Python Reference

Create lists using square brackets:

num_list = [1, 5, 11, 17]
string_list = ['hi', 'hey', 'hello']

Print the list:

print(num_list)
The output will be: [1, 5, 11, 17]

Access items - first item is in the 0 position!

print(string_list[0])
The output will be: hi

Change items

string_list[0] = "yo"

Add item to the end of the list

string_list.append("What's Up?")

Add item to a specified spot in the list

string_list.append(2,"What's Up?")

Remove item

del string_list[3]

Checking to see if Item is in list
Using the in reserved word - returns True if the item is in the list and False if not.

if "hey" in string_list:
   print("Yep, hey is in there")

Using the index method to see where the item is

num_list = [1, 5, 11, 17]
print(num_list.index("5"))
The output will be:1

Length of List use len()

list_len=len(winter_list)
print("This list has ",list_len,"items in it)

Sort list use .sort()

print("string_list.sort()

Reverse Sort list use .sort()

print("string_list.sort(reverse=True)

The basic syntax for using slice with strings is:

string[startposition:endposition]

Here are some examples:

myString = "EvaMargotMLHadleyMaxwellKaramLuke"

#returns first three characters
mystring[0:3]
'Eva'

#You can also do it like this
mystring[:3]
'Eva'

mystring[9:11]
'ML'

#returns last four characters
mystring[-4:]
'Luke'

Reverse Strings

This is even cooler - you can reverse a list by using this:

string[::-1]

Working with Lists

It works similarly with lists Here is the basic syntax:

list[startitem:enditem]

Here are some examples:

food = [“hot dog”, “cookie”, “eggs”, “bread”, “apple”]

# prints out the first item 
print(food[0:1])
hot dog

# prints out the 3rd and 4th item 
print(food[2:4])
eggs bread

# prints out the last item 
print(food[-2:])
 bread apple

for loops are one of the simplest types of loops - it is a loop that will repeat a set number of times. Here is the basic syntax:

for index_value in range (start_value, end_value, step_value):
    # repeat this code until index_value = end_value
    # can be a single line of code or multiple lines 
    # but see how each line is indented over 4 spaces

• Only the end_value is required

Simple for Loop Example

In this for loop only the end_value has been set - which means the start_value and the step_value will default to 1. That means the loop will repeat 3 times.

for x in range (3):
    print('hello - x is ',x)
Using Start and Step Values
for x in range(0, 5, 2):
    print(x)
The output will be:
   0
   2
   4
This tells it start at 0, end at 5 and to skip 2 every time.  A negative step value will make it go in reverse.
				

With Strings

This allows you to loop through every character in a string.

animal = "cat
#this examples uses the variable a as the index value"
for a in animal:    
    
    print(a)

The output will be:
   c
   a
   t

With Lists

It works slightly different with lists. Instead of just looping through a range of numbers - you can loop through a list you have already created.

color_list = ['blue', 'red', 'teal', 'orange', 'black']
#this examples uses the variable i as the index value"
for i in color_list:
    print(i)

The output will be:
  blue
  red
  teal
  orange
  black

View Full Topic

while loops are very similar to for loops except for one critical difference - you can tell a for loop how many times to loop, but a while loop will continue until a condition is met.

The loop will run the block of code as long as the condition is true. Here's an example:

   $x  =  1
   while $x <= 3:
       print(“The number is", $x)
       x +=1

  Output:
The number is 0
The number is 1
The number is 2
The number is 3

Breaking the loop

If you want to escape a loop - you can use the break keyword.

i = 0
while i < 10:
   print("this is number", i)
   check = input("Are you tired yet? (y/n) ")
   if check == "y":
       break
   i +=  1

Skipping

You can also use the continue keyword to skip the rest of the code in the loop and continue on to the next value in the loop.

for i in range(1,20): 
    answer = input("Skip this one? (y/n)")   
    if answer == "y": 
        print("Skipping") 
        continue 
    print("the number is ",i)

What happens here? If you opt to skip it, it goes back to the top of the loop to continue on but doesn't execute any of the code below it.

There are several different ways of importing a module into your code and accessing the functions that it contains. Let's look at the methods:

Import the Entire Module

This imports all the functions in a module. You have to use the module name in front of the function so you know where it comes from.

import random
print(random.randint(1,100))

The output will be:
22

Import a Single Function

Importing a single function from a module can be more efficient in terms of resources used and you do not need to define the module name.

from random import randint
print(randint(1,100))

The output will be:
77

Import all the Functions

Import all the functions from the random module means you do not have to define the module name but it can be confusing where the function came from.

from random import *
print(randint(1,100))

The output will be:
31

• Allows you to get a random number generated by the computer.
• You access it by:

import random

How to use the radint() function

This function will return a random number between the two numbers provided to it. The code below imports a random number between 1 and 100.

print(random.randint(1,100))

choice function

The choice function returns a random item from a list, tuple or string. In the example below, we imported just the single choice function from a random module which can be more efficient

from random import choice
# create a list
weather_list = [‘cloudy’, ‘snowy’, ‘sunny’, ‘rainy’, ‘foggy’]
print(choice(weather_list))

shuffle function

This function randomizes or "shuffles" items in a list. Kind of like shuffling a deck of cards!

import random
# create a list
weather_list = [‘cloudy’, ‘snowy’, ‘sunny’, ‘rainy’, ‘foggy’]
random.shuffle(weather_list)
print(weather_list)

time.time()

Returns number of seconds since January 1, 1970 at 00:00:00 Am.

Using time.time()

import time
max = 10000
t1 = time.time()
for x in range(0,max):
    print(x)
t2 = time.time()
total_time = t2-t1
print(“It took",total_time,"seconds”)

#BEGIN FunctionLand - group all your functions here
def hello():
    print("Hello!")

#MAIN program starts here
hello()

What do you notice?

• After you have defined your function: You must have a colon and all the lines of code which are in that function are indented four spaces over.
• To call a function, you simply use the function name followed by two parentheses.

Both these methods are useful for serving as a placeholder for your function code as well as serving other purposes.

Docstrings

This is a special comment that is used with functions - to explain what the function does. It is particularly useful with bigger and more complicated ones.

• The main purpose of a docstring is to explain what the function is suppose to do.
• Helps other programmers understand your function.
• To create one, add a string enclosed in three quotes as the first line of the function's code.

 def guess.py():
‘’’A game whereby the user tries to
guess the randomly generated number.’’’

• If you type help(guess.py) at the console the docstring will be displayed.

Function Stubs

If you know you are going to need a function but aren’t sure exactly what it will look like – you can create a function stub as a placeholder.

It looks like this:

def calculate_time():
pass

You pass data to functions through parameters.

• A function can have 0, 1 or multiple parameters passed to it
• You have already seen what zero parameters looks like back in the last lesson - just hello().
• Multiple parameters are separated by commas

def hello(first_name, last_name):
    print("Hello,", first_name," ",last_name,"!")

And you would call the function like this:

hello("Han", "Solo")

Often you want the function to perform some sort of operation or logical test and return some information. You do this by using the return keyword.

def function_name(parameter1, parameter2,...)
    #Code here 
    return return_value

Simple Example

def calc_area(width, length):
    area = width * height
    return area

# MAIN
# You can store the value that the function returns in a variable 
living_room_area=calc_area(75, 30)
print("The area of our living room is", living_room_area)

# Or simply directly print the return value 
print("The area of our kitchen is", calc_area(40,30))