Python

About this note

The notes are from Zero to Mastery’s complete Python course. I didn’t take the whole thing as I already know Python, but it was a good refresher! Hence, the notes below are only to refresh my personal memory and not as comprehensive as other notes.

Introduction

What is a programming language?

flowchart LR
id1(code script) ---> id2(interpreter)
id1 ---> id3(compiler)
id2 ---> id4(computer)
id3 ---> id4

• Interpreter: Goes line by line and executes the code (python usually uses this)
• Compiler: Takes the entire file and turns it into executable machine code called binary

Python interpreter

• When most people talk about Python, they’re talking about cpython that translate your code script into machine executable code — ie: cpython is a interpreter

Fundamental Data Types

• int: integer
• float: decimal numbers
• complex: complex numbers (barely used)
• str: strings (ordered sequence of characters)
• bool: true (1) or false (0)
• list: ordered sequence of objects (like arrays in other languages)
• None: absence of values (like NULL in other languages)
• dict: unordered key value pair
• tuple: immutable lists
• set: unordered collection of unique objects

Math function examples

• **: power of
• //: integer division
• %: modulo (remainder)
• round(number): rounds the number up or down
• abs(number): absolute value of the number

Operator precedence

• ()
• **
• * and /
• + and -

Type conversion

• int('string')
• str(int)

Escape sequence

• \ allows you to use special characters without breaking format
• \n line break

Formatted strings

print(f'hi {name}! you are {age} years old.')

String indexing

[start:stop:stepover]

• Start: where you want the string to start (default is 0)
• Stop: where you want to stop (default is the end)
• Step-over: the size of the step (default is 1)

Negative index means that you start from the end

Immutability

• Strings are immutable
• Lists are mutable

Built in functions and methods

• len()
• .upper()
• Methods vs functions
  • Method: owned by something .method()

List slicing

• Slicing a list creates a new list
• new = original will just point to the original
• new = original[:] will copy the original list

Matrix

• Basically nested lists

List methods

• list.append(object) adds object to the end
• list.insert(index, object) adds the object at the specified index
• list.extend(list) extends the list by adding the list
• list.pop(index) removes the object at index (default is at the end) and returns the object you removed
• list.remove(object) removes the first instance of the object and doesn’t return anything
• list.clear() clears the list completely
• list.index(object, start, end) returns the index of the object if it is within the range that’s specified (default range is the entire list if no start and end is specified)
  • i in 'hi my name is meg will return True because in is a special word
• list.count(object) count how many instances of object is in the list
• list.sort() sorts in place ‘alphabetically’
• sorted(list) sorts the list and returns a new list
• list.reverse reverses the list in place (note that it doesn’t sort it)

List tricks

list = [1, 2, 3, 4, 5]
 
# Both reverse the list BUT
list.revers() #reverses in place 
list[::-1] #reverses and returns a new list 
 
# copying a list 
list.copy()
list[:]
 
# creating a list from a range 
list2 = list(range(2, 8)) # [2, 3, 4, 5, 6, 7]
 
# creating a string from list 
separator = ' '
sentence = separator.join(['hi', 'I', 'am', 'Meg'])
print(sentence) #hi I am Meg
# short hand version
new_sentence = ' '.join(['hi', 'I', 'am', 'Meg'])

List unpacking

a,b,c,*other,d= [1,2,3,4,5,6,7,8,9]
 
print(a) # 1
print(other) # [4,5,6,7,8]
print(d) # 9

Dictionary

dictionary = {
	"a": 1,
	"b": 2
}

Dictionary keys

• Dictionary key has to be immutable and unique
  • Usually a string
  • If there’s a repeat, you’re replacing the old key with the new one
• dictionary.get(key) checks if the key even exists and if it exists then it returns its value

user = {
	'basket': [1,2,3],
	'greet': 'hello',
	'age': 20
}
 
print('greet' in user) #True
print('basket' in user.keys()) #True
print('hello' in user.values()) #True
print(user.items()) 

Sets

• Basically can do the mathematical operations
  • set1.difference(set2) tells you the difference between set1 and set2 are
  • set1.discard(value) discards the value from set1
  • set1.difference_update(set2) removes the difference from set1
  • set1.intersection(set2) what’s the intersection
  • set1.isdisjoint(set2) checks if sets are disjoint
  • set1.issubset(set2) checks if set1 is a subset of set2
  • set1.issuperset(set2) checks superset (superset is the opposite of subset)
  • set1.union(set2) combines the two sets

Vocabulary

• Variables: store data in different variables
  • Rules:
    • snake_case
    • start with lowercase or underscore
    • letters, numbers, underscores
    • case sensitive
    • can’t overwrite key words
  • Constants are usually written in all caps
  • __ variables should be left alone
• Expression: Piece of code that produces a value
• Statement: Entire line of code that performs some kind of action
• Augmented assignment operator: instead of writing out x = x + 2 you can write in short hand as x += 2

Conditional logic

if condition: 
	expression
else:
	expression 

Truthy vs Falsey

• Python evaluates everything to be booleans when comparing them so even though strings aren’t booleans, it is converted to be booleans behind the scenes
  • Falsey would be 0 or ''
  • Truthy would be like 'hello'

Ternary operator

• AKA Conditional expression
• Shorthand: condition_if_true if condition else condition_if_false

Short circuiting

• when using or it only evaluates the first condition if it’s true

Loops

for i in [1,2,3,4,5]:

Random Notes

---

• Tuple vs Lists
  • Tuples are immutable while lists are mutable

long_string = '''
	WOW 
	0 0
	___
'''

Exercises

---

Age guessing game

# Age guessing game 
birth_year = input('What year were you born?')
age = 2024 - int(birth_year)
print(f'You are {age} years old')

• Note: input() is always a string

Password checker

# Password checker
username = input('Enter your username: ')
password = input('Enter your password: ')
password_len = len(password)
 
print(f'{username}, your password {'*' * password_len} is {password_len} letters long')

Logical operators

is_magician = False
is_expert = True
 
if is_magician and is_expert:
	print("you are a master magician!")
elif is_magician and is_expert == False:
	print("at least you're getting there!")
elif is_magician == False:
	print("you need magic powers!")

is vs ==

print(True == 1) #True 
print('' == 1) #False 
print([] == 1) #False 
print(10 == 10.0) #True 
print([] == []) #True 
 
print(True is 1) #False 
print('' is 1) #False 
print([] is 1) #False 
print(10 is 10.0) #False 
print([] is []) #False 

• is checks the location of the item whereas equality checks just the values

Counter

my_list = [1,2,3,4,5,6,7,8,9,10]
sum = 0
for i in my_list:
	sum += i
print(sum)