Code Academy - Python

Reference : http://www.codecademy.com/tracks/python

Python Syntax

Multi-Line Comments

"""Sipping from your cup 'til it runneth over,
Holy Grail.
"""

Strings & Console Output

Escaping characters

'There\'s a snake in my boot!'

String methods

• len()
• lower()
• upper()
• str()

"Ryan".lower()
"Ryan".upper()
str(3.14)
len("roar")

String Formatting with %

print "The %s who %s %s!" % ("Knights", "say", "Ni")
# This will print "The Knights who say Ni!"

Date and Time

Getting the Current Date and Time

from datetime import datetime

now = datetime.now()
print now

Extracting Information

from datetime import datetime
now = datetime.now()

current_year = now.year
current_month = now.month
current_day = now.day

Pretty Time

from datetime import datetime
now = datetime.now()

print '%s:%s:%s' % (now.hour, now.minute, now.second)

Conditionals & Control Flow

And

• 1 < 2 and 2 < 3 is True
• 1 < 2 and 2 > 3 is False

This and That (or This, But Not That!)

Boolean operators aren’t just evaluated from left to right. Just like with arithmetic operators, there’s an order of operations for boolean operators:

• not is evaluated first
• and is evaluated next
• or is evaluated last

The Big If

if this_might_be_true():
    print "This really is true."
elif that_might_be_true():
    print "That is true."
else:
    print "None of the above."

PygLatin

Input!

name = raw_input("What's your name?")
print name

Check Yourself… Some More

x = "J123"
x.isalpha()  # False

Functions

Generic Imports

# Ask Python to print sqrt(25) on line 3.
import math

print math.sqrt(25)

Function Imports

from module import function

print sqrt(25)

Universal Imports

from module import *

Here Be Dragons

import math            # Imports the math module
everything = dir(math) # Sets everything to a list of things from math
print everything       # Prints 'em all!

['__doc__', '__name__', '__package__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'hypot', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc']
None

On Beyond Strings

def biggest_number(*args):
    print max(args)
    return max(args)

biggest_number(-10, -5, 5, 10)

Built-in Functions

maximum = max(1, 1.1, -3.2)
print maximum

minimum = min(-100, 0, 44, 100.3)
print minimum

absolute = abs(-42)
print absolute

print type(4)     # <type 'int'>
print type(4.2)   # <type 'float'>
print type('hello') # <type 'str'>

Python Lists and Dictionaries

Introduction to Lists

Lists are a datatype you can use to store a collection of different pieces of information as a sequence under a single variable name.

Late Arrivals & List Length

letters = ['a', 'b', 'c']
letters.append('d')
print len(letters)
print letters

List Slicing

suitcase = ["sunglasses", "hat", "passport", "laptop", "suit", "shoes"]

first  = suitcase[0:2]  # The first and second items (index zero and one)
middle = suitcase[2:4]  # Third and fourth items (index two and three)
last   = suitcase[4:6]  # The last two items (index four and five)

Slicing Lists and Strings

animals = "catdogfrog"
cat  = animals[:3]   # The first three characters of animals
dog  = animals[3:6]              # The fourth through sixth characters
frog = animals[6:]              # From the seventh character to the end

Maintaining Order

animals = ["ant", "bat", "cat"]
print animals.index("bat")

animals.insert(1, "dog")
print animals

For One and All

my_list = [1,9,3,8,5,7]

for number in my_list:
    # Your code here
    print 2 * number

More with ‘for’

animals = ["cat", "ant", "bat"]
animals.sort()

for animal in animals:
    print animal

This Next Part is Key

# Assigning a dictionary with three key-value pairs to residents:
residents = {'Puffin' : 104, 'Sloth' : 105, 'Burmese Python' : 106}

print residents['Puffin'] # Prints Puffin's room number

# Your code here!
print residents['Sloth']
print residents['Burmese Python']

New Entries

dict_name[new_key] = new_value

Changing Your Mind

del dict_name[key_name]

dict_name[key] = new_value

Remove a Few Things

beatles = ["john","paul","george","ringo","stuart"]
beatles.remove("stuart")
print beatles
>> ["john","paul","george","ringo"]

It’s Dangerous to Go Alone! Take This

my_dict = {
    "fish": ["c", "a", "r", "p"],
    "cash": -4483,
    "luck": "good"
}
print my_dict["fish"][0]

A Day at the Supermarket

This is KEY!

# A simple dictionary
d = {"foo" : "bar"}

for key in d: 
    print d[key]  # prints "bar"

Student Becomes the Teacher

Lesson Number One

animal_sounds = {
    "cat": ["meow", "purr"],
    "dog": ["woof", "bark"],
    "fox": [],
}
print animal_sounds["cat"]

Lists and Functions

Appending to a list

n = [1, 3, 5]
# Append the number 4 here
n.append(4)

Removing elements from lists

This exercise will expand on ways to remove items from a list. You actually have a few options. For a list called n:

1. n.pop(index) will remove the item at index from the list and return it to you:

   n = [1, 3, 5]
   n.pop(1)
   # Returns 3 (the item at index 1)
   print n
   # prints [1, 5]
   

2. n.remove(item) will remove the actual item if it finds it:

   n.remove(1)
   # Removes 1 from the list,
   # NOT the item at index 1
   print n
   # prints [3, 5]
   

3. del(n[1]) is like .pop in that it will remove the item at the given index, but it won’t return it:

   del(n[1])
   # Doesn't return anything
   print n
   # prints [1, 5]
   

Passing a range into a function

range(6) # => [0,1,2,3,4,5]
range(1,6) # => [1,2,3,4,5]
range(1,6,3) # => [1,4]

• range(stop)
• range(start, stop)
• range(start, stop, step)

Iterating over a list in a function

Now that we’ve learned about range, we have two ways of iterating through a list.

• Method 1 : for item in list

  for item in list:
   print item
  

• Method 2 : iterate through indexes

  for i in range(len(list)):
   print list[i]
  

Method 1 is useful to loop through the list, but it’s not possible to modify the list this way. Method 2 uses indexes to loop through the list, making it possible to also modify the list if needed. Since we aren’t modifying the list, feel free to use either one on this lesson!

Using two lists as two arguments in a function

a = [1, 2, 3]
b = [4, 5, 6]
print a + b
# prints [1, 2, 3, 4, 5, 6]

Battleship!

Make a List

print ["O"] * 5
# prints ['O', 'O', 'O', 'O', 'O']

Printing Pretty

letters = ['a', 'b', 'c', 'd']
print " ".join(letters)
print "---".join(letters)

Hide…

from random import randint
coin = randint(0, 1) # 0, 1
dice = randint(1, 6) # 1, 2, 3, 4, 5, 6

Loops

While / else

while condition:
    pass
else:
    pass

For your strings

word = "eggs!"
for c in word:
    print c

Looping over a dictionary

d = {'x': 9, 'y': 10, 'z': 20}
for key in d:
    if d[key] == 10
        print "This dictionary has the value 10!"

Counting as you go

choices = ['pizza', 'pasta', 'salad', 'nachos']

print 'Your choices are:'
for index, item in enumerate(choices):
    print index + 1, item

Multiple lists

It’s also common to need to iterate over two lists at once. This is where the built-in zip function comes in handy.

zip will create pairs of elements when passed two lists, and will stop at the end of the shorter list.

zip can handle three or more lists as well!

For / else

names = ['Hyde', 'Jack', 'Dorothy', 'Jane']

for name in names:
    print name
else:
    print "No more name"

Advanced Topics in Python

keys() and values()

my_dict = {
    "Name" : "Junggu Lee",
    "Company" : "LGE",
    "Age" : 41
}

print my_dict.keys()
print my_dict.values()
print my_dict.items()

The ‘in’ Operator

for number in range(5):
    print number,

d = { "name": "Eric", "age": 26 }
for key in d:
    print key, d[key],

for letter in "Eric":
    print letter,  # note the comma!

List Comprehension Syntax

new_list = [x for x in range(1,6)]
# => [1, 2, 3, 4, 5]

doubles = [x*2 for x in range(1,6)]
# => [2, 4, 6, 8, 10]

doubles_by_3 = [x*2 for x in range(1,6) if (x*2)%3 == 0]
# => [6]

List Slicing Syntax

[start:end:stride]

Reversing a List

letters = ['A', 'B', 'C', 'D', 'E']
print letters[::-1]

Anonymous Functions

See the lambda bit? Typing

lambda x: x % 3 == 0

Is the same as

def by_three(x):
    return x % 3 == 0

Lambda Syntax

my_list = range(16)
print filter(lambda x: x % 3 == 0, my_list)

Try it!

cubes = [x**3 for x in range(1, 11)]
filter(lambda x: x % 3 == 0, cubes)

Binary Representaion

Lesson 10:The Base 2 Number System

print 0b1,    #1
print 0b10,   #2

The bin() Function

• bin() function
• oct() function
• hex() function

print bin(1)
print bin(2)

int()’s Second Parameter

int("42")
# ==> 42

int("110", 2)
# ==> 6

Introduction to Classes

Class Syntax

class NewClass(object):
    # Class magic here
    pass

Let’s Not Get Too Selfish

class Animal(object):
    def __init__(self, name):
        self.name = name

Inheritance Syntax

class DerivedClass(BaseClass):
    # code goes here

Override!

class Employee(object):
    def __init__(self, name):
        self.name = name
    def greet(self, other):
        print "Hello, %s" % other.name

class CEO(Employee):
    def greet(self, other):
        print "Get back to work, %s!" % other.name

ceo = CEO("Emily")
emp = Employee("Steve")
emp.greet(ceo)
# Hello, Emily
ceo.greet(emp)
# Get back to work, Steve!

``` This Looks Like a Job For …

class Derived(Base):
   def m(self):
       return super(Derived, self).m()

File Input/Output

See It to Believe It

my_list = [i**2 for i in range(1,11)]
# Generates a list of squares of the numbers 1 - 10

f = open("output.txt", "w")

for item in my_list:
    f.write(str(item) + "\n")

f.close()

Reading

my_file = open("output.txt", "r")
print my_file.read()
my_file.close()

Reading Between the Lines

my_file = open("text.txt", "r")

print my_file.readline()
print my_file.readline()
print my_file.readline()

my_file.close()

The ‘with’ and ‘as’ Keywords

Programming is all about getting the computer to do the work. Is there a way to get Python to automatically close our files for us?

Of course there is. This is Python.

You may not know this, but file objects contain a special pair of built-in methods: __enter__() and __exit__(). The details aren’t important, but what is important is that when a file object’s __exit__() method is invoked, it automatically closes the file. How do we invoke this method? With with and as.

The syntax looks like this:

with open("file", "mode") as variable:
    # Read or write to the file

with open("text.txt", "w") as textfile:
    textfile.write("Success!")

Case Closed?

f = open("bg.txt")
f.closed
# False
f.close()
f.closed
# True