class-08.md

Reading-Notes

Code Fellows Python 401

Read: 08 - Ten Thousand Game 3

List Comprehensions

• In Python, List Comprehensions are a more compact way to create and manage lists
• faster and more flexible than for loops

Syntax

my_new_list = [ expression for item in list ]

three main parts(in square brackets):

1. expression - expression we’d like to carry out
2. item - object that the expression will work on
3. list - iterable list of objects to build our new list from

Create a List with range()

Creating a list with list comprehensions

# construct a basic list using range() and list comprehensions
# syntax
# [ expression for item in list ]
digits = [x for x in range(10)]

print(digits)

Output

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

• the first x doesn't do anything(just records a number)
• second x is the item created

Create a List Using Loops and List Comprehension in Python

Comparing list creation methods in Python

For Loop

# create a list using a for loop
squares = []

for x in range(10):
    # raise x to the power of 2
    squares.append(x**2)

print(squares)

List Comp

# create a list using list comprehension
squares = [x**2 for x in range(10)]

print(squares)

Output

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

• List comprehension reduces the amount of code needed to produce the same output

Multiplying Parts of a List

# create a list with list comprehensions
multiples_of_three = [ x*3 for x in range(10) ]

print(multiples_of_three)

Output

[0, 3, 6, 9, 12, 15, 18, 21, 24, 27]

# create a list with list even numbers
even_numbers = [ x for x in range(1,20) if x % 2 == 0]

Show the first letter of each word using Python

# a list of the names of popular authors
authors = ["Ernest Hemingway","Langston Hughes","Frank Herbert","Toni Morrison",
    "Emily Dickson","Stephen King"]

# create an acronym from the first letter of the author's names
letters = [ name[0] for name in authors ]
print(letters)

Output

['E', 'L', 'F', 'T', 'E', 'S']

Separating the characters in a string

# use list comprehension to print the letters in a string
letters = [ letter for letter in "20,000 Leagues Under The Sea"]

print(letters)

Output

['2', '0', ',', '0', '0', '0', ' ', 'L', 'e', 'a', 'g', 'u', 'e', 's', ' ', 'U', 'n', 'd', 'e', 'r', ' ', 'T', 'h', 'e', ' ', 'S', 'e', 'a']

Lower/Upper case converter using Python

lower_case = [ letter.lower() for letter in ['A','B','C'] ]
upper_case = [ letter.upper() for letter in ['a','b','c'] ]

print(lower_case, upper_case)

Output

['a', 'b', 'c'] ['A', 'B', 'C']

Print numbers only from a given string

# user data entered as name and phone number
user_data = "Elvis Presley 987-654-3210"
phone_number = [ x for x in user_data if x.isdigit()]

print(phone_number)

Output

['9', '8', '7', '6', '5', '4', '3', '2', '1', '0']

Parsing a file using list comprehension

• you can even read files usung list comprehension

Hold fast to dreams
For if dreams die
Life is a broken-winged bird
That cannot fly.
-Langston Hughes

# open the file in read-only mode
file = open("dreams.txt", 'r')
poem = [ line for line in file ]

for line in poem:
    print(line)

Output

Hold fast to dreams

For if dreams die

Life is a broken-winged bird

That cannot fly.

-Langston Hughs

Using functions in list comprehensions

adding arguments

# list comprehension with functions
# create a function that returns a number doubled
def double(x):
    return x*2

nums = [double(x) for x in range(1,10)]
print(nums)

Output

[2, 4, 6, 8, 10, 12, 14, 16, 18]

using filters

# add a filter so we only double even numbers
even_nums = [double(x) for x in range(1,10) if x%2 == 0]
print(even_nums)

Output

[4, 8, 12, 16]

complex arguments

nums = [x+y for x in [1,2,3] for y in [10,20,30]]
print(nums)

Output

[11, 21, 31, 12, 22, 32, 13, 23, 33]

Audio: Debugging with PySnooper

Bookmark and Review

• Primer on Decorators