reference/python.md at main · heyaco/reference

title

Python

date

2020-12-23 10:41:20 -0800

icon

icon-python

background

bg-blue-600

Getting Started {.cols-3}

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

x = 4        # x is of type int
x = "Sally"  # x is now of type str
print(x)

Python has no command for declaring a variable.

Data Types {.row-span-2}


`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

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

See: Strings

Lists

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

See: Lists

If Else

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

See: Flow control

Loops

for x in range(6):
    if x == 3: break
    print(x)
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 {.col-span-2}

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

See: File Handling

Arithmetic

result = 10 + 30  # =>  40
result = 40 - 10  # =>  30
result = 50 * 5   # =>  250
result = 16 / 4   # =>  4
result = 25 % 2   # =>  1
result = 5 ** 3   # =>  125

Plus-Equals

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

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

Python Data Types {.cols-3}

Strings

s = "Hello World"
s = 'Hello World'

a = """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

a = True 
b = 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

a = (1, 2, 3)
a = 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 Strings {.cols-3}

Array-like

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

Get the character at position 1

Looping

>>> for x in "abc":
...     print(x)
a
b
c

Loop through the letters in the word "banana"

Slicing string {.row-span-4}

 ┌───┬───┬───┬───┬───┬───┬───┐
 | 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

>>> a = "Hello, World!"
>>> print(len(a))
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 {.col-span-2}

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 Lists {.cols-3}

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 {.col-span-2}

>>> 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 {.col-span-2 .row-span-3}

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 {.row-span-2}

>>> 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 {.row-span-2}

>>> 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 {.cols-3}

Basic

a = 5
if a > 10:
    print("a is totally bigger than 10.")
elif a < 10:
    print("a is smaller than 10.")
else:
    print("a 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 {.cols-3}

Basic

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

With index

animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
    print(i, value)

While

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

Break

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

Continue

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

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()

name = ['Pete', 'John', 'Elizabeth']
age = [6, 23, 44]
for n, a in zip(name, age):
    print('%s is %d years old' %(n, a))

List Comprehension {.col-span-2}

result = [x**2 for x in range(10) if x % 2 == 0]
 
print(result)
# [0, 4, 16, 36, 64]

Python Functions {.cols-3}

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 {.cols-3}

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 {.cols-3}

Read file

Line by line

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

With line number

input = open('myfile.txt', 'r')
for i,line in enumerate(input, 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 {.cols-3}

Defining

class MyNewClass:
    '''This is a docstring.'''
    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 {.row-span-2}

class my_class:
    class_variable = "A class variable!"
  
x = my_class()
y = my_class()
 
# => A class variable!
print(x.class_variable)

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

Super() Function {.row-span-2}

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 {.cols-3}

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 {.col-span-3}

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")

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Getting Started {.cols-3}

Introduction

Hello World

Variables

Data Types {.row-span-2}

Slicing String

Lists

If Else

Loops

Functions

File Handling {.col-span-2}

Arithmetic

Plus-Equals

Python Data Types {.cols-3}

Strings

Numbers

Booleans

Lists

Tuple

Set

Dictionary

Casting

Integers

Floats

Strings

Python Strings {.cols-3}

Array-like

Looping

Slicing string {.row-span-4}

With a stride

String Length

Multiple copies

Check String

Concatenates

Formatting {.col-span-2}

format() Method

Input

Join

Endswith

Python Lists {.cols-3}

Defining

Generate {.col-span-2}

Append

List Slicing {.col-span-2 .row-span-3}

Slicing

Omitting index

With a stride

Remove

Access

Concatenating {.row-span-2}

Sort & Reverse {.row-span-2}

Count

Repeating

Python Flow control {.cols-3}

Basic

One line

else if

Python Loops {.cols-3}

Basic

With index

While

Break

Continue

Range

With zip()

List Comprehension {.col-span-2}

Python Functions {.cols-3}

Basic

Return

Positional arguments

Keyword arguments

Returning multiple

Default Value

Anonymous functions