🐍 Python Refresher

A concise Python refresher for developers with experience in other languages 🚀

This guide is for programmers who already know the basics of programming and want a fast overview of Python’s syntax and essential features. Use it to prepare for interviews, start a new project, or quickly transition from another language.

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Built-in Data Types in Python

Category	Data Types
Text Type	str
Numeric Types	int, float, complex
Sequence Types	list, tuple, range
Mapping Type	dict
Set Types	set, frozenset
Boolean Type	bool
Binary Types	bytes, bytearray, memoryview
None Type	NoneType

Numeric Variables in Python

Integers and Floats:
Python supports numeric variables like integers (int) and floating-point numbers (float).

cost = 10           # integer
tax_percent = 0.25  # float
tax = cost * tax_percent
price = cost + tax
print(price)        # Output: 12.5

Assignment-01

Task:
Write a Python program that simulates buying an item with tax and prints the remaining money.

Instructions:

• You have $50.
• You buy an item that costs $15 with a 3% tax.
• Print how much money you have left after the purchase.

Solution:

wallet = 50
item_price = 15
tax_rate = 0.03
wallet -= item_price + (item_price * tax_rate)
print(wallet)  # Output: 34.55

Strings in Python

Single and Double Quotes:
Strings can be defined using either single (') or double (") quotes.

username = "mirzaree"
first_name = 'Mohibul'
print(username + " " + first_name)  # Output: mirzaree Mohibul

Comments in Python

Single-line Comments:
Use # for single-line comments.

# This is a single-line comment
print("Hello, World!")  # This prints a message

Multi-line Comments (Docstrings):
Triple quotes (''' or """) are used for multi-line comments or docstrings.

"""
This is a multi-line comment (docstring).
It can span multiple lines.
"""

String Formatting in Python

F-Strings:
Introduced in Python 3.6, f-strings provide a concise way to embed expressions inside string literals.

first_name = "John"
last_name = "Doe"
age = 30
print(f"Hello, my name is {first_name} {last_name} and I am {age} years old.")
# Output: Hello, my name is John Doe and I am 30 years old.

str.format() Method:
The str.format() method allows you to insert values into a string using curly braces {} as placeholders.

first_name = "John"
last_name = "Doe"
sentence = "Hi {} {}"
print(sentence.format(first_name, last_name))
# Output: Hi John Doe

User Input in Python

Getting Input from the User:
Use the input() function to prompt the user for input. The input is always returned as a string.

first_name = input("Enter your first name: ")
last_name = input("Enter your last name: ")
age = input("Enter your age: ")

Type Casting Input:
If you expect a numeric value, convert the input string to an integer (or float) using int() or float().

age = int(age)  # Converts the input string to an integer

Example:

first_name = input("Enter your first name: ")
last_name = input("Enter your last name: ")
age = input("Enter your age: ")
age = int(age)  # Convert age to integer

print(f"Hello, {first_name} {last_name}. You are {age} years old.")

Assignment-02: String Assignment

Task:
Ask the user how many days until their birthday and print the approximate number of weeks and days left.

Solution:

days_until_birthday = int(input("How many days until your birthday? "))

weeks_until = days_until_birthday // 7
rem_days = days_until_birthday % 7
print(f"There are approximately {weeks_until} weeks and {rem_days} days until your birthday.")

# "/"" is True division and "//"" Floor Division

Lists in Python

Lists are collections of items, similar to arrays in other programming languages. Lists can store elements of different data types and are mutable (can be changed after creation).

my_list = [2, 5, 3, 4, 1]

people_list = ["Alice", "Bob", "Charlie", "David"]

mixed_list = [1, "Hello", 3.14, True]

Indexing:
List indices start at 0.

print(mixed_list[0])  # Output: 1
print(mixed_list[1])  # Output: Hello
print(mixed_list[2])  # Output: 3.14
print(mixed_list[3])  # Output: True

Slicing:
Get a sublist using slicing (list[start:end+1]).

print(mixed_list[1:3])  # Output: ['Hello', 3.14]

Adding Elements:

• append() adds to the end.
• insert(index, item) adds at a specific position.

mixed_list.append("New Item")
print(mixed_list)  # Output: [1, "Hello", 3.14, True, "New Item"]

mixed_list.insert(2, "Inserted Item")
print(mixed_list)  # Output: [1, "Hello", "Inserted Item", 3.14, True, "New Item"]

Removing Elements:

• remove(item) removes the first occurrence.
• pop() removes and returns the last item or item at a given index.

mixed_list.remove("Hello")
print(mixed_list)  # Output: [1, "Inserted Item", 3.14, True, "New Item"]

popped_item = mixed_list.pop()
print(popped_item)  # Output: New Item
print(mixed_list)   # Output: [1, "Inserted Item", 3.14, True]

mixed_list.pop(1)
print(mixed_list)   # Output: [1, 3.14, True]

Sorting and Length:

• Use sort() to arrange the list in ascending or descending order.
• Use len() to get the number of elements in a list.

my_list.sort()
print(my_list)  # Output: [1, 2, 3, 4, 5]

my_list.sort(reverse=True)
print(my_list)  # Output: [5, 4, 3, 2, 1]

print(len(my_list))  # Output: 5

Sets in Python

Sets are similar to lists but are unordered collections that do not allow duplicate elements. Sets are defined using curly braces {}.

my_set = {1, 2, 5, 9, 3, 4, 5}
print(my_set)  # Output: {1, 2, 3, 4, 5, 9}
# Note: Duplicate '5' is removed and order is not guaranteed.

Looping Through a Set:

for item in my_set:
    print(item)

Adding Elements:

• Use add() to insert a single element.
• Use update() to add multiple elements.

my_set.add(7)
print(my_set)  # Output: {1, 2, 3, 4, 5, 7, 9}

my_set.update([6, 8, 10])
print(my_set)  # Output: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

Removing Elements:

• remove(item) deletes an element but raises an error if the element is not found.
• discard(item) deletes an element if present, but does nothing if not found.

my_set.remove(10)
# my_set.remove(11)  # Raises KeyError if uncommented

my_set.discard(9)
my_set.discard(11)  # No error if 11 is not present
print(my_set)  # Output: {1, 2, 3, 4, 5, 6, 7, 8}

Clearing a Set:

• Clear all

my_set.clear()
print(my_set)  # Output: set()

Tuples in Python

Tuples are ordered, immutable collections of items.
They are defined using parentheses ().

my_tuple = (1, 2, 2, 6, 3, 4, 5)
print(my_tuple)  # Output: (1, 2, 2, 6, 3, 4, 5)

Immutability:
Tuples cannot be changed after creation.

my_tuple[0] = 10  # Raises TypeError

Accessing Elements:
Access tuple elements by index.

print(my_tuple[0])  # Output: 1

Unchangeable Nature:
You cannot add, remove, or modify items in a tuple after it is created.

Tuple Packing and Unpacking:

# Packing
person = ("Alice", 30, "Engineer")

# Unpacking
name, age, profession = person
print(name)       # Output: Alice
print(age)        # Output: 30
print(profession) # Output: Engineer

Assignment-03: List Assignment

Task:

• Create a list of 5 animals called zoo
• Delete the animal at the 3rd index
• Append a new animal at the end of the list
• Delete the animal at the beginning of the list
• Print all the animals
• Print only the first 3 animals

Solution:

zoo = ["Lion", "Tiger", "Elephant", "Giraffe", "Zebra"]

# Delete the animal at the 3rd index
zoo.pop(3)
print(zoo)  # Output: ['Lion', 'Tiger', 'Elephant', 'Zebra']

# Append a new animal at the end of the list
zoo.append("Deer")
print(zoo)  # Output: ['Lion', 'Tiger', 'Elephant', 'Zebra', 'Deer']

# Delete the animal at the beginning of the list
zoo.pop(0)
print(zoo)  # Output: ['Tiger', 'Elephant', 'Zebra', 'Deer']

# Print all the animals
for animal in zoo:
    print(animal)  # Output: 'Tiger', 'Elephant', 'Zebra', 'Deer'

# Print only the first 3 animals
print(zoo[:3]) #print(zoo[0:3]) # Output: ['Tiger', 'Elephant', 'Zebra']

Booleans and Operators in Python

Boolean Type:
Booleans represent truth values: True or False.

is_liked = True
print(type(is_liked))  # Output: <class 'bool'>

Comparison Operators:
Used to compare values.

print(1 == 1)   # Equal: True
print(1 != 1)   # Not equal: False
print(1 > 2)    # Greater than: False
print(2 >= 1)   # Greater than or equal: True
print(1 < 5)    # Less than: True
print(1 <= 7)   # Less than or equal: True

Logical Operators:
Combine multiple conditions.

print(2 > 1 and 5 < 7)    # Both must be True: True
print(2 >= 1 or 5 < 7)    # At least one is True: True
print(not(1 == 1))        # Negates the condition: False

Flow Control: If, Elif, Else

Python uses indentation to define blocks of code for flow control statements.

If Statement:

age = 1
if age < 3:
    print("Baby")  # Output: Baby

If-Else Statement:

is_boy = False
if is_boy:
    print("Boy")
else:
    print("Girl")  # Output: Girl

Elif (Else If):

hour = 16
if hour < 12:
    print("Good Morning")
elif hour >= 12 and hour <= 17:
    print("Good Afternoon")  # Output: Good Afternoon
else:
    print("Good Evening")

Assignment-04: If-Else Assignment

Task:

• Create a variable grade holding an integer between 0 - 100.
• Use if, elif, else statements to print the letter grade based on the number grade.

Grades:

• A = 90 - 100
• B = 80 - 89
• C = 70 - 79
• D = 60 - 69
• F = 0 - 59

Solution:

grade = 87
if grade >= 90 and grade <= 100:
    print("A")
elif 80 <= grade <= 89:  # If two variables are the same, you can write "80 <= grade <= 89"
    print("B")
elif 70 <= grade <= 79:
    print("C")
elif 60 <= grade <= 69:
    print("D")
else:
    print("F")