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Python Quick Reference
https://github.com/steincastillo/python-reference.git
Edited by: Stein Castillo
http://www.stein-castillo.com
forked from:  https://github.com/justmarkham/python-reference
By Kevin Markham (kevin@dataschool.io)
http://www.dataschool.io
Table of Contents:
    Imports
    Data Types
    Comparisons and Boolean Operations
    Conditional Statements
    Strings
    Dictionaries
    Defining Functions
    Anonymous (Lambda) Functions
    For Loops and While Loops
    Exception handling
    Comprehensions
    Map and Filter
    Collections
    Classes and objects
###############
### IMPORTS ###
###############
# 'generic import' of math module
import math
math.sqrt(25)
# import a function
from math import sqrt
sqrt(25)    # no longer have to reference the module
# import multiple functions at once
from math import cos, floor
# import all functions in a module (generally discouraged)
from csv import *
# define an alias
import datetime as dt
# show all functions in math module
##################
### DATA TYPES ###
##################
# determine the type of an object
type(2)         # returns 'int'
type(2.0)       # returns 'float'
type('two')     # returns 'str'
type(True)      # returns 'bool'
type(None)      # returns 'NoneType'
# check if an object is of a given type
isinstance(2.0, int)            # returns False
isinstance(2.0, (int, float))   # returns True
# convert an object to a given type
# zero, None, and empty containers are converted to False
bool('')    # empty string
bool([])    # empty list
bool({})    # empty dictionary
# non-empty containers and non-zeros are converted to True
bool('two')
############
### MATH ###
############
# basic operations
10 + 4          # add (returns 14)
10 - 4          # subtract (returns 6)
10 * 4          # multiply (returns 40)
10 ** 4         # exponent (returns 10000)
5 % 4           # modulo (returns 1) - computes the remainder
10 / 4          # divide (returns 2 in Python 2, returns 2.5 in Python 3)
10 / float(4)   # divide (returns 2.5)
# force '/' in Python 2 to perform 'true division' (unnecessary in Python 3)
from __future__ import division
10 / 4          # true division (returns 2.5)
10 // 4         # floor division (returns 2)
##########################################
### COMPARISONS AND BOOLEAN OPERATIONS ###
##########################################
# assignment statement
# comparisons (these return True)
# boolean operations (these return True)
5 > 3 and 6 > 3
5 > 3 or 5 < 3
False or not False and True     # evaluation order: not, and, or
##############################
### CONDITIONAL STATEMENTS ###
##############################
# if statement
    print('positive')
# if/else statement
    print('positive')
    print('zero or negative')
# if/elif/else statement
    print('positive')
elif x == 0:
    print('zero')
    print('negative')
# single-line if statement (sometimes discouraged)
if x > 0: print('positive')
# single-line if/else statement (sometimes discouraged)
# known as a 'ternary operator'
'positive' if x > 0 else 'zero or negative'
#############
### LISTS ###
#############
## properties: ordered, iterable, mutable, can contain multiple data types
# create an empty list (two ways)
empty_list = []
empty_list = list()
# Create a list eliminating duplicated elements
new_list = list(set(list_of_values))
# create a list
simpsons = ['homer', 'marge', 'bart']
# examine a list
simpsons[0]     # print element 0 ('homer')
len(simpsons)   # returns the length (3)
# modify a list (does not return the list)
simpsons.append('lisa')                 # append element to end
simpsons.extend(['itchy', 'scratchy'])  # append multiple elements to end
simpsons.insert(0, 'maggie')            # insert element at index 0 (shifts everything right)
simpsons.remove('bart')                 # search for first instance and remove it
simpsons.pop(0)                         # remove element 0 and return it
del simpsons[0]                         # remove element 0 (does not return it)
simpsons[0] = 'krusty'                  # replace element 0
simpsons.reverse()                  # reverses the list
# concatenate lists (slower than 'extend' method)
neighbors = simpsons + ['ned', 'rod', 'todd']
# find elements in a list
simpsons.count('lisa')      # counts the number of instances
simpsons.index('itchy')     # returns index of first instance
# list slicing [start:end:step]
weekdays = ['mon', 'tues', 'wed', 'thurs', 'fri']
weekdays[0]         # element 0
weekdays[0:3]       # elements 0, 1, 2
weekdays[:3]        # elements 0, 1, 2
weekdays[3:]        # elements 3, 4
weekdays[-1]        # last element (element 4)
weekdays[::2]       # every 2nd element (0, 2, 4)
weekdays[::-1]      # backwards (4, 3, 2, 1, 0)
# alternative method for returning the list backwards
list(reversed(weekdays))
# sort a list in place (modifies but does not return the list)
simpsons.sort()
simpsons.sort(reverse=True)     # sort in reverse
simpsons.sort(key=len)          # sort by a key
# return a sorted list (does not modify the original list)
sorted(simpsons)
sorted(simpsons, reverse=True)
sorted(simpsons, key=len)
# insert into an already sorted list, and keep it sorted
num = [10, 20, 40, 50]
from bisect import insort
insort(num, 30)
# create a second reference to the same list
same_num = num
same_num[0] = 0         # modifies both 'num' and 'same_num'
# copy a list (two ways)
new_num = num[:]
new_num = list(num)
# examine objects
num is same_num         # returns True (checks whether they are the same object)
num is new_num          # returns False
num == same_num         # returns True (checks whether they have the same contents)
num == new_num          # returns True
# Build-in functions with list
all(list_values)    # Return true if all the elements of the lists are true (or if the list is empty)
len(list_values)    # Return the length (number of items) in the list
sum(list_values)    # Return the sum of all the elments in the list
##############
### TUPLES ###
##############
## properties: ordered, iterable, immutable, can contain multiple data types
## like lists, but they don't change size
# create a tuple
digits = (0, 1, 'two')          # create a tuple directly
digits = tuple([0, 1, 'two'])   # create a tuple from a list
zero = (0,)                     # trailing comma is required to indicate it's a tuple
# examine a tuple
digits[2]           # returns 'two'
len(digits)         # returns 3
digits.count(0)     # counts the number of instances of that value (1)
digits.index(1)     # returns the index of the first instance of that value (1)
# elements of a tuple cannot be modified
digits[2] = 2       # throws an error
# concatenate tuples
digits = digits + (3, 4)
# create a single tuple with elements repeated (also works with lists)
(3, 4) * 2          # returns (3, 4, 3, 4)
# sort a list of tuples
tens = [(20, 60), (10, 40), (20, 30)]
sorted(tens)        # sorts by first element in tuple, then second element
                    #   returns [(10, 40), (20, 30), (20, 60)]
# tuple unpacking
bart = ('male', 10, 'simpson')  # create a tuple
(sex, age, surname) = bart      # assign three values at once
###############
### STRINGS ###
###############
## properties: iterable, immutable
# create a string
s = str(42)         # convert another data type into a string
s = 'I like you'
# Declaring a long string
longstring = 'foooo'\
'this is a long string'\
'really long'
# examine a string
s[0]                # returns 'I'
len(s)              # returns 10
# string slicing is like list slicing
s[:6]               # returns 'I like'
s[7:]               # returns 'you'
s[2:6]              # returns 'like'
s[-1]               # returns 'u'
s[-3]               # returns 'y'
# basic string methods (does not modify the original string)
s.lower()           # returns 'i like you'
s.casefold()        # returns 'i like you'
s.upper()           # returns 'I LIKE YOU'
s.startswith('I')   # returns True
s.endswith('you')   # returns True
s.isdigit()         # returns False (returns True if every character in the string is a digit)
s.find('like')      # returns index of first occurrence (2), but doesn't support regex
s.find('hate')      # returns -1 since not found
s.replace('like', 'love')    # replaces all instances of 'like' with 'love'
s.rjust(10, '-')        # returns '----------I like you'
s.capitalize()      # returns 'I Like You'
s.upper()           # returns 'I LIKE YOU'
s.lower()           # returns 'i like you'
s.rstrip()          # Remove trailing spaces of a string
s.lstrip()          # Remove leading spaces of a string
s.strip()           # Remove leading AND trailing spaces of a string
s.encode(encoding = 'utf-8', errors='ignore') # Encodes the string using the specified encoding if not specified, utf-8
                                              # errors: 'backslachreplace, ignore, namreplace, strict, replace, xmlcharrefreplace'
# split a string into a list of substrings separated by a delimiter
s.split(' ')        # returns ['I', 'like', 'you']
s.split()           # equivalent (since space is the default delimiter)
s2 = 'a, an, the'
s2.split(',')       # returns ['a', ' an', ' the']
# join a list of strings into one string using a delimiter
stooges = ['larry', 'curly', 'moe']
' '.join(stooges)   # returns 'larry curly moe'
'<->'.join(stooges) # returns 'larry<->curly<->moe'
' '.join(reversed(stooges)) # returns 'moe curly larry'
# concatenate strings
s3 = 'The meaning of life is'
s3 + ' ' + s4       # returns 'The meaning of life is 42'
# Multiply strings
start * 10  # Returns '**********'
# remove whitespace from start and end of a string
s5 = '  ham and cheese  '
s5.strip()          # returns 'ham and cheese'
s5.strip('and')
# String conditional statement
if 'meaning' in s3:                 # Checks if the substring is containd in the string
    print ('found meaning')
# String loop
for s in s3:                    # Iterates the string and prints each character
    print(s)
# string substitutions: all of these return 'raining cats and dogs'
'raining %s and %s' % ('cats', 'dogs')                       # old way
'raining {} and {}'.format('cats', 'dogs')                   # new way
'raining {arg1} and {arg2}'.format(arg1='cats', arg2='dogs') # named arguments
'message: {s3} is {s4}'.format(**locals())  # returns message: the meaning of life is 42
# string formatting
# more examples: https://mkaz.tech/python-string-format.html
'pi is {:.2f}'.format(3.14159)      # returns 'pi is 3.14'
# normal strings versus raw strings
print('first line\nsecond line')    # normal strings allow for escaped characters
print(r'first line\nfirst line')    # raw strings treat backslashes as literal characters
####################
### DICTIONARIES ###
####################
## properties: unordered, iterable, mutable, can contain multiple data types
## made of key-value pairs
## keys must be unique, and can be strings, numbers, or tuples
## values can be any type
# create an empty dictionary (two ways)
empty_dict = {}
empty_dict = dict()
# create a dictionary (two ways)
family = {'dad':'homer', 'mom':'marge', 'size':6}
family = dict(dad='homer', mom='marge', size=6)
# convert a list of tuples into a dictionary
list_of_tuples = [('dad', 'homer'), ('mom', 'marge'), ('size', 6)]
family = dict(list_of_tuples)
# examine a dictionary
family['dad']       # returns 'homer'
len(family)         # returns 3
'mom' in family     # returns True
'marge' in family   # returns False (only checks keys)
# returns a list (Python 2) or an iterable view (Python 3)
family.keys()       # keys: ['dad', 'mom', 'size']
family.values()     # values: ['homer', 'marge', 6]
family.items()      # key-value pairs: [('dad', 'homer'), ('mom', 'marge'), ('size', 6)]
sorted(family.keys())   # Returns a sorted list of the dict keys. -> good to use in dict iterations
# modify a dictionary (does not return the dictionary)
family['cat'] = 'snowball'              # add a new entry
family['cat'] = 'snowball ii'           # edit an existing entry
del family['cat']                       # delete an entry
family['kids'] = ['bart', 'lisa']       # dictionary value can be a list
family.pop('dad')                       # remove an entry and return the value ('homer')
family.update({'baby':'maggie', 'grandpa':'abe'})   # add multiple entries
# access values more safely with 'get'
family['mom']                       # returns 'marge'
family.get('mom')                   # equivalent
family['grandma']                   # throws an error since the key does not exist
family.get('grandma')               # returns None instead
family.get('grandma', 'not found')  # returns 'not found' (the default)
# access a list element within a dictionary
family['kids'][0]                   # returns 'bart'
family['kids'].remove('lisa')       # removes 'lisa'
# string substitution using a dictionary
'youngest child is %(baby)s' % family   # returns 'youngest child is maggie'
# Iterating over dictionaries
for key in sorted(family.keys()):
    print (key, family[key])
#nested dictionaries
w = {"id": 2340108, "name": "my city", "main":{"temp":5.67, "humidity": 86, "pressure": 1009}}
w["id"]     #returns 2340108
w["name"]       # returns "my city"
w["main"]       #  returns {"temp":5.67, "humidity": 86, "pressure": 1009}
w["main"]["temp"]               #returns 5.67
w["main"]["pressure"]       #returns 1009
############
### SETS ###
############
## properties: unordered, iterable, mutable, can contain multiple data types
## made of unique elements (strings, numbers, or tuples)
## like dictionaries, but with keys only (no values)
# create an empty set
empty_set = set()
# create a set
languages = {'python', 'r', 'java'}         # create a set directly
snakes = set(['cobra', 'viper', 'python'])  # create a set from a list
# examine a set
len(languages)              # returns 3
'python' in languages       # returns True
# set operations
languages & snakes          # returns intersection: {'python'}
languages | snakes          # returns union: {'cobra', 'r', 'java', 'viper', 'python'}
languages - snakes          # returns set difference: {'r', 'java'}
snakes - languages          # returns set difference: {'cobra', 'viper'}
# modify a set (does not return the set)
languages.add('sql')        # add a new element
languages.add('r')          # try to add an existing element (ignored, no error)
languages.remove('java')    # remove an element
languages.remove('c')       # try to remove a non-existing element (throws an error)
languages.discard('c')      # remove an element if present, but ignored otherwise
languages.pop()             # remove and return an arbitrary element
languages.clear()           # remove all elements
languages.update(['go', 'spark'])  # add multiple elements (can also pass a set)
# get a sorted list of unique elements from a list
sorted(set([9, 0, 2, 1, 0]))    # returns [0, 1, 2, 9]
##########################
### DEFINING FUNCTIONS ###
##########################
# define a function with no arguments and no return values
def print_text():
    print('this is text')
# call the function
print_text()
# define a function with one argument and no return values
def print_this(x):
    print(x)
# call the function
print_this(3)       # prints 3
n = print_this(3)   # prints 3, but doesn't assign 3 to n
                    # because the function has no return statement
# define a function with one argument and one return value
def square_this(x):
    return x**2
# include an optional docstring to describe the effect of a function
def square_this(x):
    """Return the square of a number."""
    return x**2
# call the function
square_this(3)          # prints 9
var = square_this(3)    # assigns 9 to var, but does not print 9
# define a function with two 'positional arguments' (no default values) and
# one 'keyword argument' (has a default value)
def calc(a, b, op='add'):
    if op == 'add':
        return a + b
    elif op == 'sub':
        return a - b
        print('valid operations are add and sub')
# call the function
calc(10, 4, op='add')   # returns 14
calc(10, 4, 'add')      # also returns 14: unnamed arguments are inferred by position
calc(10, 4)             # also returns 14: default for 'op' is 'add'
calc(10, 4, 'sub')      # returns 6
calc(10, 4, 'div')      # prints 'valid operations are add and sub'
# use 'pass' as a placeholder if you haven't written the function body
def stub():
# return two values from a single function
def min_max(nums):
    return min(nums), max(nums)
# return values can be assigned to a single variable as a tuple
nums = [1, 2, 3]
min_max_num = min_max(nums)         # min_max_num = (1, 3)
# return values can be assigned into multiple variables using tuple unpacking
min_num, max_num = min_max(nums)    # min_num = 1, max_num = 3
####################################
### ANONYMOUS (LAMBDA) FUNCTIONS ###
####################################
## primarily used to temporarily define a function for use by another function
# define a function the "usual" way
def squared(x):
    return x**2
# define an identical function using lambda
squared = lambda x: x**2
# sort a list of strings by the last letter (without using lambda)
simpsons = ['homer', 'marge', 'bart']
def last_letter(word):
    return word[-1]
sorted(simpsons, key=last_letter)
# sort a list of strings by the last letter (using lambda)
sorted(simpsons, key=lambda word: word[-1])
#################################
### FOR LOOPS AND WHILE LOOPS ###
#################################
# range returns a list of integers (Python 2) or a sequence (Python 3)
range(0, 3)     # returns [0, 1, 2]: includes start value but excludes stop value
range(3)        # equivalent: default start value is 0
range(0, 5, 2)  # returns [0, 2, 4]: third argument is the step value
# Python 2 only: use xrange to create a sequence rather than a list (saves memory)
xrange(100, 100000, 5)
# for loop (not the recommended style)
fruits = ['apple', 'banana', 'cherry']
for i in range(len(fruits)):
    print(fruits[i].upper())
# for loop (recommended style)
for fruit in fruits:
    print(fruit.upper())
# iterate through two things at once (using tuple unpacking)
family = {'dad':'homer', 'mom':'marge', 'size':6}
for key, value in family.items():
    print(key, value)
# use enumerate if you need to access the index value within the loop
for index, fruit in enumerate(fruits):
    print(index, fruit)
# for/else loop
for fruit in fruits:
    if fruit == 'banana':
        print('Found the banana!')
        break    # exit the loop and skip the 'else' block
    # this block executes ONLY if the for loop completes without hitting 'break'
    print("Can't find the banana")
# while loop
while count < 5:
    print('This will print 5 times')
    count += 1    # equivalent to 'count = count + 1'
##########################
### EXCEPTION HANDLING ###
##########################
# When an error occours, or exception as it is called, Python will normally stop the execution
# and generate and error message.
# These exceptions can be handled using the try statement:
    print(x)
    print('An exception has ocourred')
# else can be used to define a block of code to be executed if no errors were raised:
    print('Hello')
    print ('Something went wrong')
    print('Nothing went wrong')
######################
### COMPREHENSIONS ###
######################
# A list comprehension is an expression and loop (with an optional condition)
# enclosed in brackets:
# [item for item in iterable]
# [expression for item in iterable]
# [expression for item in iterable if condition]
# examples:
a = [2**i for i in range(13)]
a = [1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096]
a = [y for y in range(1900, 1940) if y%4 == 0]
a = [1900, 1904, 1908, 1912, 1916, 1920, 1924, 1928, 1932, 1936]
# for loop to create a list of cubes
nums = [1, 2, 3, 4, 5]
for num in nums:
    cubes.append(num**3)
# equivalent list comprehension
cubes = [num**3 for num in nums]    # [1, 8, 27, 64, 125]
# for loop to create a list of cubes of even numbers
cubes_of_even = []
for num in nums:
    if num % 2 == 0:
        cubes_of_even.append(num**3)
# equivalent list comprehension
# syntax: [expression for variable in iterable if condition]
cubes_of_even = [num**3 for num in nums if num % 2 == 0]    # [8, 64]
# for loop to cube even numbers and square odd numbers
cubes_and_squares = []
for num in nums:
    if num % 2 == 0:
        cubes_and_squares.append(num**3)
        cubes_and_squares.append(num**2)
# equivalent list comprehension (using a ternary expression)
# syntax: [true_condition if condition else false_condition for variable in iterable]
cubes_and_squares = [num**3 if num % 2 == 0 else num**2 for num in nums]    # [1, 8, 9, 64, 25]
# for loop to flatten a 2d-matrix
matrix = [[1, 2], [3, 4]]
for row in matrix:
    for item in row:
        items.append(item)
# equivalent list comprehension
items = [item for row in matrix
              for item in row]      # [1, 2, 3, 4]
# set comprehension
fruits = ['apple', 'banana', 'cherry']
unique_lengths = {len(fruit) for fruit in fruits}   # {5, 6}
# dictionary comprehension
fruit_lengths = {fruit:len(fruit) for fruit in fruits}              # {'apple': 5, 'banana': 6, 'cherry': 6}
fruit_indices = {fruit:index for index, fruit in enumerate(fruits)} # {'apple': 0, 'banana': 1, 'cherry': 2}
######################
### MAP AND FILTER ###
######################
# 'map' applies a function to every element of a sequence
# ...and returns a list (Python 2) or iterator (Python 3)
simpsons = ['homer', 'marge', 'bart']
map(len, simpsons)                      # returns [5, 5, 4]
map(lambda word: word[-1], simpsons)    # returns ['r', 'e', 't']
# equivalent list comprehensions
[len(word) for word in simpsons]
[word[-1] for word in simpsons]
# 'filter' returns a list (Python 2) or iterator (Python 3) containing
# ...the elements from a sequence for which a condition is True
nums = range(5)
filter(lambda x: x % 2 == 0, nums)      # returns [0, 2, 4]
# equivalent list comprehension
[num for num in nums if num % 2 == 0]
###################
### COLLECTIONS ###
###################
# Python's collections module implements specialized container data types
# providing high-performance alternatives to the general purpose built-in containers.
# A specialized Counter type (subclass for counting objects) is provided by Python's collections.Counter:
from collections import Counter
seq1 = [1, 2, 3 ,5, 1, 2, 5, 5, 2, 1, 4]
seq_count = Counter(seq1)       # Counter({1: 3, 2: 3, 5: 3, 3: 1, 4: 1})
seq_count.most_common()     #  [(1, 3), (2, 3), (5, 3), (3, 1), (4, 1)] -> returns the count of the elements in the dict
seq_count.most_common(2)    # [(1, 3), (2, 3)] -> returns the 2 most commo elements
#########################################################
### CLASSES AND OBJECTS (Object Oriented Programming) ###
#########################################################
# Class definition
class myClass(self):
# Constructor definiton
# This method is executed when an instance of the class is created
class myClass(self):
    def __init__ (self, parm1, parm2):
        self.parm1 = parm1
        self.parm2 = parm2
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