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Python

Data

  • All data in a Python program is stored in objects.
  • These objects have three components: id, type, and value.
  • Each object has a unique identifier.
  • Object’s type determines what functions can operate on that particular data type
print(id(50))
print(type(50))

Variables

  • A Python variable is not an object and does not actually store data; it stores an id that refers to an object that stores data.
  • Python standard for naming varibale is to use underscore.

Determining types of data values

print(type('hello'))
print(type(1))
print(type(1.64))
print(type(True))

Printing

  • Syntax: print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)
print('My name is Ambreen.')
print(1, 2, 3, 4)
print(1, 2, 3, 4, sep='*')
print(1, 2, 3, 4, sep='#', end='&')

Output Formatting

x = 5; y = 10
print('The value of x is {} and y is {}'.format(x,y))
# Output: The value of x is 5 and y is 10
print('I like {0} and {1}'.format('mango','orange'))
# Output: I like mango and orange
print('Hello {name}, {greeting}'.format(greeting = 'Good Morning', name = 'John'))
# Output: Hello John, Good Morning
x = 12.3456789
print('The value of x is %3.2f' %x)
# Output: The value of x is 12.35
print('The value of x is %3.4f' %x)
# Output: The value of x is 12.3457

Type Casting

a = int(1)        # a will be 1
b = int(2.5)      # b will be 2
c = int("3")      # c will be 3
#c1 = int("3.4")   # c1 will be... This will result in an error; replace it with below statement to work:
c1 = int(float("3.4"))
d = float(1)      # d will be 1.0
e = float(2.5)    # e will be 2.5
f = float("3")    # f will be 3.0
g = float("4.23") # g will be 4.23
h = str("80s")    # h will be '80s'
i = str(22)       # i will be '22'
j = str(3.01)     # j will be '3.01'
print([a,b,c,c1,d,e,f,g,h,i,j])

Eval

# int('2+3')  - This will result in an error
eval('2+3') 
# Output: 5

Arithmatic Operators

a=10
b=3
print('Addition : ', a + b)
print('Subtraction : ', a - b)
print('Multiplication : ', a * b)
print('Division (float) : ', a / b)
print('Division (floor) : ', a // b)
print('Modulus : ', a % b)
print('Exponent : ', a ** b)

Getting user input values

num1=input('Enter a digit: ')
num2=input('Enter a second number:')
answer=float(num1)+float(num2)
print(answer)

Example

# - Create a distance converter converting Km to miles
# - Take two inputs from user: Their first name and the distance in km
# - Print: Greet user by name and show km, and mile values
# - 1 mile is 1.609 kilometers
name= input('What is your name?: ')
distance = input('Enter the distance you can run?')
print(f'{name.capitalize()} you can run {distance} kms and that is equivalent to {round(float(distance)*1/1.609)} miles')

Working with Strings

msg='welcome to Python 101: Strings'
print(msg)
print(msg.upper())
print(msg.lower())
print(msg.capitalize()) #capitalize first letter on the sentence
print(msg.title()) #capitalize first letter of each word
print(len(msg)) 
print(msg.count('Python'))
print(msg.count('o'))
# Slicing
print(msg[4])
print(msg[-7])
# To grab everthing after 2
print(msg[2:])
# from position -> to upto (character at last position is not included)
print(msg[2:15])
# Starting everything from 0 to indicated number-1
print(msg[:7])
# Challenge: Write text '1 Welcome Ring To Tyler' using given variable 'msg'
msg1=msg[18]+' '+msg[:8]+msg[25:29]+msg[7:11]+msg[13]+msg[12]+msg[2]+msg[1]+msg[-5]  
print(msg1.title()) 
# Print the same string backward
print(msg1[::-1])
#Other way to flip the list
my_list = [1,5,3,7,2]
print(list(reversed(my_list))) #output: [2, 7, 3, 5, 1]
# multi-line strings
msg2="""Dear Terry,,
You must cut down the mightiest 
tree in the forest with…
a herring! <3"""
print(msg2.find("mightiest"))
# Find/Replace in Python
msg3='welcome to Python 101: Strings'
print(msg3.find('Python'))
print(msg3.replace('Python', 'Java'))
# Check if text exists in given string
msg4='welcome to Python 101: Strings'
print('Python' in msg)
print('Java' not in msg)
# String Formatting using variables
name1='terry'
color1 = 'RED'
msg6 = '[' + name1 + '] loves the color ' + color1.lower() + '!'
msg7 = f'[{name1.capitalize()}] loves the color {color1.lower()}!'
print(msg6)
print(msg7)

Working with Lists

friends = ['John','Michael','Terry','Eric','Graham']
# 2nd element and 5th element
print(friends[1],friends[4])
# last element
print(friends[-1])
#  3rd to 5th element
print(friends[2:4])
# first element to fourth element
print(friends[:4])
# reverse the list
print(friends[::-1])
# whole list
print(friends[:]) 
# number of elements in list
print(len(friends))
# Determine index of element in list
print(friends.index('Eric'))
# Count # of certain elements
print(friends.count('Eric'))
# Generate certain number of values
alot_of_zeros = [0] * 20
print(alot_of_zeros)
# Sorting a list 
friends.sort()
print(friends)
# Sort in descending order
friends.sort(reverse=True)
print(friends)
# reverse the list
friends.reverse()
print(friends)
# Sort numeric array
cars = [911,130,328,535,740,308]
cars.sort()
print(cars)
# Find minimum/maximum value in an array
print(min(cars))
print(max(cars))
# Sum of the elements in an array
print(sum(cars))
# Add new elements in the list
friends.append('TerryG')
print(friends)
# Specify the position where to insert the element
friends.insert(1,'TerryV')
print(friends)
# Replacing the value in the list
friends[2]='TerryG'
print(friends)
# Extend the lists (combines 2 lists)
friends.extend(cars)
print(friends)
# Remove an elements
friends.remove('Terry')
# Pop the last element from the list. This element can be assigned to any variable
friends.pop()
# Pop last element - alternate way
friends.pop(-1)
# pop specific element
second_element = friends.pop(2)
print(second_element)
# Remove all elements from the list and returns an empty list
friends.clear()
# Delete a list
del friends
# removes specific index value
del friends[2]
# Copy List - Method 1
new_friends = friends.copy()
# Copy List - Method 2
new_friends = list(friends)
# Copy List - Method 3
new_friends = friends[:]
# Un-packing the list
items = ['laptop', 'mouse', 'keyboard']
laptops, mouse, keyboard = items #generates variables using items from the list
print(laptops)
mobile = ['Pixel3', 'Android 12', 'Android 13', 'Android 14']
mobile_type, *ios_version = items #generates variables using items from the list
print(mobile_type) # prints Pixel3
print(ios_version) #prints other items from the list
# Creating a list from a string
name = list('Ambreen')
print(name)
# Output
['A', 'm', 'b', 'r', 'e', 'e', 'n']
# Creating a list by giving a range
my_list = list(range(0, 10))
# Output:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# Generating a list by giving range and increment value
my_list = list(range(2, 22, 2))
print(my_list)
#Output:
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

Example List

# Lemonade sales for week 1 & week 2
sales_w1 = [7,3,42,19,15,35,9]
sales_w2 = [12,4,26,10,7,28]
# Get the number of lemonades sold on 7th day in second week
new_day = input('Enter number of lemonades sold on 7th day in second week')
sales_w2.append(int(new_day))
# Combine the sales for week 1 & week 2
#sales.extend(sales_w1)
#sales.extend(sales_w2)
sales = sales_w1 + sales_w2
print(sales)
# How much you earned on Best & Worst Days
worst_day_prof = min(sales) * 1.5
best_day_prof = max(sales) * 1.5
print(f'Worst day profit:$ {worst_day_prof}')
print(f'Best day profit:$ {best_day_prof}')
print(f'Combined profit:$ {worst_day_prof + best_day_prof}')

Split and Join

Split

  • Split turn data into a list
msg ='Welcome to Python 101: Split and Join'
names = 'Edward,Gor,Eileen,Andy,Abdul'
print(msg.split()) #Splits by spaces
print(names.split(',')) #Splits by Commas

Join

  • Join turns data into a string
name_list = ['Edward','Gor','Eileen','Andy','Abdul']
print('-'.join(name_list))
# Output: 
# Edward-Gor-Eileen-Andy-Abdul
  • To remove the spaces from a string and join it back 
print(''.join(msg.split())) 
# Alternate approach
print(msg.replace(' ', ''))

Example

  • Make a list from the names given in csv variable
csv = 'Eric,John,Michael,Terry,Graham:TerryG;Brian'
friends_list = (','.join((','.join(csv.split(';'))).split(':'))).split(',')
print(friends_list)
# Alternate way using replace method
print(csv.replace(';',',').replace(':',',').split(','))

Tuples

  • Tuple is read only list that can't be changed
  • Tuples are faster in performace than lists
  • Tuple can have duplicate items
i_am_tuple = ('1', '2', '2','3')
# To get an element from tuple, use same method as list
print(i_am_tuple[0])
print(i_am_tuple[1:3])

Sets

  • Sets are unordered list and remove duplicate date
  • Sets are faster in finding the element than the list
i_am_set = {'1','2','3'}
  • Applying intersection, difference and union on two Sets
friends_set = {'John','Michael','Terry','Eric','Graham','Eric'}
my_friends_set = {'Reg','Loretta','Colin','Eric','Graham'}
print(friends_set.intersection(my_friends_set)) # What's common in two sets
print(friends_set & my_friends_set) #Alternate way of writing intersection statement
print(friends_set.difference(my_friends_set)) # What's different in two sets
print(friends_set - my_friends_set) #Alternate way of writing difference statement
print(friends_set.union(my_friends_set)) #Union of two sets removing dupliactes
print(friends_set | my_friends_set) #Alternate way of writing union statement
  • Symmetric Difference - data that exists only in one of the set
print(friends_set.symmetric_difference(my_friends_set))
print(friends_set ^ my_friends_set) #Alternate way of writing smmetric difference statement

Creating empty list, tuple and set

#empty Lists
empty_list = []
empyt_list = list()
#empty Tuple
empty_tuple = ()
empty_tuple = tuple()
#empty Set
empty_set = {} # this is wrong, this is a dictionary
empty_set = set()

Truthy and Falsy Values

  • Empty objects and 0 evaluates to false, everything else evaluates to true
if 1:
    print('I am a truthy value') 
0 #Falsy 
"" #Falsy
[] #Falsy

Mutable Vs immutable

What is not immutable?

  • Data Types: bool, int, float, tuple, strings
  • you can freely change the value of new variable created from immutable data type as it won't change the value of the original variable.

What is immutable?

  • Data Types: lists, sets, dictionaries
  • Stores values by reference
  • If you change the value of new variable created from a mutable data type, it will change the value of the original variable.

If, Else, Elif Statement

  • Example
age = 18
if age > 70:
    print("You are old. Don't take a risk to try this ride") 
elif age > 18:
    print("You can try this ride")   
else:
    print("You are too young to try this ride")
  • Another Example:
def num_days(month):
    days = 31
    if month in {'apr','jun','sep','nov'}:
    #if month == 'apr' or month =='jun' or month =='sep' or month =='nov':
        days = 30
    elif month == 'feb':
        days = 28
    print('number of days in',month,'is',days)
num_days('jan')
  • pass statement If statements cannot be empty, there must be at least one statement in every if and elif block. You can use the pass statement to do nothing and avoid getting an error.
a_number = 9
if a_number % 2 == 0:
    pass
elif a_number % 3 == 0:
    print('{} is divisible by 3 but not divisible by 2'.format(a_number))

Loops

Three Loop Questions:

  1. What do I want to repeat?
  2. What do I want to change each time?
  3. How long should we repeat?

For loops

students = ['Ambreen', 'Abdul', 'Mariam', 'David']
for student in students:
    print(student)
   
for i in range(0, 20): #running loop 20 times
    print(f'I am running for {i} time')
name = 'Ambreen'
for letter in name:
    print(letter) #prints the letters in the name 
#output   
# A
# m
# b
# r
# e
# e
# n   
for table_of_two in range(2,21,2):   #performs the action in step of 2
    print(table_of_two)
# Use of break to get out of loop
friends = ['Ambreen', 'Abdul', 'Mariam', 'David', 'Michael', 'Andy']
for friend in friends:
    if friend == 'Mariam':
        print('Found ' + friend + '!')
        break
    print(friend)
# Continue the loop by going to top of the loop on continue condition
friends = ['Ambreen', 'Abdul', 'Mariam', 'David', 'Michael', 'Andy']
for friend in friends:
    if friend == 'Mariam':
        print('Found ' + friend + '!')
        continue
    print(friend)

While Loop

age = 10
while age < 18:
    print(f'You are not allowed to take this ride! at age {age}')
    age += 1
    if age == 18:
        print(f'Congrats, you are {age} and now you are eligible to try this ride')
#Output
# You are not allowed to take this ride! at age 10
# You are not allowed to take this ride! at age 11
# Congrats, you are 12 and now you are eligible to try this ride
# Contine the loop
kid_age = 10
while kid_age < 18:
    age += 1
    if age == 12:
        continue #doesn't perform the action mentioned below for value 12
    print(f'You are not allowed to take this ride! at age {age}')
# Output
# You are not allowed to take this ride! at age 11
# You are not allowed to take this ride! at age 13
# You are not allowed to take this ride! at age 14
# You are not allowed to take this ride! at age 15
# You are not allowed to take this ride! at age 16
# You are not allowed to take this ride! at age 17
# You are not allowed to take this ride! at age 18

Functions

  • Always declare the function before using it
def multiplyByTen(number):
    return number * 10 
number_returned = multiplyByTen(20)
   
print(number_returned)
def add(number, by=1): #providing default value
    return number+by
print(add(10, 5)) #prints 15
print(add(10))   #prints 11
  • Functions - Named Notation allows to send parameters in any order while calling a function
def greeting(name, age=28, color="red"):
 print(f"Hello {name.capitalize()}, you will be {age+1} next birthday!")
 print(f"We hear you like the color {color.lower()}!")
greeting(age=27, name="brian",color="Blue")
  • Functions can return multiple values and any data type. 
def value_added_tax(amount):
    tax = amount * 0.25
    total_amount = amount * 1.25
    return [amount, tax, total_amount]
    
price = value_added_tax(100)    
print(price, type(price)) # output: [100, 25.0, 125.0] <class 'list'>
# or return a particluar value from list as below
print(price[1], type(price)) # output: 25.0 <class 'list'>

Comparisions and Booleans

a=7
b=3
print('a == b is', a == b)
print('a != b is', a != b)
print('a > b is', a > b)
print('a < b is', a < b)
print('a >= b is', a >= b)
print('a <= b is', a <= b)
print('o in John is ','o' in 'John') #membership
print('o in John is ','o' not in 'John') #non membership
print('John is John ','John' is 'John') #identity
print('John is not John is ','John' is not 'John') # negative identity

Enumerate (use of counter)

  • Use enumerate() to get a counter in a loop
  • enumerate function goes to the list and turns all elements into tuples
friends = ['Ambreen', 'Abdul', 'Mariam', 'David', 'Michael', 'Andy']
for num, friend in enumerate(friends,1):
    print(num, friend)
for num, friend in enumerate(friends,51):
    print(num, friend)
for friend in enumerate(friends,51):
    print(friend)
for friend in enumerate(enumerate(friends,51),-100):
    print(friend)   
for num, letter in enumerate('python',start = 5):
    print(num, letter)
print(type(enumerate(friends)))
print(list(enumerate(friends)))

Sort() and Sorted()

  • sort() function just performs the sorting operation but doesn't return anything. It will modify the original list
  • sorted() function yields a new sorted list. The actual list doesn't change.
my_list = [1,5,3,7,2]
print(my_list,'original')
print(sorted(my_list)) #returns a new sorted list
print(my_list,'new') #original list remains unchanged
  • If sorted() function is performed on a tuple, it returns a list
my_tuple = ('d','c','e','a','b')
print(sorted(my_tuple)) #output: ['a', 'b', 'c', 'd', 'e']
#
  • If sorted() function is performed on a string, it returns a sorted list
my_string = 'python' 
print(sorted(my_string)) #output: ['h', 'n', 'o', 'p', 't', 'y']
  • If sorted() function is performed on a dictionary, it only sorts key values tuples
my_dict = {'car':4,'dog':2,'add':3,'bee':1}
print(sorted(my_dict)) #output: ['add', 'bee', 'car', 'dog']
  • If sorted() function is performed on items in a dictionary, it returns a list of tuples that is sorted on the keys.
my_dict = {'car':4,'dog':2,'add':3,'bee':1}
print(sorted(my_dict.items())) #output: [('add', 3), ('bee', 1), ('car', 4), ('dog', 2)]
  • To get dictionary values in ascending order
print(sorted(my_dict.values())) #output: [1, 2, 3, 4]
  • To get dictionary values in descending order
print(sorted(my_dict.values(), reverse=True)) #output: [4, 3, 2, 1]
  • Sorted on absolute values
my_list = [1,5,-3,7,-2]
print(sorted(my_list, key = abs)) #output: [1, -2, -3, 5, 7]
  • A list of list is sorted by the first element
my_llist=[['car',4,65],['dog',2,30],['add',3,10],['bee',1,24]]
print(sorted(my_llist)) #output: [['add', 3, 10], ['bee', 1, 24], ['car', 4, 65], ['dog', 2, 30]]
  • Applying lambda function while sorting a list of list
my_llist=[['car',4,65],['dog',2,30],['add',3,10],['bee',1,24]]
print(sorted(my_llist, key = lambda item :item[1])) #output: [['bee', 1, 24], ['dog', 2, 30], ['add', 3, 10], ['car', 4, 65]]
print(sorted(my_llist, key = lambda item :item[2])) #output: [['add', 3, 10], ['bee', 1, 24], ['dog', 2, 30], ['car', 4, 65]]

Dictionaries

  • Used to store data values as key value pairs.
  • A dictionary is a collection which is ordered (starting version 3.7), changeable and does not allow duplicates.
car = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(car["brand"])
  • To get the keys in the dictionary:
for key in car:
    print(key) 
#Output: 
# Ford
# brand
# model
# year
# Alternate to get Keys
print(car.keys()) # Returns an array - dict_keys(['brand', 'model', 'year'])
  • To get the items (i.e. values) in the dictionary:
print(car.items()) #Returns a list of tuples - dict_items([('brand', 'Ford'), ('model', 'Mustang'), ('year', 1964)])
  • To get key and values from a dictionary: 
for key, values in car.items():
    print(key)
    print(values)
  • To check if key exists in the dictionary:
if "brand" in car:
    print('The brand exists: ')  
    print(car["brand"])
  • To check if key doesn't exist in the dictionary:
if "brand" not in car:
    print('The brand doesn\'t exist')
  • If a key doesn't exist in a dictionary and we try to get it using bracket notation, it results in unknown error. To avoid the error, use get() instead.
car = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(car.get('owner')) #This will return 'None' which is an empty object.
# To return a default value set by user when an key is not found, set default value like below:
print(car.get('owner', 'not found')) #returns 'not found' if the value is not found.
  • To update the value of a key, use bracket notation:
print(car["brand"] = 'Toyota')
  • To add a new key value, you can use bracket notation.
car["color"] = 'Red')
  • To update the whole dictionary, you can use update command
movie = {
    'title' : 'Life of Brian',
    'year' : 1979,
    'cast' : ['John','Eric','Michael','George','Terry']
}
movie.update({'title' : 'The Holy Grail','year':1975,'cast':['John','Eric','Michael','George','Terry']})
  • To delete an item from the dictionary, use del command
del movie['year']
  • Use pop command to remove the value from dictionary and assign it to some variable for a later use
year = movie.pop('year')
  • The dictionaries can be Concatenated in 3 different ways:
user_list_1 = {'John':35,'Eric':36,'Michael':35,'Terry':38,'Graham':37,'TerryG':34}
user_list_2 = {'Kevin':40,'Tim':35,'Nike':39,'Ahmed':40, 'Kabir':17}
user_list_3 = {'Brian':33,'Reg':35,'Saima':32,'Phillip':45}
users1= {}
users2 = {}
users3 = {}
#method 1: using update
users1.update(user_list_1)
users1.update(user_list_2)
users1.update(user_list_3)
print(users1)
#method 2: comprehension
for groups in (user_list_1,user_list_2,user_list_3) : users2.update(groups)
print(users2)
#method 3: unpacking, only works for 3.5 & later
users3 = {**user_list_1,**user_list_2,**user_list_3}
print(users3)
  • To get the sum of the values:
print('The sum of the ages: ', sum(user_list_1.values()))

Try, Except & Raise

#try:
    #code you want to run
#except:
    #executed if error occurs
#else:
    #executed if no error
#finally:
    #always executed 
try:
    num=int(input('Enter a number between 1 and 30: '))
    num1 = 30/num
    if num > 30:
        raise ValueError(num)
except ZeroDivisionError as err:
    print(err, "You can't divide by Zero!!!")
except ValueError as err:
    print(err,num, "Bad Value not between 1 and 30!")
except:
    print("Invalid Input!")
else:
    print("30 divided by",num, "is: ", 30/num)
finally:
    print("**Thank you for playing!**")

Classes and Objects

  • Keyword self refers to the newly created object that is active at the moment. It is used to represent the instance of a class.

  • By using the "self" keyword we access the attributes and methods of the class in python.

  • __init__ is a reserved method in Python classes. It is called as a constructor when an object is created from a class.

  • __init__ allows the class to initialize the attributes of the class.

  • Example:

class Movie:
    def __init__(self,title,year,imdb_score,have_seen):
        self.title = title
        self.year = year
        self.imdb_score = imdb_score
        self.have_seen = have_seen
    
    def nice_print(self):
        print("Title: ", self.title)
        print("Year of production: ", self.year)
        print("IMDB Score: ", self.imdb_score)
        print("I have seen it: ", self.have_seen)
        
film_1 = Movie("Life of Brian",1979,8.1,True)
film_2 = Movie("The Holy Grail",1975,8.2,True)
films = [film_1,film_2]
print(films[1].title,films[0].title)
films[0].nice_print()

Inheritance

class Person:
    def move(self):
        print("Moves 4 paces")
    def rest(self):
        print("Gains 4 health points")
class Doctor(Person):
    def heal(self):
        print("Heals 10 health points")
class Fighter(Person):
    def fight(self):
        print("Do 10 health points of damage")
    def move(self):
        print("Moves 6 paces")
        
class Wizard(Doctor,Fighter):
    def cast_spell(self):
        print("Turns invisble")
    def heal(self):
        print("Heals 15 health points")
    
    
character1=Wizard()
character1.move()

Modules

  • Python Module Index
  • Using dir on a module name provides the name of all the methods available in a module.
  • While importing a module, Python looks at several places defined in sys.path. We can also add our own location to this list.
import platform
print(dir(platform))
print(platform.python_version())
  • The imports can be renamed: 
import platform as pl
print(pl.python_version())
  • To import a specific method from a module:
from platform import python_version, system
print(python_version())
print(system())
  • The specific method imported from a module can also be renamed as below:
from platform import python_version as pv
print(pv())
  • Another Example:
import random
random_number = random.randint(0,20)
print(random_number)

Packing & Unpacking (zip & unzip)

  • Example:
nums = '1234' 
letters = ['a','b','c','d']
names =['John','Eric','Michael','Graham','Joe']
combo = list(zip(nums,letters,names))
print(combo)
# Now unzip the combo by following command
num,let,nam =zip(*combo)
print(num,let,nam)
  • Another example using dictionary
keys = 'this parrot is deceased'
values = 'denna papegojan är avliden'
keys = keys.split()
values = values.split()
print(keys,values)
en_sv_dict = dict(zip(keys,values))
print(en_sv_dict)
# Alternate method (known as dictionary comprehension)
new_dict = {key:value for key,value in zip(keys,values)}
print(new_dict)
# Unzipping a dictionary
en,sv = list(en_sv_dict.keys()),list(en_sv_dict.values())
print(en,sv)
# Alternate way to unzip the dictionary (returns a tuple)
en1,sv1 = zip(*en_sv_dict.items())
print(en1,sv1)

Lambda Functions

  • Lambda functions are single line functions that can have a name or can be anonymous
  • A lambda function always returns a value.
# A simple function
def square(x):
    return x*x
# Using lambda syntax to define above method:    
#name = lambda parameter(s): expression
square1 = lambda x: x*x
print(square1(3))

Reading & Writing files

def read_files(path):
    file_countries_names = open(path, 'r')
    if((file_countries_names.readable())):
        for lines in file_countries_names.readlines():
            print(lines)
    file_countries_names.close()
def write_file(path):
    file_name = open(path, 'w')
    file_name.write('Write something')
def append_file(path):
    file_countries_names = open(path, 'a')
    file_countries_names.write('\nI am new here!')
read_files('read-write-files/countries.txt')
write_file('read-write-files/new_file.txt')
append_file('read-write-files/countries.txt')

PyPI

  • The official Python package repository is called PyPI, which stands for Python Package Index.
  • Use pip to install any package from PyPI.

Virtual Environment

  • It is a complete copy of the Python interpreter.
  • Allows to have different versions of packages for different applications.
  • Use a different virtual environment for each application.
  • Support for virtual environments is included in all recent versions of Python
  • By running below command Python will use venv package, which will create a virtual environment named venv. Here:
    • The first venv in the command is the name of the Python virtual environment package
    • The second venv is the virtual environment name and this can be anything. 
python3 -m venv venv
  • After the command completes, a directory named venv is created where the virtual environment files are stored.
  • To use this virtual environment, you need to activate it using below command:
source venv/bin/activate

How to declare a package?

  • In Python, a sub-directory that includes a __init__.py file is considered a package, and can be imported.
  • When you import a package, the init.py executes and defines what symbols the package exposes to the outside world.

Resources: