Learning Python 3

all_the_thingz.md

Code for integer division in Python 2:

3 / 4

Code for float division in Python 2:

3 / 4.

Code for integer division in Python 3:

3 // 4

Code for float division in Python 3:

3 / 4

Create a string with single quotes in Python:

my_ball = "Will's ball"

Create a multiline string in Python:

multiline = """Will's ball
is red
and bouncy!"""

Code to escape a single quote in Python:

print("Will's ball is red")

or

print('Will\'s ball is red')

or

print("""Will's ball is red""")

Code to print tab characters in Python:

print("Will's\tball\tis\tred")

Code to use variable substitution in a string:

item = 'ball'
color = 'red'
print("Will's %s is %s." % (item, color))

Code to use variable substitution with the string format method:

item = 'ball'
color = 'red'
print("Will's {0} is {1}".format(item, color))

Code to use the .endswith method:

"The ball is red".endswith("red")

Code to find a string within a string:

"The ball is red".find("is")

Code to join strings together in Python:

" ".join(["the", "ball", "is", "red"])

Code to strip spaces from the beginning and end of a string in Python:

" Will ".strip()

Code to strip spaces from the beginning of a string in Python:

" Will ".lstrip()

Code to strip spaces from the end of a string in Python:

" Will ".rstrip()

Use the dir method to find the methods available:

dir("Will")

Use the help method to find out how to use a method:

help("Will".rstrip)

Use an if statement in Python:

x = int(input("enter an integer: "))
if x < 0:
    x = 0
    print("Negative number changed to zero")

Use an elif statement in Python:

x = int(input("enter an integer: "))
if x < 0:
    x = 0
    print("Negative number changed to zero")
elif x == 0:
    print("zero")
elif x == 1:
    print("one")
else:
    print("Some other number")

Use a for statement in Python:

pets = ['cat', 'dog', 'elephant']
for pet in pets:
    print('I have a pet {0}.format(pet))

Use for with a range of numbers in Python:

for i in range(5):
    print(i)

Use a while statement in Python:

x = 1
while x < 10:
    print(x)
    x = x + 1

Terminate a loop with a break statement in Python:

pets = ['cat', 'dog', 'elephant']
for pet in pets:
    if pet == 'dog':
        print("No dogs allowed")
        break
    else:
        print("We love {0}.format(pet))

Test for equality in Python:

5 == 5 returns True

Test for inequality in Python:

5 != 4 returns True

Test for greater than in Python:

5 > 3 returns True

Test for less than in Python:

3 < 5 returns True

Test for greater than or equal to in Python:

5 >= 5 returns True

Use chaining to test multiple conditions in Python:

3 < x < 5 returns True

Test to see if an object is a given type:

isinstance("will", str) returns True

isinstance("will", int) returns False

Test to see if an object is the exact same with the is operator:

a = True
b = True
a is b # returns True
x = [1, 2, 3]
y = [1, 2, 3]
x is y # returns Flase

Use the in operator to check if a value is present:

x = [1, 2, 3]
3 in x # returns True
5 in x # returns False

Iterate over a list and test for conditional values:

x = [1, 2, 3]
for value in x:
    if value == 2:
        print('Value is 2!')

car = {'model': 'Chevy', 'year': 1970, 'color': 'red'}
if 'model' in car:
    print('This is a {0}'.format(car['model']))

Create a list in Python:

a = [1, 2, 3, 4]
b = [1, 'cat', 7, {'car': 'chevy'}]

Access an individual item by index number in Python:

b[0] # prints the first item in the list
b[1] # prints the second item in the list

Append items to a list:

pets = []
pets.append('cat')
pets.append('dog')
pets.append('bear')
pets.append('shark')

Remove an item from a list:

pets.remove('dog')

Use the pop method to remove the last item from a list:

pets.pop()

Use the pop method to specify an item to remove by index number:

pets.pop(0) # removes cat from the list

Use the sort method to sort items in a list:

pets = ['cat', 'dog', 'bear', 'shark']
pets.sort()
pets # returns ['bear', 'cat', 'dog', 'shark']

Use the reverse method to reverse the list:

pets.reverse() # returns ['shark', 'dog', 'cat', 'bear']

Get the number of items in a list:

len(pets) # returns 4

Get the number of occurrences of an item in a list:

pets.count('bear') # returns 1

Slice a list in Python:

a = [1, 2, 3, 4, 5, 6, 7, 8, 9]
a[2:5] # returns [3, 4, 5]
a[2:]  # returns [3, 4, 5, 6, 7, 8, 9]
a[:5]  # returns [1, 2, 3, 4, 5]
a[-4:] # returns [6, 7, 8, 9]

Use slice to replace part of a list in Python:

a = [1, 2, 3, 4, 5, 6, 7, 8, 9]
a[2:5] = ['foo', 'bar', 'baz']
a # returns [1, 2, 'foo', 'bar', 'baz', 6, 7, 8, 9]

Use list comprehension to create a new list with items not found in the second list:

zoo_animals = ['giraffe', 'monkey', 'elephant', 'lion', 'bear', 'pig', 'horse', 'aardvark']
my_animals = ['monkey', 'bear', 'pig']

other_animals = [animal for animal in zoo_animals if animal not in my_animals]

Use list comprehension to create a new list after each list item is modified:

sales = [3.14, 7.99, 10.99, 0.99, 1.24]

sales = [sale * 1.07 for sale in sales]

Create a dictionary and add values to it in Python:

age = {}
age['will'] = 40
age['bob'] = 30
age['john'] = 20
age # returns {'will': 40, 'bob': 30, 'john': 20}

Access the value for a specific key in the dictionary:

print(age['will']) # returns 40

Check to see if a key is present in the dictionary:

'will' in age # returns True
'austin' in age # returns False

Delete a key/value pair from a dictionary:

del age['bob']

Use the get method to retrieve a value or a default value if not found:

print(age.get('will', 'Key not found')) # returns 40
print(age.get('austin', 'Key not found')) # returns Key not found

Iterate the key value pairs in a dictionary:

for key, value in age.items():
    print(key, value)

Create a tuple in Python:

t = 'dog', 'cat', 12345
t # returns ('dog', 'cat', 12345)

Access a tuple value by index id:

t[0] # returns 'dog'
t[1] # returns 'cat'

Create a set in Python:

animals = {'monkey', 'bear', 'dog', 'monkey', 'cat', 'bear', 'gorilla'}
animals # returns {'monkey', 'bear', 'gorilla', 'dog', 'cat'}

Check if a value is present in a set:

'monkey' in animals # returns True
'shark' in animals # returns False

Create an empty set in Python:

fish = set()

Add items to a set:

fish.add('shark')
fish.add('guppy')
fish.add('whale')

Remove items from a set:

fish.remove('whale')

Use union to combine two sets into a single, deduplicated set:

fish.union(animals) # returns {'guppy', 'monkey', 'bear', 'shark', 'gorilla', 'dog', 'cat'}

Display a list of methods and operators for a string in Python:

dir('foo')

Display the help page for a given method:

help("foo".upper)

data.csv

data.json

[
    {"name": "Will", "type": "human", "housebroken": "True"},
    {"name": "Dennis", "type": "dog", "housebroken": "True"},
    {"name": "Phelps", "type": "toad", "housebroken": "False"},
    {"name": "Hootie", "type": "owl", "housebroken": "False"},
    {"name": "Vonni", "type": "human", "housebroken": "True"}
]

executeable.md

Code to create an executable Python script:

#! /usr/bin/env python

print("Hello world") Command to make the script executable:

chmod +x hello.py

Command to execute the script:

./hello.py

read_file.py

#! /usr/bin/env python
# import csv

# with open('data.csv', 'r') as f:
#     data = csv.reader(f)
#     for row in data:
#         if row[-1] == 'True':
#             print("{0} is a {1} and is housrbroken!".format(row[0], row[1]))
#         elif row[-1] == 'False':
#             print("{0} is a {1} and is NOT housrbroken!".format(
#                 row[0], row[1]))

import json

with open('data.json', 'r') as f:
    data = json.load(f)
    for row in data:
        outcome = "housebroken" if row['housebroken'] == 'True' else "not housebroken"
        print("{0} is a {1} and is {2}".format(
            row["name"], row["type"], outcome))

virtualenv.md

Command to install virtualenv:

pip install virtualenv

Steps to create a new virtual environment:

mkdir my_project
cd my_project
virtualenv py3 -p /usr/local/bin/python3
source py3/bin/activate

Command to freeze requirements into a file:

pip freeze > requirements.txt

Command to install dependencies from a requirements file:

pip install -r requirements.txt

Command to leave a virtual environment:

deactivate