Python Lecture 1
Basic Python programs,
defining functions
Python!
• Created in 1991 by Guido van Rossum (now at Google)
– Named for Monty Python
• Useful as a scripting language
– script: A small program meant for one-time use
– Targeted towards small to medium sized projects
• Used by:
– Google, Yahoo!, Youtube
– Many Linux distributions
– Games and apps (e.g. Eve Online)
Installing Python
Windows: Mac OS X:
• Download Python from • Python is already installed.
http://www.python.org • Open a terminal and run python
• Install Python. or run Idle from Finder.
• Run Idle from the Start Menu.
Linux:
• Chances are you already have
Python installed. To check, run
python from the terminal.
• If not, install from your
distribution's package system.
Note: For step by step installation
instructions, see the course web site.
Interpreted Languages
• interpreted
– Not compiled like Java
– Code is written and then directly executed by an interpreter
– Type commands into interpreter and see immediate results
Java: Runtime
Code Compiler Computer
Environment
Python: Code Interpreter Computer
The Python Interpreter
• Allows you to type commands one-at-a-time and see results
• A great way to explore Python's syntax
– Repeat previous command: Alt+P
Basic Java code
• File name == class_name.java
• Console output: System.out.println
• Methods: public static void name() { ...
• main: function where the execution begins
• > javac Hello2.java
• >java Hello2
Hello2.java
1 public class Hello2 {
2 public static void main(String[] args) {
3 hello();
4 }
5
6 public static void hello() {
7 System.out.println("Hello, world!");
8 }
9 }
Our First Python Program
• Python does not have a main method like Java
– The program's main code is just written directly in the file
• Python statements do not end with semicolons
hello.py
1 print "Hello, world!"
The print Statement
print "text"
print (a blank line)
– Escape sequences such as \" are the same as in Java
– Strings can also start/end with '
swallows.py
1 print "Hello, world!"
2 print
3 print "Suppose two swallows \"carry\" it together."
4 print 'African or "European" swallows?'
Comments
• Syntax:
# comment text (one line)
swallows2.py
1 # Student, CS G523
2 # This program prints important messages.
3 print "Hello, world!"
4 print # blank line
5 print "Suppose two swallows \"carry\" it together."
6 print 'African or "European" swallows?'
Functions
• Function: Equivalent to a static method in Java.
• Syntax:
hello2.py
def name():
statement 1 # Prints a helpful message.
2 def hello():
statement 3 print "Hello, world!"
... 4
5 # main (calls hello twice)
statement 6 hello()
7 hello()
– Must be declared above the 'main' code
– Statements inside the function must be indented
Whitespace Significance
• Python uses indentation to indicate blocks, instead of {}
– Makes the code simpler and more readable
– In Java, indenting is optional. In Python, you must indent.
hello3.py
1 # Prints a helpful message.
2 def hello():
3 print "Hello, world!"
4 print "How are you?"
5
6 # main (calls hello twice)
7 hello()
8 hello()
Exercise
• Write a optimized program in Python (using function calls).
Its output should be:
______
/ \
/ \
\ /
\______/
\ /
\______/
+--------+
______
/ \
/ \
| STOP |
\ /
\______/
______
/ \
/ \
+--------+
Exercise Solution
def egg(): def top():
top() print " ______"
bottom() print " / \\"
print print "/ \\"
def cup(): def bottom():
bottom() print "\\ /"
line() print " \\______/"
print
def line():
def stop(): print "+--------+"
top()
print "| STOP |" # main
bottom() egg()
print cup()
stop()
def hat(): hat()
top()
line()
print
General Information
• Unlike C/C++ or Java, Python statements do not end in a
semicolon
• In Python, indentation is the way you indicate the scope of
a conditional, function, etc.
• Look, no braces!
• Python is interpretive, meaning you don’t have to write
programs.
• You can just enter statements into the Python environment
and they’ll execute
• For the most part, we’ll be writing programs
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The Python Shell
• Because Python is interpretive, you can do simple
things with the shell
• At the prompt, type Python
• You should have a >>> prompt
• Type in:
print(“hello, world”)
• You have written your first Python program
• Keep the shell up; we’ll be using it
15
The Python Shell
• This is good for simple calculations but not for real
programming
• For programming, we’ll use Idle
• There are two versions: Idle for Python 2.7 and Idle3 for
Python 3.2
• For most of what we do, we’ll have to use 2.7 because
Bluetooth doesn’t seem to work with 3.2
• You’ll run as the “superuser” because otherwise you won’t
have access to the GPIO pins
• Idle will give you access to a shell but also to an IDE for
writing and saving programs
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Python Modules
• In practice, only the simplest programs are run in the shell
• You can create a module by going to the File->New Window
menu option
• This brings up a text editor that lets you create a Python
program and run it
• Write your first “Hello World!” program thus:
print(“Hello, World!”)
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Python Modules
• Press F5
• It will ask you to save the file before you run it
• Save it to your home directory as HelloWorld.py
• You must provide the .py extension
• If you want to run it outside of the development
environment simply type:
python HelloWorld.py
• Note that Linux is case sensitive
18
Variables
• As in every language, a variable is the name of a memory
location
• Python is weakly typed
• That is, you don’t declare variables to be a specific type
• A variable has the type that corresponds to the value you
assign to it
• Variable names begin with a letter or an underscore and can
contain letters, numbers, and underscores
• Python has reserved words that you can’t use as variable
names
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Variables
• At the >>> prompt, do the following:
x=5
type(x)
x=“this is text”
type(x)
x=5.0
type(x)
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Printing
• You’ve already seen the print statement
• You can also print numbers with formatting
• These are identical to Java or C format specifiers
21
Comments
• All code must contain comments that describe what it does
• In Python, lines beginning with a # sign are comment lines
You can also have comments on the same line as a statement
# This entire line is a comment
x=5 # Set up loop counter
22
Operators
• Arithmetic operators we will use:
– + - * / addition, subtraction/negation, multiplication,
division
– % modulus, a.k.a. remainder
– ** exponentiation
• precedence: Order in which operations are computed.
– * / % ** have a higher precedence than + -
1 + 3 * 4 is 13
– Parentheses can be used to force a certain order of
evaluation.
(1 + 3) * 4 is 16
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Expressions
• When integers and reals are mixed, the result is a real
number.
– Example: 1 / 2.0 is 0.5
– The conversion occurs on a per-operator basis.
– 7 / 3 * 1.2 + 3 / 2
– 2 * 1.2 + 3 / 2
– 2.4 + 3 / 2
– 2.4 + 1
– 3.4
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Math Functions
– Use this at the top of your program: from math import *
25
Relational Operators
• Many logical expressions use relational operators:
26
Logical Operators
• These operators return true or false
27
The if Statement
• Syntax:
if <condition>:
<statements>
x = 5
if x > 4:
print(“x is greater than 4”)
print(“This is not in the scope of the if”)
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The if Statement
• The colon is required for the if
• Note that all statement indented one level in from the if
are within it scope:
x = 5
if x > 4:
print(“x is greater than 4”)
print(“This is also in the scope of the if”)
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The if/else Statement
if <condition>:
<statements>
else:
<statements>
• Note the colon following the else
• This works exactly the way you would expect
30
The for Loop
• This is similar to what you’re used to from C or Java, but not the
same
• Syntax:
for variableName in groupOfValues:
<statements>
• variableName gives a name to each value, so you can refer to it
in the statements.
• groupOfValues can be a range of integers, specified with the
range function.
• Example:
for x in range(1, 6):
print x, "squared is", x * x
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Range
• The range function specifies a range of integers:
range(start, stop) - the integers between start
(inclusive) and stop (exclusive)
• It can also accept a third value specifying the change
between values.
range(start, stop, step) - the integers between
start (inclusive) and stop (exclusive) by step
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The while Loop
• Executes a group of statements as long as a condition is
True.
• Good for indefinite loops (repeat an unknown number of
times)
• Syntax:
while <condition>:
<statements>
• Example:
number = 1
while number < 200:
print number,
number = number * 2
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Exercise
• Write a Python program to compute and display the first 16
powers of 2, starting with 1
• Do this in the Python shell
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