Transcript Sudden Python

Sudden Python
Drinking from the Fire Hose
27-Mar-16
Get Python (and IDLE)
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We will be using Python 2 (version 2.7.3), not
Python 3
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Get it from www.python.org
The download includes an Integrated
Development Environment (IDE), named
“IDLE”
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Running IDLE
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IDLE opens a window in which you can
enter and run Python commands
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This window is called a REPL (Read-EvalPrint-Loop)
Choose File -> New Window to open a
window in which you can type entire
programs
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To execute the program, hit the F5 key
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Commands and data
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A program consists of commands (more commonly
called “statements”) that manipulate data
Here are the four most common kinds of data:
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Integers (whole numbers), such as 5, 17, or -300
“Floating point” numbers, such as 3.1416
“Strings” are character sequences enclosed in either single
quotes or double quotes, such as "Madam, I'm Adam" and
'"Too soon," she said.'
 Straight quotes only (" and '), not curly quotes ( “” ‘’)
“Boolean” (logical) values—there are only two of these,
True and False
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Program components
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Programs can read in data
Programs can write results
In between reading and writing, programs can
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compute, that is, do arithmetic (or logic)
test, that is, decide what to do next
loop, that is, do the same actions a number of times
delegate, that is, ask other parts of the program to
perform some task
Also, programs can ignore comments
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Doing simple arithmetic
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Here are the arithmetic operators:
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+
*
/
performs addition
performs subtraction
performs multiplication
performs division
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When dividing two integers, the result is an integer: 14 / 5 is 2
% performs modulus (remainder of division): 14 % 5 is 4
** performs exponentiation
The result of doing arithmetic is often assigned to a variable:
sum = 10 + 22 + 13 + 44 + 72
Variables can be used in arithmetic:
average = sum / 5
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Reading in data
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Here’s how to ask the user to enter a string:
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Here’s how to ask the user to enter a number:
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name = raw_input("What is your name? ")
Whatever the user types in, up to a press of the Enter key, is
a string that is assigned to the variable name
age = input("What is your age? ")
Whatever the user types in, up to a press of the Enter key, is
converted to a number and assigned to the variable age
input and raw_input are functions (or methods)
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For now, we will treat “function” and “method” as synonyms
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Printing results
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To print results, use the print statement
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You can print multiple things separated by commas
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print "Two plus two is four"
print 2, "plus", 2, "is", 2 + 2
Each print statement writes a “newline” at the end (so
that the next print statement goes to a new line)
You can omit the newline by ending with a comma:
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print 2, "plus", 2,
print "is", 2 + 2
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Comments
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A comment is a note to any human looking at the program;
comments are ignored by the computer.
A comment begins with # and extends to the end of the line
Good uses of comments:
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At the beginning of a program, to tell what the program does
When using someone else’s code, to say where you got it from
To explain any code that’s hard to understand
Bad uses of comments:
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To explain something that’s obvious anyway
To explain code that’s hard to understand, but could be made simpler
To add irrelevant comments, like # Go Eagles!
When you should instead use a doc string (described on a later slide)
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Layout
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Every statement goes on a line by itself
Put spaces around operators, including the assignment
operator (=)
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Put spaces after commas (but not before commas)
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average = sum / 5
print 2, "plus", 2, "is", 2 + 2
When using a function, do not put spaces on either side
of the parentheses
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age = input("What is your age? ")
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Decisions and tests
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Your program can decide what to do by making a test
The result of a test is a boolean value, True or False
Here are tests on numbers:
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< means “is less than”
<= means “is less than or equal to”
== means “is equal to”
!= means “is not equal to”
>= means “is greater than or equal to”
< means “is greater than”
These same tests work on strings
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All capital letters are “less than” all lowercase letters
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Compound tests
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Boolean values can be combined with these operators:
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and – gives True if both sides are True
or – gives True if at least one side is True
not – given True, this returns False, and vice versa
Examples
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score > 0 and score <= 100
name == "Joe" and not score > 100
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The if statement
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The if statement evaluates a test, and if it is True,
performs the following indented statements; but if the
test is False, it does nothing
Examples:
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if grade == "A+":
print "Congratulations!"
if score < 0 or score > 100:
print "That’s not possible!"
score = input("Enter a correct value: ")
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if with else
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The if statement can have an optional else part, to be
performed if the test result is False
Example:
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if grade == "A+":
print "Congratulations!"
else:
print "You could do so much better."
print "Your mother will be disappointed."
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if with elif
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The if statement can have any number of elif tests
Only one group of statements is executed—those
controlled by the first test that passes
Example:
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if grade == "A":
print "Congratulations!"
elif grade == "B":
print "That's pretty good."
elif grade == "C":
print "Well, it's passing, anyway."
else:
print "You really blew it this time!"
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Indentation
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Indentation is required and must be consistent
Standard indentation is 4 spaces or one tab
IDLE does this pretty much automatically for you
Example:
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if 2 + 2 != 4:
print "Oh, no!"
print "Arithmethic doesn't work!"
print "Time to buy a new computer."
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Lists and ranges
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A list is a sequence of values enclosed in brackets
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You can refer to an individual value by putting a bracketed
number (starting from 0) after the list
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Example: len(courses) is 3
range is a function that creates a list of integers, from the first
number up to but not including the second number
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Example: courses[2] is 'CIT 593'
The len function tells you how many things are in a list
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Example: courses = ['CIT 591', 'CIT 592', 'CIT 593']
Example: range(0, 5) creates the list [0, 1, 2, 3, 4]
If you give range a third number, it is used as the step size
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Example: range(2, 10, 3) creates the list [2, 5, 8]
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The for loop
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A for loop performs the same statements for each value
in a list
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Example:
for n in range(1, 4):
print "This is the number", n
prints
This is the number 1
This is the number 2
This is the number 3
The for loop uses a variable (in this case, n) to hold the
current value in the list
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The while loop
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A while loop performs the same statements over and over until
some test becomes False
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Example:
n = 3
while n > 0:
print n, "is a nice number."
n = n – 1
prints
3 is a nice number.
2 is a nice number.
1 is a nice number.
If the test is initially False, the while loop doesn't do anything.
If the test never becomes False, you have an "infinite loop."
This is usually bad.
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Calling a function
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A function is a section of code that either (1) does some input or
output, or (2) computes some value.
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A function can do both, but it's bad style.
Good style is functions that are short and do only one thing
Most functions take one or more arguments, to help tell them what to do
Here's a function that does some input:
age = input("How old are you? ")
The argument, "How old are you?", is shown to the user
Here's a function that computes a value (a list):
odds = range(1, 100, 2)
The arguments are used to tell what to put into the list
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Defining a function
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def sum(numbers):
"""Finds the sum of the numbers in a list."""
total = 0
for number in numbers:
total = total + number
return total
def defines a function
numbers is a parameter: a variable used to hold an argument
This doc string tells what the function does
A function that computes a value must return it
sum(range(1, 101)) will return 5050
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Summary
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Arithmetic: + - * / %
< <= == != >= >
Logic (boolean): True False and or not
Strings: "Double quoted" or 'Single quoted'
Lists: [1, 2, 3, 4] len(lst) range(0, 100, 5)
Input: input(question)
raw_input(question)
Decide: if test: elif test: else:
For loop: for variable in list:
While loop: while test:
Calling a function: sum(numbers)
Defining a function: def sum(numbers): return result
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Advice to beginners
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Programming is hard!
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You will make many mistakes, and they will (almost) all be
stupid mistakes
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The individual building blocks are all pretty simple, but they go together in
complex patterns
That doesn’t mean you are stupid!
Experts make just as many stupid mistakes, but they are more experienced
at finding and correcting them
Therefore: Don’t be shy about letting other people see your mistakes
In a few weeks you will find that it suddenly all starts to make
sense, and you'll wonder what the problem was
Don’t panic!
There’s lots of help available
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Advice to non-beginners
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A lot is known about how to program well
You probably have a lot of bad habits to unlearn
The following are important habits to learn:
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Thorough testing is essential; anything less is amateurish
Concentrate on clarity (not efficiency) at all times
If code is hard to understand, simplify it
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Use comments to explain code you aren’t able to simplify
Remember that the best way to learn something is to
teach it
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The End
“Programming is an art form that fights back.”
-- Anonymous
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