Python Cheat Sheet (Module 5 version)

Contents:

• Input
• Processing using =
  • Arithmetic Expressions
• Output using print
• Selection using if
  • Relational expressions
  • Logical or Boolean Expressions
• Repetition using while
• Repetition using for
  • The range function
• Object-based programming
• Strings
• Lists

Input

Use input to get numerical data from the user,

name = input( "Prompt" )

This displays Prompt and then reads a line of input. By default the line is read from the keyboard, and reading is triggered when the Enter key is pressed. input evaluates the line it reads as a Python expression and stores the result as name.

Example:

distance = input( "Enter the distance in miles: " )

Notes:

• Note the space before the second ". This results in better spacing when the user enters their value.

Processing using =

Processing is specified in Python using assignment statements with the form,

name = expression

The value of expression is evaluated and stored in memory as name.

Notes:

• name must appear on the left and expression on the right, i.e. you cannot write,

  expression = name

Arithmetic Expressions

Arithmetic processing is expressed using standard mathematical notation within the limitations imposed by a keyboard, e.g.

3*(x + y)**4

Arithmetic Operators:

In addition the abs() function for absolute value is built-in, and less common mathematical functions can be accessed by importing the math library, e.g.

import math
print "The value of pi is", math.pi
print "The square root of pi is", math.sqrt(math.pi)

Output using print

Use print to display stored values:

print list

displays the value of each item in the list. Puts a space between each pair of values. Example:

print "The answer is", 5 + 2, "."

displays:

The answer is 7 .

Appending a comma to the end of a print statement holds the current output line open, e.g. the code,

print "The answer is",
print 5 + 2, "."

displays

The answer is 7 .

on a single line.

For more control over output appearance embed formatting codes into output strings. See section 2.2.6.2 String Formatting Operations of the Python Library Reference for the gory details.

Selection using if

Use if to execute one block of code or another, but not both.

if test:
    statement
elif test:
    statement
else:
    statement

N.B. the elif and else statements are optional as shown in the first two examples below.

Examples:

if x < 0:
    print x, "is negative"

if flip == 1:
    print "You got heads"
else:
    print "You got tails"

if num < 0:
    print "The number", num, "is negative."
elif num == 0:
    print "The number", num, "is neither positive nor negative."
else:
    print "The number", num, "is positive."

Relational Expressions

Look like standard arithmetic comparisons (but again within the limitations imposed by using a standard keyboard), e.g.

x <= 4

Relational Operators:

Logical or Boolean Expressions

Repetition using while

Use while to execute a block of code multiple times.

while test:
    statement

Examples.

Common components:

x = 1 # initialization expression
while x < 10: # test expression
    print x
    x = x + 1 # increment expression

Input validation:

num = input( "Enter a number between 1 and 100: " )
while num < 1 or num > 100:
    print "Oops, your input value (", num, ") is out of range."
    num = input( "Be sure to enter a value between 1 and 100: " )

Repeating a program:

again = "y"
while again == "y" or again == "Y" or again == "yes" or again == "Yes":
    #
    # Put the body of your program here
    #
    again = raw_input( "Play again (y/n)? " )
print "Thanks for playing"

Repetition using for

Use for to repeat a block of code for every member of a list.

print "Table of squares for 1-10"
print "    x    x**2"
for x in (1,2,3,4,5,6,7,8,9,10):
    print "%5d%8d", % (x, x**2)

The range function

The built-in function range creates arithmetic lists, and is often used to produce the lists to drive for loops.

range(start_value, stop_value, stepsize)

Example:

>>> range(1, 20, 5)
[1, 6, 11, 16]

Notes:

• the start_value defaults to 0.
• the stepsize defaults to 1.
• the stop_value is not included in the list.

Using range the example above can be rewritten,

print "Table of squares for 1-10"
print "    x    x**2"
for x in range(1,11):
    print "%5d%8d", % (x, x**2)

Object-based programming

An object contains both data and instructions on how to process the data. The data are referred to as the attribues of the object. The instructions are referred to as the methods. The attributes and methods available depend on the type of the object. The syntax to invoke an object's method is,

object_name.method_name(method_params)

Examples:

math.sqrt(100)
str.center(30)
names.sort()

Related built-in functions:

dir( object_name ) # lists both attributes and methods of object_name
type( object_name) # displays the type of object_name

Strings

Lists