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Python | Basics

Setup virtual environment

# Install venv module
python -m pip install virtualenv

# Setup virtual env for new project
cd <project_folder>
python -m venv .venv

# Activate virtual environment (Linux - like OS)
source ./.venv/bin/activate

# Activate virtual environment (Windows)
.\.venv\Scripts\activate

# Deactivate virtual environment
deactivate

# Store installed packages into requirements.txt
python -m pip freeze > requirements.txt

# Install packages from requirements.txt
python -m pip install -r requirements.txt

Basic / Primitive Types

In Python everything is an object with hell number of properties and methods.

Primitive Types.

1. Integers (int) - Represents whole numbers without any decimal points.
2. Floats (float) - Represents decimal numbers.
3. Booleans (bool) - Represents either True or False (actually implemented as ints 1 and 0 respectively).
4. Strings (str) - Represents sequences of characters.
5. NoneType (None) - Represents the absence of a value or a null value.

Variables.

# Declaring variables of basic types
temperature = -22 # int
income = 220000.10 # float
name = "Albert" # str
is_active = True # bool
result = None # none, null

# Using constructor functions / conversion
age = int(26)
length = float("2.5")
index_text = str(5)

# Multiple assignments
age, name, ratio = 2, "Bla Bla", 4.5
is_active = is_ready = is_valid = True

# Constants
PI = 3.14

Decision-making statements.

# if
if a > 10:
  # some logic

# if-else
if isActive:
  print("Active")
else:
  print("Inactive")

# if-elif-else
if score >= 0 and score < 20:
  # do something
elif score >= 20 and score < 50 :
  # do something
elif score == 9999 or score == -1:
  # do something
else:
  # do something

# Conditional Expression
c = a if a > b else b

# Python does not have a built-in switch statement
# switch-like approach using dict and lambda
def switch_grade_dict(val):
    switch_dict = {
        'A': lambda: print("Grade A : [0 , 20)"),
        'B': lambda: print("Grade B : [20 , 50)"),
        'default': lambda: print("Invalid grade")
    }

    switch_dict.get(val, switch_dict['default'])()

# Usage
switch_grade_dict('B')

Loop statements.

# for loop
for i in range(5):
  print(i)
# Output: 0, 1, 2, 3, 4
for fruit in ["apple", "banana", "cherry"]:
  print(fruit)

# while loop
while not is_empty and step != 10:
  # do something

# continue, break, else - loop control statements
for i in range(10):
  if i < 5:
    continue
  if i > 5:
    break
  print(i)
# Output: 5

for j in range(3, 5):
  print(j)
else:
  print("The End")
# Output: 3, 4, 'The End'

# List comprehensions
squares = [x**2 for x in range(5)]
# Output: [0, 1, 4, 9, 16]

Naming Convention.

General:

• Use descriptive names.
• Start with a letter or underscore.
• Consists of letters, numbers, and underscores.
• Case-sensitive (count and Count are different).

Variables, Functions, Modules:

• snake_case
• Lowercase letters

total_count = 100
is_valid = True

def calculate_summary(order_list):
  # Function implementation

import math_module
from my_module import greet, pi

Constants:

• snake_case
• Uppercase letters

MAX_SIZE = 50
SEARCH_URL = "https://google.com/"

Classes:

• CamelCase

class UserPreferences:
  # Class definition

Private Variables, Constants, Methods and Dunder Methods:

class Order:
  _NEW = "New"

  def __init__(self):
    self._status = self._NEW
    self._creation_date = datetime.now()

  def _is_new(self):
    return self._status == self._NEW

REPL

REPL stands for Read-Eval-Print Loop.

Useful commands:

# type() - to check variable type
num = 25
type(num) # Output: <class 'int'>

# dir() - prints list of methods and properties for type or var.
dir(num)
dir(str)

# help() - prints pydoc.help for the python object.
help(None)
help(11)