Getting Started

Introduction

• Python (python.org)
• Learn X in Y minutes (learnxinyminutes.com)
• Regex in python (quickref.me)

Hello World

>>> print("Hello, World!")
Hello, World!

The famous "Hello World" program in Python

Variables

age = 18      # age is of type int
name = "John" # name is now of type str
print(name)

Python can't declare a variable without assignment.

Data Types

str	Text
int, float, complex	Numeric
list, tuple, range	Sequence
dict	Mapping
set, frozenset	Set
bool	Boolean
bytes, bytearray, memoryview	Binary
See: Data Types

Slicing String

>>> msg = "Hello, World!"
>>> print(msg[2:5])
llo

See: Strings

Lists

mylist = []
mylist.append(1)
mylist.append(2)
for item in mylist:
    print(item) # prints out 1,2

See: Lists

If Else

num = 200
if num > 0:
    print("num is greater than 0")
else:
    print("num is not greater than 0")

See: Flow control

Loops

for item in range(6):
    if item == 3: break
    print(item)
else:
    print("Finally finished!")

See: Loops

Functions

>>> def my_function():
...     print("Hello from a function")
...
>>> my_function()
Hello from a function

See: Functions

File Handling

with open("myfile.txt", "r", encoding='utf8') as file:
    for line in file:
        print(line)

See: File Handling

Arithmetic

result = 10 + 30 # => 40
result = 40 - 10 # => 30
result = 50 * 5  # => 250
result = 16 / 4  # => 4.0 (Float Division)
result = 16 // 4 # => 4 (Integer Division)
result = 25 % 2  # => 1
result = 5 ** 3  # => 125

The / means quotient of x and y, and the // means floored quotient of x and y, also see StackOverflow

Plus-Equals

counter = 0
counter += 10           # => 10
counter = 0
counter = counter + 10  # => 10

message = "Part 1."

# => Part 1.Part 2.
message += "Part 2."

f-Strings (Python 3.6+)

>>> website = 'Quickref.ME'
>>> f"Hello, {website}"
"Hello, Quickref.ME"

>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'

See: Python F-Strings

Python Built-in Data Types

Strings

hello = "Hello World"
hello = 'Hello World'

multi_string = """Multiline Strings
Lorem ipsum dolor sit amet,
consectetur adipiscing elit """

See: Strings

Numbers

x = 1    # int
y = 2.8  # float
z = 1j   # complex

>>> print(type(x))
<class 'int'>

Booleans

my_bool = True 
my_bool = False

bool(0)     # => False
bool(1)     # => True

Lists

list1 = ["apple", "banana", "cherry"]
list2 = [True, False, False]
list3 = [1, 5, 7, 9, 3]
list4 = list((1, 5, 7, 9, 3))

See: Lists

Tuple

my_tuple = (1, 2, 3)
my_tuple = tuple((1, 2, 3))

Similar to List but immutable

Set

set1 = {"a", "b", "c"}   
set2 = set(("a", "b", "c"))

Set of unique items/objects

Dictionary

>>> empty_dict = {}
>>> a = {"one": 1, "two": 2, "three": 3}
>>> a["one"]
1
>>> a.keys()
dict_keys(['one', 'two', 'three'])
>>> a.values()
dict_values([1, 2, 3])
>>> a.update({"four": 4})
>>> a.keys()
dict_keys(['one', 'two', 'three', 'four'])
>>> a['four']
4

Key: Value pair, JSON like object

Casting

Integers

x = int(1)   # x will be 1
y = int(2.8) # y will be 2
z = int("3") # z will be 3

Floats

x = float(1)     # x will be 1.0
y = float(2.8)   # y will be 2.8
z = float("3")   # z will be 3.0
w = float("4.2") # w will be 4.2

Strings

x = str("s1") # x will be 's1'
y = str(2)    # y will be '2'
z = str(3.0)  # z will be '3.0'

Python Advanced Data Types

Heaps

import heapq

myList = [9, 5, 4, 1, 3, 2]
heapq.heapify(myList) # turn myList into a Min Heap
print(myList)    # => [1, 3, 2, 5, 9, 4]
print(myList[0]) # first value is always the smallest in the heap

heapq.heappush(myList, 10) # insert 10
x = heapq.heappop(myList)  # pop and return smallest item
print(x)                   # => 1

Negate all values to use Min Heap as Max Heap

myList = [9, 5, 4, 1, 3, 2]
myList = [-val for val in myList] # multiply by -1 to negate
heapq.heapify(myList)

x = heapq.heappop(myList)
print(-x) # => 9 (making sure to multiply by -1 again)

Heaps are binary trees for which every parent node has a value less than or equal to any of its children. Useful for accessing min/max value quickly. Time complexity: O(n) for heapify, O(log n) push and pop. See: Heapq

Stacks and Queues

from collections import deque

q = deque()          # empty
q = deque([1, 2, 3]) # with values

q.append(4)     # append to right side
q.appendleft(0) # append to left side
print(q)    # => deque([0, 1, 2, 3, 4])

x = q.pop() # remove & return from right
y = q.popleft() # remove & return from left
print(x)    # => 4
print(y)    # => 0
print(q)    # => deque([1, 2, 3])

q.rotate(1) # rotate 1 step to the right
print(q)    # => deque([3, 1, 2])

Deque is a double-ended queue with O(1) time for append/pop operations from both sides. Used as stacks and queues. See: Deque

Python Strings

Array-like

>>> hello = "Hello, World"
>>> print(hello[1])
e
>>> print(hello[-1])
d

Get the character at position 1 or last

Looping

>>> for char in "foo":
...     print(char)
f
o
o

Loop through the letters in the word "foo"

Slicing string

 ┌───┬───┬───┬───┬───┬───┬───┐
 | m | y | b | a | c | o | n |
 └───┴───┴───┴───┴───┴───┴───┘
 0   1   2   3   4   5   6   7
-7  -6  -5  -4  -3  -2  -1

---

>>> s = 'mybacon'
>>> s[2:5]
'bac'
>>> s[0:2]
'my'

>>> s = 'mybacon'
>>> s[:2]
'my'
>>> s[2:]
'bacon'
>>> s[:2] + s[2:]
'mybacon'
>>> s[:]
'mybacon'

>>> s = 'mybacon'
>>> s[-5:-1]
'baco'
>>> s[2:6]
'baco'

With a stride

>>> s = '12345' * 5
>>> s
'1234512345123451234512345'
>>> s[::5]
'11111'
>>> s[4::5]
'55555'
>>> s[::-5]
'55555'
>>> s[::-1]
'5432154321543215432154321'

String Length

>>> hello = "Hello, World!"
>>> print(len(hello))
13

The len() function returns the length of a string

Multiple copies

>>> s = '===+'
>>> n = 8
>>> s * n
'===+===+===+===+===+===+===+===+'

Check String

>>> s = 'spam'
>>> s in 'I saw spamalot!'
True
>>> s not in 'I saw The Holy Grail!'
True

Concatenates

>>> s = 'spam'
>>> t = 'egg'
>>> s + t
'spamegg'
>>> 'spam' 'egg'
'spamegg'

Formatting

name = "John"
print("Hello, %s!" % name)

name = "John"
age = 23
print("%s is %d years old." % (name, age))

format() Method

txt1 = "My name is {fname}, I'm {age}".format(fname="John", age=36)
txt2 = "My name is {0}, I'm {1}".format("John", 36)
txt3 = "My name is {}, I'm {}".format("John", 36)

Input

>>> name = input("Enter your name: ")
Enter your name: Tom
>>> name
'Tom'

Get input data from console

Join

>>> "#".join(["John", "Peter", "Vicky"])
'John#Peter#Vicky'

Endswith

>>> "Hello, world!".endswith("!")
True

Python F-Strings (Since Python 3.6+)

f-Strings usage

>>> website = 'Quickref.ME'
>>> f"Hello, {website}"
"Hello, Quickref.ME"

>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'

>>> f"""He said {"I'm John"}"""
"He said I'm John"

>>> f'5 {"{stars}"}'
'5 {stars}'
>>> f'{{5}} {"stars"}'
'{5} stars'

>>> name = 'Eric'
>>> age = 27
>>> f"""Hello!
...     I'm {name}.
...     I'm {age}."""
"Hello!\n    I'm Eric.\n    I'm 27."

it is available since Python 3.6, also see: Formatted string literals

f-Strings Fill Align

>>> f'{"text":10}'     # [width]
'text      '
>>> f'{"test":*>10}'   # fill left
'******test'
>>> f'{"test":*<10}'   # fill right
'test******'
>>> f'{"test":*^10}'   # fill center
'***test***'
>>> f'{12345:0>10}'    # fill with numbers
'0000012345'

f-Strings Type

>>> f'{10:b}'        # binary type
'1010'
>>> f'{10:o}'        # octal type
'12'
>>> f'{200:x}'       # hexadecimal type
'c8'
>>> f'{200:X}'
'C8'
>>> f'{345600000000:e}' # scientific notation
'3.456000e+11'
>>> f'{65:c}'       # character type
'A'
>>> f'{10:#b}'      # [type] with notation (base)
'0b1010'
>>> f'{10:#o}'
'0o12'
>>> f'{10:#x}'
'0xa'

F-Strings Others

>>> f'{-12345:0=10}'  # negative numbers
'-000012345'
>>> f'{12345:010}'    # [0] shortcut (no align)
'0000012345'
>>> f'{-12345:010}'
'-000012345'
>>> import math       # [.precision]
>>> math.pi
3.141592653589793
>>> f'{math.pi:.2f}'
'3.14'
>>> f'{1000000:,.2f}' # [grouping_option]
'1,000,000.00'
>>> f'{1000000:_.2f}'
'1_000_000.00'
>>> f'{0.25:0%}'      # percentage
'25.000000%'
>>> f'{0.25:.0%}'
'25%'

F-Strings Sign

>>> f'{12345:+}'      # [sign] (+/-)
'+12345'
>>> f'{-12345:+}'
'-12345'
>>> f'{-12345:+10}'
'    -12345'
>>> f'{-12345:+010}'
'-000012345'

Python Lists

Defining

>>> li1 = []
>>> li1
[]
>>> li2 = [4, 5, 6]
>>> li2
[4, 5, 6]
>>> li3 = list((1, 2, 3))
>>> li3
[1, 2, 3]
>>> li4 = list(range(1, 11))
>>> li4
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Generate

>>> list(filter(lambda x : x % 2 == 1, range(1, 20)))
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

>>> [x ** 2 for x in range (1, 11) if  x % 2 == 1]
[1, 9, 25, 49, 81]

>>> [x for x in [3, 4, 5, 6, 7] if x > 5]
[6, 7]

>>> list(filter(lambda x: x > 5, [3, 4, 5, 6, 7]))
[6, 7]

Append

>>> li = []
>>> li.append(1)
>>> li
[1]
>>> li.append(2)
>>> li
[1, 2]
>>> li.append(4)
>>> li
[1, 2, 4]
>>> li.append(3)
>>> li
[1, 2, 4, 3]

List Slicing

Syntax of list slicing:

a_list[start:end]
a_list[start:end:step]

Slicing

>>> a = ['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[2:5]
['bacon', 'tomato', 'ham']
>>> a[-5:-2]
['egg', 'bacon', 'tomato']
>>> a[1:4]
['egg', 'bacon', 'tomato']

Omitting index

>>> a[:4]
['spam', 'egg', 'bacon', 'tomato']
>>> a[0:4]
['spam', 'egg', 'bacon', 'tomato']
>>> a[2:]
['bacon', 'tomato', 'ham', 'lobster']
>>> a[2:len(a)]
['bacon', 'tomato', 'ham', 'lobster']
>>> a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[:]
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

With a stride

['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[0:6:2]
['spam', 'bacon', 'ham']
>>> a[1:6:2]
['egg', 'tomato', 'lobster']
>>> a[6:0:-2]
['lobster', 'tomato', 'egg']
>>> a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[::-1]
['lobster', 'ham', 'tomato', 'bacon', 'egg', 'spam']

Remove

>>> li = ['bread', 'butter', 'milk']
>>> li.pop()
'milk'
>>> li
['bread', 'butter']
>>> del li[0]
>>> li
['butter']

Access

>>> li = ['a', 'b', 'c', 'd']
>>> li[0]
'a'
>>> li[-1]
'd'
>>> li[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

Concatenating

>>> odd = [1, 3, 5]
>>> odd.extend([9, 11, 13])
>>> odd
[1, 3, 5, 9, 11, 13]
>>> odd = [1, 3, 5]
>>> odd + [9, 11, 13]
[1, 3, 5, 9, 11, 13]

Sort & Reverse

>>> li = [3, 1, 3, 2, 5]
>>> li.sort()
>>> li
[1, 2, 3, 3, 5]
>>> li.reverse()
>>> li
[5, 3, 3, 2, 1]

Count

>>> li = [3, 1, 3, 2, 5]
>>> li.count(3)
2

Repeating

>>> li = ["re"] * 3
>>> li
['re', 're', 're']

Python Flow control

Basic

num = 5
if num > 10:
    print("num is totally bigger than 10.")
elif num < 10:
    print("num is smaller than 10.")
else:
    print("num is indeed 10.")

One line

>>> a = 330
>>> b = 200
>>> r = "a" if a > b else "b"
>>> print(r)
a

else if

value = True
if not value:
    print("Value is False")
elif value is None:
    print("Value is None")
else:
    print("Value is True")

Python Loops

Basic

primes = [2, 3, 5, 7]
for prime in primes:
    print(prime)

Prints: 2 3 5 7

With index

animals = ["dog", "cat", "mouse"]
# enumerate() adds counter to an iterable
for i, value in enumerate(animals):
    print(i, value)

Prints: 0 dog 1 cat 2 mouse

While

x = 0
while x < 4:
    print(x)
    x += 1  # Shorthand for x = x + 1

Prints: 0 1 2 3

Break

x = 0
for index in range(10):
    x = index * 10
    if index == 5:
    	break
    print(x)

Prints: 0 10 20 30 40

Continue

for index in range(3, 8): 
    x = index * 10
    if index == 5:
    	continue
    print(x)

Prints: 30 40 60 70

Range

for i in range(4):
    print(i) # Prints: 0 1 2 3

for i in range(4, 8):
    print(i) # Prints: 4 5 6 7

for i in range(4, 10, 2):
    print(i) # Prints: 4 6 8

With zip()

words = ['Mon', 'Tue', 'Wed']
nums = [1, 2, 3]
# Use zip to pack into a tuple list
for w, n in zip(words, nums):
    print('%d:%s, ' %(n, w))

Prints: 1:Mon, 2:Tue, 3:Wed,

for/else

nums = [60, 70, 30, 110, 90]
for n in nums:
    if n > 100:
        print("%d is bigger than 100" %n)
        break
else:
    print("Not found!")

Also see: Python Tips

Python Functions

Basic

def hello_world():  
    print('Hello, World!')

Return

def add(x, y):
    print("x is %s, y is %s" %(x, y))
    return x + y

add(5, 6)    # => 11

Positional arguments

def varargs(*args):
    return args

varargs(1, 2, 3)  # => (1, 2, 3)

Keyword arguments

def keyword_args(**kwargs):
    return kwargs

# => {"big": "foot", "loch": "ness"}
keyword_args(big="foot", loch="ness")

Returning multiple

def swap(x, y):
    return y, x

x = 1
y = 2
x, y = swap(x, y)  # => x = 2, y = 1

Default Value

def add(x, y=10):
    return x + y

add(5)      # => 15
add(5, 20)  # => 25

Anonymous functions

# => True
(lambda x: x > 2)(3)

# => 5
(lambda x, y: x ** 2 + y ** 2)(2, 1)

Python Modules

Import modules

import math
print(math.sqrt(16))  # => 4.0

From a module

from math import ceil, floor
print(ceil(3.7))   # => 4.0
print(floor(3.7))  # => 3.0

Import all

from math import *

Shorten module

import math as m

# => True
math.sqrt(16) == m.sqrt(16)

Functions and attributes

import math
dir(math)

Python File Handling

Read file

Line by line

with open("myfile.txt") as file:
    for line in file:
        print(line)

With line number

file = open('myfile.txt', 'r')
for i, line in enumerate(file, start=1):
    print("Number %s: %s" % (i, line))

String

Write a string

contents = {"aa": 12, "bb": 21}
with open("myfile1.txt", "w+") as file:
    file.write(str(contents))

Read a string

with open('myfile1.txt', "r+") as file:
    contents = file.read()
print(contents)

Object

Write an object

contents = {"aa": 12, "bb": 21}
with open("myfile2.txt", "w+") as file:
    file.write(json.dumps(contents))

Read an object

with open('myfile2.txt', "r+") as file:
    contents = json.load(file)
print(contents)

Delete a File

import os
os.remove("myfile.txt")

Check and Delete

import os
if os.path.exists("myfile.txt"):
    os.remove("myfile.txt")
else:
    print("The file does not exist")

Delete Folder

import os
os.rmdir("myfolder")

Python Classes & Inheritance

Defining

class MyNewClass:
    pass

# Class Instantiation
my = MyNewClass()

Constructors

class Animal:
    def __init__(self, voice):
        self.voice = voice
 
cat = Animal('Meow')
print(cat.voice)    # => Meow
 
dog = Animal('Woof') 
print(dog.voice)    # => Woof

Method

class Dog:

    # Method of the class
    def bark(self):
        print("Ham-Ham")
 
charlie = Dog()
charlie.bark()   # => "Ham-Ham"

Class Variables

class MyClass:
    class_variable = "A class variable!"

# => A class variable!
print(MyClass.class_variable)

x = MyClass()
 
# => A class variable!
print(x.class_variable)

Super() Function

class ParentClass:
    def print_test(self):
        print("Parent Method")
 
class ChildClass(ParentClass):
    def print_test(self):
        print("Child Method")
        # Calls the parent's print_test()
        super().print_test()

---

>>> child_instance = ChildClass()
>>> child_instance.print_test()
Child Method
Parent Method

repr() method

class Employee:
    def __init__(self, name):
        self.name = name
 
    def __repr__(self):
        return self.name
 
john = Employee('John')
print(john)  # => John

User-defined exceptions

class CustomError(Exception):
    pass

Polymorphism

class ParentClass:
    def print_self(self):
        print('A')
 
class ChildClass(ParentClass):
    def print_self(self):
        print('B')
 
obj_A = ParentClass()
obj_B = ChildClass()
 
obj_A.print_self() # => A
obj_B.print_self() # => B

Overriding

class ParentClass:
    def print_self(self):
        print("Parent")
 
class ChildClass(ParentClass):
    def print_self(self):
        print("Child")
 
child_instance = ChildClass()
child_instance.print_self() # => Child

Inheritance

class Animal: 
    def __init__(self, name, legs):
        self.name = name
        self.legs = legs
        
class Dog(Animal):
    def sound(self):
        print("Woof!")
 
Yoki = Dog("Yoki", 4)
print(Yoki.name) # => YOKI
print(Yoki.legs) # => 4
Yoki.sound()     # => Woof!

Miscellaneous

Comments

# This is a single line comments.

""" Multiline strings can be written
    using three "s, and are often used
    as documentation.
"""

''' Multiline strings can be written
    using three 's, and are often used
    as documentation.
'''

Generators

def double_numbers(iterable):
    for i in iterable:
        yield i + i

Generators help you make lazy code.

Generator to list

values = (-x for x in [1,2,3,4,5])
gen_to_list = list(values)

# => [-1, -2, -3, -4, -5]
print(gen_to_list)

Handle exceptions

try:
    # Use "raise" to raise an error
    raise IndexError("This is an index error")
except IndexError as e:
    pass                 # Pass is just a no-op. Usually you would do recovery here.
except (TypeError, NameError):
    pass                 # Multiple exceptions can be handled together, if required.
else:                    # Optional clause to the try/except block. Must follow all except blocks
    print("All good!")   # Runs only if the code in try raises no exceptions
finally:                 # Execute under all circumstances
    print("We can clean up resources here")