03_Python_Notes.md

Difference between function and method

• Functions which are defined inside a class are called methods. Functions which are defined outside a class are called functions.

• Function: A block of reusable code that performs a specific task. It can be defined using the def keyword and can be called independently. Functions can take parameters and return values.

• Method: A function that is associated with an object or class. Methods are defined within a class and are called on instances of that class. They can access and modify the object's state and are defined using the same def keyword. Methods are typically used to operate on the data contained within the object.

How to install virtual environment in python

• common way: using venv (traditional way)

python3 -m venv <name_of_your_env>

• activate the environment

source <name_of_your_env>/bin/activate

• deactivate the environment

deactivate

for activation of virtual environment in windows

<name_of_your_env>\Scripts\activate

installing packages in virtual environment

pip install <package_name>

installing packages in virtual environment from requirements.txt

pip install -r requirements.txt

requirements.txt file is a text file that contains a list of packages to be installed using pip.

the name of the file is requirements.txt by convention, but you can name it anything you want.
To install specific version of a package, you can specify the version number in the requirements.txt file like this:

package_name==1.0.0

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New way to create virtual environment is uv

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How to organize your python code

• create a folder for your project

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Difference between modules and packages in python

• Module: A single file containing Python code, which can define functions, classes, and variables. It can also include runnable code. Modules are imported using the import statement.
• Package: A directory that contains multiple modules and a special __init__.py file. The __init__.py file can be empty or execute the initialization code for the package. Packages allow for a hierarchical organization of modules, enabling a structured way to group related modules. Packages are also imported using the import statement, but they can include sub-packages and modules.

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Object and mutability

• Object: In Python, everything is an object. An object is an instance of a class and can have attributes (data) and methods (functions). Objects can be mutable or immutable.

Object have identity, type, and value.

For immutability, the object cannot be changed after it is created. For example, strings and tuples are immutable objects in Python. To checking the mutability of an object, you can use the id() function to get the identity of the object and compare it before and after an operation that is supposed to change the object.

But never use id() to check mutability in production code, as it is not a reliable method for checking mutability. Instead, you should know the type of the object and its mutability characteristics.

Never use value comparison to check mutability, as it can lead to unexpected results. Instead, use the type or identity of the object to determine its mutability.

a = [1, 2, 3]
print(id(a))  # Get the identity of the object
a.append(4)  # Modify the object
print(id(a))  # Check the identity again

If the identity remains the same, the object is mutable. If it changes, the object is immutable.

In simple terms immutability means that once an object is created, it cannot be changed. For example, strings and tuples are immutable in Python, meaning you cannot modify their content after they are created. Lists and dictionaries, on the other hand, are mutable, allowing you to change their content.

each immutable object has its own identity, which is a unique identifier for that object in memory. When you create a new immutable object, it gets a new identity, even if its value is the same as an existing object.

sugar_amount = 2
print(f"Initial sugar: {sugar_amount}") #2
sugar_amount = 12
print(f"Second Initial sugar: {sugar_amount}") #12
print(f"ID of 2: {id(2)}") #140737488346112
print(f"ID of 12: {id(12)}") #140737488346144

In this example, the identity of the integer objects 2 and 12 is different, even though they are both integers. This shows that immutable objects have their own identity.

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Numbers

• int: Integer type, used for whole numbers (e.g., 1, 2, -3).
• float: Real number(Floating-point type), used for decimal numbers (e.g., 1.5, -2.3).
• complex: Complex number type, used for numbers with a real and imaginary part (e.g., 1 + 2j).
• bool: Boolean type, used for True/False values (e.g., True, False).

The division operator / always returns a float, even if the result is a whole number. If you want to perform integer division (floor division), use the // operator.

a = 5
b = 2
print(a / b)  # Output: 2.5 (float division)
print(a // b)  # Output: 2 (integer division)

In python, the types which can be converted to False are:

False, None, 0, 0.0, 0j, [], (), {},
set(), '', b'', bytearray(), memoryview(b'')

Other than these types, everything else is considered True.

• the b here is used to denote a byte string, which is a sequence of bytes. It is often used for binary data or when working with files in binary mode.

Logical Operators in Python

• and: Returns True if both operands are True.
• or: Returns True if at least one operand is True.
• not: Returns True if the operand is False, and vice versa.

Strings

Strings are immutable sequences of characters. They can be defined using single quotes, double quotes, or triple quotes for multi-line strings.

In Python, when slicing strings using the [] operator, the end index is not inclusive—meaning the character at the end index is not included in the result. This behavior is also the same in JavaScript when using the slice() method: the substring returned includes the character at the start index but excludes the character at the end index.

Example in Python:

s = "hello"
print(s[1:4])  # Output: 'ell'

Example in JavaScript:

let s = "hello";
console.log(s.slice(1, 4)); // Output: 'ell'

there is also third parameter in slicing which is the step size. It allows you to skip characters in the string.

s= "hello"
print(s[0:5:2])  # Output: 'hlo'

the first parameter and second parameter are the start and end indices, and the third parameter is the step size.

first and second parameter are also optional, if you don't provide them, it will take the whole string.

s = "hello"
print(s[::2])  # Output: 'hlo' (every second character)
print(s[::-1])  # Output: 'olleh' (reversed string)

Membership

• in: Checks if a substring exists within a string.
• not in: Checks if a substring does not exist within a string.

s = "hello"
print("e" in s)  # Output: True
print("x" not in s)  # Output: True

Tuples

• A tuple is an ordered collection of items that can contain elements of different types, including other tuples. Tuples are immutable, meaning you cannot change their content after creation. They are defined using parentheses (), and elements are separated by commas.
• Tuples can be indexed and sliced, similar to lists, but they cannot be modified after creation.

my_tuple = (1, 2, 3, "hello", (4, 5))
print(my_tuple[0])  # Output: 1
print(my_tuple[3])  # Output: 'hello'
print(my_tuple[4][1])  # Output: 5 (accessing an element from a nested tuple)

Tuples are often used to group related data together, and they can be unpacked into variables:

x, y, z = (1, 2, 3)
print(x)  # Output: 1
print(y)  # Output: 2
print(z)  # Output: 3

Tuples can also be used as keys in dictionaries because they are immutable, while lists cannot be used as dictionary keys.

my_dict = {(1, 2): "value1", (3, 4): "value2"}
print(my_dict[(1, 2)])  # Output: 'value1

Tuples are generally used when you want to ensure that the data cannot be modified, or when you want to use them as keys in dictionaries. They are also slightly more memory-efficient than lists due to their immutability.

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List (mutable data type)

• A list is an ordered collection of items that can contain elements of different types, including other lists. Lists are mutable, meaning you can change their content after creation. Similar like arrays in other programming languages, lists can be indexed and sliced.
• Lists are defined using square brackets [], and elements are separated by commas.

my_list = [1, 2, 3, "hello", [4, 5]]
print(my_list[0])  # Output: 1
print(my_list[3])  # Output: 'hello'
print(my_list[4][1])  # Output: 5 (accessing an element from a nested list)

Set (mutable data type)

• A set is an unordered collection of unique items. Sets are mutable, meaning you can add or remove elements after creation. They are defined using curly braces {} or the set() constructor.
• Sets do not allow duplicate elements, and they are useful for membership testing and removing duplicates from a list.

my_set = {1, 2, 3, "hello"}
print(my_set)  # Output: {1, 2, 3, 'hello'}
my_set.add(4)  # Adding an element
print(my_set)  # Output: {1, 2, 3, 'hello', 4}
my_set.remove(2)  # Removing an element
print(my_set)  # Output: {1, 3, 'hello', 4}

• Sets do not support indexing or slicing, as they are unordered collections. However, you can check for membership using the in keyword.

my_set = {1, 2, 3, "hello"}
print(2 in my_set)  # Output: True
print("world" in my_set)  # Output: False

• Sets are often used for operations like union, intersection, and difference, which are common in mathematical set theory.
• Sets can also be used to remove duplicates from a list by converting the list to a set and then back to a list.

my_list = [1, 2, 2, 3, "hello", "hello"]
my_set = set(my_list)  # Convert list to set to remove duplicates
print(my_set)  # Output: {1, 2, 3, 'hello'}
my_list_no_duplicates = list(my_set)  # Convert set back to list
print(my_list_no_duplicates)  # Output: [1, 2, 3, 'hello']

frozenset (immutable data type)

• A frozenset is an immutable version of a set. It is an unordered collection of unique items, but unlike a regular set, you cannot add or remove elements after creation. Frozensets are defined using the frozenset() constructor.
• Frozensets are useful when you need a set-like structure that should not change, such as when using sets as keys in dictionaries or elements in other sets.

my_frozenset = frozenset([1, 2, 3, "hello"])
print(my_frozenset)  # Output: frozenset({1, 2, 3, 'hello'})
# my_frozenset.add(4)  # This will raise an AttributeError because frozensets are immutable

• You can perform set operations like union, intersection, and difference with frozensets, just like with regular sets, but you cannot modify the frozenset itself.

set1 = frozenset([1, 2, 3])
set2 = frozenset([3, 4, 5])
print(set1.union(set2))  # Output: frozenset({1, 2, 3, 4, 5})
print(set1.intersection(set2))  # Output: frozenset({3})
print(set1.difference(set2))  # Output: frozenset({1, 2})

• Frozensets can be used as keys in dictionaries or elements in other sets because they are immutable, while regular sets cannot be used in these contexts.

my_dict = {frozenset([1, 2]): "value1", frozenset([3, 4]): "value2"}
print(my_dict[frozenset([1, 2])])  # Output: 'value1'

NOTE : Dictionaries also have union, intersection, and difference operations, but they are not as commonly used as in sets. Dictionaries are primarily used for key-value pairs, while sets and frozensets are used for unique collections of items.

Dictionary (mutable data type)

• A dictionary is an unordered collection of key-value pairs. Each key is unique, and it maps to a specific value. Dictionaries are mutable, meaning you can change their content after creation. They are defined using curly braces {} with key-value pairs separated by colons :.

my_dict = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "hobbies": ["reading", "traveling"]
}
print(my_dict["name"])  # Output: 'Alice'
print(my_dict["age"])  # Output: 30
print(my_dict["hobbies"][0])  # Output: 'reading' (accessing a list within a dictionary)

Dictionaries can be used to store related data in a structured way, allowing you to access values using their corresponding keys. You can add, remove, or modify key-value pairs in a dictionary.

my_dict["country"] = "USA"  # Adding a new key-value pair
print(my_dict)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York', 'hobbies': ['reading', 'traveling'], 'country': 'USA'}
my_dict["age"] = 31  # Modifying an existing key-value pair
print(my_dict)  # Output: {'name': 'Alice', 'age': 31, 'city': 'New York', 'hobbies': ['reading', 'traveling'], 'country': 'USA'}
del my_dict["city"]  # Removing a key-value pair
print(my_dict)  # Output: {'name': 'Alice', 'age': 31, 'hobbies': ['reading', 'traveling'], 'country': 'USA'}

Dictionaries can also be nested, meaning you can have dictionaries within dictionaries, or dictionaries containing lists or other data structures.

nested_dict = {
    "person": {
        "name": "Bob",
        "age": 25
    },
    "skills": ["Python", "JavaScript"]
}
print(nested_dict["person"]["name"])  # Output: 'Bob'
print(nested_dict["skills"][1])  # Output: 'JavaScript'

Dictionaries are widely used in Python for various purposes, such as storing configuration settings, representing JSON data, and managing key-value pairs in applications. They provide efficient access to values based on their keys, making them a powerful data structure for many programming tasks.

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Conditionals

Coming from JS background, you might be familiar with the if, else if, and else statements. In Python, the syntax is slightly different, but the logic remains the same.

example of if-else statement in python

age = 18
if age < 18:
    print("You are a minor.")
elif age == 18:
    print("You are exactly 18 years old.")
else:
    print("You are an adult.")

Output: You are exactly 18 years old.

match

Match is just like switch statement in JS, but it is more powerful and flexible. It allows you to match patterns in data structures, making it easier to work with complex data.

A simple example of match statement in python

seat_type = input("Enter seat type (sleeper/AC/general/luxury): ").lower()

match seat_type:
    case "sleeper":
        print("Sleeper - No AC, beds available")
    case "ac":
        print("AC - Air conditioned, comfy ride")
    case "general":
        print("General - Cheapest option, no reservation")
    case "luxury":
        print("Luxury - Premium seats with meals")
    case _:
        print("Invalid seat type")

Output: AC - Air conditioned, comfy ride

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Loops

In python there are two types of loops: for and while. for loop: Used to iterate over a sequence (like a list, tuple, or string) or any iterable object.

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

range, enumerate, and zip are built-in functions that can be used with for loops. range: Generates a sequence of numbers, often used for iterating a specific number of times.

for i in range(5):
    print(i)

Output:

0
1
2
3
4

range(start, stop, step) can also be used to specify a starting point, an ending point, and a step size.

example,

for i in range(1, 10, 2):
    print(i)

Output:

1
3
5
7
9

enumerate: Adds a counter to an iterable, returning both the index and the value in the form of tuples.

fruits = ["apple", "banana", "cherry"]
for index, fruit in enumerate(fruits):
    print(f"Index: {index}, Fruit: {fruit}")

Output:

Index: 0, Fruit: apple
Index: 1, Fruit: banana
Index: 2, Fruit: cherry

zip: Combines multiple iterables (like lists or tuples) into a single iterable of tuples, pairing elements from each iterable.

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
for name, age in zip(names, ages):
    print(f"{name} is {age} years old.")

Output:

Alice is 25 years old.
Bob is 30 years old.
Charlie is 35 years old.

while loop

A while loop continues to execute as long as a specified condition is true. It is useful when you don't know beforehand how many iterations you need.

# You want to simulate tea heating.
# It starts at 40°C and boils at 100°C.
# Task:
# Use a while loop.
# Increase temperature by 15 until it reaches or exceeds 100.
# Print each temperature step.
temperature = 40


while temperature < 100:
    print(f"Current temperature: {temperature}")
    temperature += 15

print("Tea is ready to boil")

Output:

Current temperature: 40
Current temperature: 55
Current temperature: 70
Current temperature: 85
Current temperature: 100
Tea is ready to boil

break and continue

• break: Exits the loop immediately, regardless of the loop's condition.
• continue: Skips the current iteration and moves to the next iteration of the loop.

flavours = ["Ginger", "Out of Stock", "Lemon", "Discontinued", "Tulsi"]

for flavour in flavours:
    if flavour == "Out of Stock":
        continue
    if flavour == "Discontinued":
        print(f"{flavour} item found")
        break
    print(f"{flavour} item found")

print("Out side of loop")

Output:

Ginger item found
Lemon item found
Discontinued item found
Out side of loop

for-else

A for-else loop executes the else block when the loop completes normally (i.e. it does not encounter a break statement). If the loop is exited early using break, the else block is skipped.

numbers = [1, 2, 3, 4, 5]
for number in numbers:
    if number == 3:
        print("Found 3, breaking the loop.")
        break
else:
    print("Loop completed without finding 3.")

Output:

Found 3, breaking the loop.

In the above example, the else block is not executed because the loop was exited with a break statement. If the loop completes without encountering a break, the else block will execute.
Example:

numbers = [1, 2, 4, 5]
for number in numbers:
    if number == 3:
        print("Found 3, breaking the loop.")
        break
else:
    print("Loop completed without finding 3.")

Output:

Loop completed without finding 3.

In this case, the else block is executed because the loop completed normally without encountering a break statement.

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Statement, expression & Walrus Operator

• Statement: A statement is a complete instruction that performs an action. It can be a single line of code or a block of code. Examples include variable assignments, function definitions, loops, and conditionals. Statements do not return a value.

  Example:

  x = 5  # This is a statement

• Expression: An expression is a piece of code that evaluates to a value. It can be as simple as a single value or a more complex combination of values, operators, and function calls. Expressions can be used within statements.

  Example:

  y = x + 10  # This is an expression that evaluates to a value

• Walrus Operator (:=): Introduced in Python 3.8, the walrus operator allows you to assign a value to a variable as part of an expression. This can be useful for reducing code duplication and improving readability.

  Example:

     if (n := len(my_list)) > 5:
         print(f"List is too long with {n} elements.")

  In this example, the length of my_list is assigned to the variable n while checking if it is greater than 5. This avoids the need to call len(my_list) twice.

Note: The walrus operator is particularly useful in situations where you want to use the result of an expression immediately after assigning it to a variable, such as in loops or conditionals.

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Using Dictionaries instead of repeated cases

Instead of using multiple if-elif statements to handle different cases, you can use a dictionary to map keys to values or functions. This can make your code cleaner and more maintainable.

users = [
    {"id": 1, "total": 100, "coupon": "P20"},
    {"id": 2, "total": 140, "coupon": "F10"},
    {"id": 3, "total": 80, "coupon": "P50"},
]

discounts = {"P20": (0.2, 0), "F10": (0.5, 0), "P50": (0, 10)}


for user in users:
    percent, fixed = discounts.get(user["coupon"], (0, 0))
    discount = user["total"] * percent + fixed
    print(
        f"id of {user['id']} paid {user['total']} and got discount for next visit of rupees {discount}"
    )

Output:

id of 1 paid 100 and got discount for next visit of rupees 20.0
id of 2 paid 140 and got discount for next visit of rupees 70.
id of 3 paid 80 and got discount for next visit of rupees 50

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Scopes and Name Resolution

• Local - inside a function or method
• Enclosing - from outer function if nested
• Global - at the top level of a module or script
• Built-in - names provided by Python (like print, len, etc.)

Global and nonlocal

• Global: Variables defined at the top level of a module or script. They can be accessed from anywhere in the module.
• Nonlocal: Variables defined in an enclosing scope (like an outer function) but not in the global scope. They can be accessed from nested functions.
• To modify a global variable inside a function, you need to use the global keyword. Similarly, to modify a nonlocal variable inside a nested function, you need to use the nonlocal keyword.
• Example of global and nonlocal:

# Global variable
x = 10
def outer_function():
    # Nonlocal variable
    y = 20

    def inner_function():
        global x  # Accessing global variable
        nonlocal y  # Accessing nonlocal variable
        x += 5  # Modifying global variable
        y += 5  # Modifying nonlocal variable
        print(f"Global x: {x}, Nonlocal y: {y}")

    inner_function()
outer_function()
print(f"Global x after function call: {x}")  # Output: Global x after function call: 15

Output:

Global x: 15, Nonlocal y: 25
Global x after function call: 15

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Default traps

• Default arguments in Python functions are evaluated only once when the function is defined, not each time the function is called. This can lead to unexpected behavior if mutable objects (like lists or dictionaries) are used as default values.

Let's see an example of this trap:

def my_function(arg=[]):
    arg.append(1)
    return arg
print(my_function())  # Output: [1]
print(my_function())  # Output: [1, 1] (same list is modified)

• In the above example, the default argument arg is a list that is created only once when the function is defined. Therefore, every time the function is called without an argument, it modifies the same list, leading to unexpected results.
• To avoid this trap, it is recommended to use None as the default value for mutable objects and then check for it inside the function. This way, a new object is created each time the function is called without an argument.

def my_function(arg=None):
    if arg is None:
        arg = []  # Create a new list if no argument is provided
    arg.append(1)
    return arg
print(my_function())  # Output: [1]
print(my_function())  # Output: [1] (new list created each time)

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args and kwargs

• *args: Allows you to pass a variable number of positional arguments to a function. It collects the extra positional arguments into a tuple.
• **kwargs: Allows you to pass a variable number of keyword arguments to a function. It collects the extra keyword arguments into a dictionary.
• Example of using *args and **kwargs:

def my_function(*args, **kwargs):
    print("Positional arguments:", args)
    print("Keyword arguments:", kwargs)
my_function(1, 2, 3, name="Alice", age=30)

Output:

Positional arguments: (1, 2, 3)
Keyword arguments: {'name': 'Alice', 'age': 30}

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After learning python

• PEP 8 – Style Guide for Python Code

  • 4 spaces, never tabs
  • meaningful variable names
  • use snake_case for variable and function names
  • use CamelCase for class names
  • use UPPERCASE for constants
  • formatters like black and isort can help maintain code style

• PEP 20 – The Zen of Python

  • Readability counts
  • Simple is better than complex
  • Explicit is better than implicit
  • There should be one-- and preferably only one --obvious way to do it
  • Errors should never pass silently
  • In the face of ambiguity, refuse the temptation to guess
  • Namespaces are one honking great idea -- let's do more of those!
  • In terminal, enter python3 and type import this to see the Zen of Python

• PEP 484 – Type Hints

  • Type hints are optional annotations that can be added to function signatures to indicate the expected types of parameters and return values.
  • They help with code readability and can be checked using static type checkers like mypy.
  • Example:

def add(a: int, b: int) -> int:
    return a + b