
Students
Program area
Program Area
- A simple editor for
programs
Command area
Command Area
- Interaction with Jython
Python interaction through
commands
Anything you type in command area is
evaluated and its value is displayed
• Example:
prompt
>>> 5 + 3
8
>>> ‘spam’
‘spam’
>>> “spam”
‘spam’
>>> “spam and more spam”
‘spam and more spam’
>>> ‘spam’ + ‘spam’
‘spamspam’
print displays the value of an
expression
• In many cases it makes no difference whether you
use print - the result gets displayed anyway.
>>> print 5 + 3
8
>>> print ‘spam’
spam
>>> ‘spam’ + ‘spam’
‘spamspam’
>>> print ‘spam’ + ‘spam’
Spamspam
Note: no quotes!
Command Area Editing
• Up/down arrows walk through command
history
• You can edit the line at the bottom
– Just put the cursor at the end of the line
before hitting Return/Enter.
Variables are names for data
Example:
a= 3
b= -1
c=2
x=0
f = a*x*x + b*x + c
Variables -more examples
• Variables of other types
newsItem = “You’ve got spam!”
• Variables keep their value for the
duration of a program or until they get a
new value through a new assignment
a=3
b = a *2 + 5
a=0
Up to this point
the value of a is
still 3, but then
it changes to 0
Python functions
• Python has a lot of built-in functions for
general mathematical manipulation, eg:
• sqrt(4) - computes the square root of 4
• ord(‘A’) - computes the ASCII code for
character ‘A’
• Example: print (-b+sqrt(b*b - 4*a*c))/2*a
But what exactly is a function?
4
‘a’
F(a,b)
side-effects
F(4, ‘a’)
Functions in general
4
‘a’
F(a,b)
side-effects
F(4, ‘a’)
Example: sqrt()
4
sqrt(a)
side-effects
2
JES Functions
• A bunch of functions are pre-defined in JES
for sound and picture manipulations
–
–
–
–
–
pickAFile()
makePicture()
makeSound()
show()
play()
• Some of these functions accept input values
theFile = pickAFile()
pic = makePicture(theFile)
Example: pickAFile()
pickAFile()
Filename
(eg: C:\Documents and Settings\mpapalas\My Documents\greenroom.jpg)
Pops up a dialog
box for the user to
select a file
Picture Functions
• makePicture(filename) creates and
returns a picture object, from the JPEG
file at the filename
• show(picture) displays a picture in a
window
• We’ll learn functions for manipulating
pictures later, like getColor(),
setColor(), and repaint()
Example: makePicture()
theFile
makePicture(filename)
Picture object corresponding to
image that is saved in theFile
makePicture(filename)
• creates and returns a picture object, from the JPEG file at the filename
Example: show()
Picture object corresponding to
image that is saved in theFile
show(picture-obj)
Pops up a new window
displaying image stored
in the the picture object
Demonstrating picture
manipulation with JES
>>>
>>> print pickAFile()
C:\Documents and Settings\mpapalas\Desktop\sample.jpg
>>> theFile = "C:\Documents and Settings\mpapalas\Desktop\sample.jpg"
>>> makePicture(theFile)
Picture, filename C:\Documents and Settings\mpapalas\Desktop\sample.jpg
height 1200 width 1600
>>> print makePicture(theFile)
Picture, filename C:\Documents and Settings\mpapalas\Desktop\sample.jpg
height 1200 width 1600
>>> pic = makePicture(theFile)
>>> print pic
Picture, filename C:\Documents and Settings\mpapalas\Desktop\sample.jpg
height 1200 width 1600
>>> show(pic)
Writing a recipe:
Making our own functions
def fcnName (input1, input2,...) :
block describing what the function should do
return value
• To make a function, use the command def
• Then, the name of the function, and the names of
the input values between parentheses (“(input1)”)
• Important: End the line with a colon (“:”)
• The body of the recipe is indented (Hint: Use two
spaces)
– That’s called a block
– Optionally, the function can return a value
Writing a recipe:
Making our own functions
def fcnName (input1, input2,...) :
block describing what the function should do
return value
Optional
• To make a function, use the command def
• Then, the name of the function, and the names of
the input values between parentheses (“(input1)”)
• Important: End the line with a colon (“:”)
• The body of the recipe is indented (Hint: Use two
spaces)
– That’s called a block
– Optionally, the function can return a value
A recipe for displaying picked
picture files
def pickAndShow():
theFile = pickAFile()
theFile
pickAFile()
Pops up a dialog
box for the user to
select a file
Defines a new function
A recipe for displaying picked
picture files
def pickAndShow():
theFile = pickAFile()
pic = makePicture(theFile)
theFile
pickAFile()
makePicture(filename)
Pops up a dialog
box for the user to
select a file
pic
A recipe for displaying picked
picture files
def pickAndShow():
theFile = pickAFile()
pic = makePicture(theFile)
show(pic)
theFile
pickAFile()
makePicture(filename)
Pops up a dialog
box for the user to
select a file
pic
show(picture-obj)
Pops up a new window
displaying image stored
in the the picture object
A recipe for displaying picked
picture files
def pickAndShow():
theFile = pickAFile()
pic = makePicture(theFile)
show(pic)
pickAndShow()
theFile
pickAFile()
makePicture(filename)
Pops up a dialog
box for the user to
select a file
pic
show(picture-obj)
Pops up a new window
displaying image stored
in the the picture object
A recipe for playing picked
sound files
def pickAndPlay():
myfile = pickAFile()
mysound = makeSound(myfile)
play(mysound)
Note: myfile and mysound, inside pickAndPlay(), are
completely different from the same names in the command
area.
Anything complicated is best
done in the Program Area
Program area
Program Area
- A simple editor for
programs
Command area
Command Area
Using the Program Area
• Type your program (or cut and paste from the
command area)
• Save (or Save As) - use .py for file extension
• Load Program (click on button between command
and program areas)
–
–
–
–
Before you load it, the program is just a bunch of characters.
Loading encodes it as an executable function
You must Save before Loading
You must Load before you can use your function
Making functions the easy
way
• Get something working by typing
commands
• Enter the def command.
• Copy-paste the right commands up into
the recipe
Names for variables and
functions can be (nearly)
anything!
• Must start with a letter (but can contain numerals)
• Can’t contain spaces
– myPicture is okay but my Picture is not
• Be careful not to use command names as your own names
– print = 1 won’t work
– (Avoid names that appear in the editor pane of JES highlighted in
blue or purple)
• Case matters
– MyPicture is not the same as myPicture or mypicture
• Sensible names are sensible
– E.g. myPicture is a good name for a picture, but not for a picture
file.
– x could be a good name for an x-coordinate in a picture, but
probably not for anything else
Blocking is indicated for you in
JES
• Statements that are
indented the same,
are in the same
block.
• Statements that are
in the same block as
where the line
where the cursor is
are enclosed in a
blue box.
Now what?
• When you load your program, what happens
with the function?
• After a program with a function definition is
loaded, that function can be used either from
the command or the program area
• Try using your function by typing
pickAndShow() in the command area
Exploring more functions
• The JES Functions menu has functions
arranged by type - check them out!
• Can you find a function that inputs a
number?
• Can you find under what category
pickAFile() is listed?
Reading
• Chapters 1, 2 from “Introduction to
Computing and Programming in Python”
Assignment
Create a function that…
– Causes an interactive input window to pop up and
request some form of input from the user and
stores it in a variable
– Displays the value stored in the variable
– Displays an image from a file
• Save your function in a file
• Load it
• Use the function in the command area to
make sure that it works and does what you
want it to do
• Submit your file through webct