cheetsheetz/python.md

Python Cheat Sheet

Table of Contents

1. Variables

2. Data Types

3. Operators

4. Control Structures

5. Functions

6. Lists

7. Strings

8. Dictionaries

9. Tuples

10. Sets

11. File Handling

12. Exception Handling

13. Classes and Objects

14. SQLite3 Database Connection

    14.1 Connecting to a Database

    14.2 Creating a Table

    14.3 Inserting Data

    14.4 Querying Data

    14.5 Updating Data

    14.6 Deleting Data

    14.7 Closing the Connection

15. JSON Data Connection

    15.1 Loading JSON Data

    15.2 Writing JSON Data

Variables

Declaration and Assignment

variable_name = value

Variable Types

• String: string_variable = "Hello"
• Integer: int_variable = 42
• Float: float_variable = 3.14
• Boolean: bool_variable = True

Data Types

Basic Types

• String
• Integer
• Float
• Boolean

Compound Types

• List
• Tuple
• Set
• Dictionary

Operators

Arithmetic Operators

+, -, *, /, %, ** (exponentiation), //

Comparison Operators

==, !=, <, >, <=, >=

Logical Operators

and, or, not

Assignment Operators

=, +=, -=, *=, /=, %=

Control Structures

If Statement

if condition:
    # code to be executed if the condition is true
elif another_condition:
    # code to be executed if the previous condition was false and this one is true
else:
    # code to be executed if all conditions are false

For Loop

for i in range(5):
    # code to be executed in each iteration

While Loop

while condition:
    # code to be executed as long as the condition is true

Functions

Function Declaration

def function_name(param1, param2):
    # code to be executed
    return result  # optional

Function Call

result = function_name(arg1, arg2)

Lists

List Declaration

my_list = [1, 2, 3, "hello", True]

List Operations

my_list.append(4)  # Add an element to the end
my_list.remove("hello")  # Remove a specific element

Strings

String Concatenation

string1 = "Hello"
string2 = "World"
concatenated = string1 + " " + string2

String Methods

len(my_string)  # Length of the string
my_string.find("find")  # Index of the first occurrence of "find"
my_string.split(",")  # Split the string into a list

Dictionaries

Dictionary Declaration

my_dict = {"key1": "value1", "key2": "value2"}

Dictionary Operations

my_dict["new_key"] = "new_value"  # Add a new key-value pair
del my_dict["key1"]  # Remove a key-value pair

Tuples

Tuple Declaration

my_tuple = (1, 2, 3, "hello", True)

Immutable Nature

# Tuple elements cannot be changed once set
my_tuple[0] = 4  # This will result in an error

Sets

Set Declaration

my_set = {1, 2, 3, 3, 4}

Set Operations

my_set.add(5)  # Add an element to the set
my_set.remove(3)  # Remove an element from the set

File Handling

Reading from a File

with open("filename.txt", "r") as file:
    file_content = file.read()

Writing to a File

with open("filename.txt", "w") as file:
    file.write("Hello, World!")

Exception Handling

Try-Except Block

try:
    # code that may raise an exception
except Exception as e:
    # handle the exception
    print("Error:", e)

Classes and Objects

Class Declaration

class MyClass:
    def __init__(self, name):
        self.name = name

    def say_hello(self):
        print(f"Hello, {self.name}!")

Object Instantiation

obj = MyClass("John")
obj.say_hello()  # Outputs: Hello, John!

Working with Data

# Import necessary modules
import sqlite3
import json

SQLite3 Database Connection

Connecting to a Database

# Connect to a database (creates a new database if it doesn't exist)
conn = sqlite3.connect('example.db')

# Create a cursor object to execute SQL commands
cursor = conn.cursor()

Creating a Table

# Create a table
cursor.execute('''
    CREATE TABLE IF NOT EXISTS users (
        id INTEGER PRIMARY KEY,
        username TEXT NOT NULL,
        email TEXT NOT NULL
    )
''')

# Commit the changes
conn.commit()

Inserting Data

# Insert data into the table
cursor.execute('''
    INSERT INTO users (username, email)
    VALUES ('john_doe', 'john@example.com')
''')

# Commit the changes
conn.commit()

Querying Data

# Fetch all rows from the table
cursor.execute('SELECT * FROM users')
rows = cursor.fetchall()

# Print the results
for row in rows:
    print(row)

Updating Data

# Update data in the table
cursor.execute('''
    UPDATE users
    SET email = 'john.doe@example.com'
    WHERE username = 'john_doe'
''')

# Commit the changes
conn.commit()

Deleting Data

# Delete data from the table
cursor.execute('''
    DELETE FROM users
    WHERE username = 'john_doe'
''')

# Commit the changes
conn.commit()

Closing the Connection

# Close the cursor and connection when done
cursor.close()
conn.close()

JSON Data Connection

Loading JSON Data

# Load JSON data from a file
with open('data.json', 'r') as json_file:
    data = json.load(json_file)
    print(data)

Writing JSON Data

# Write Python data to a JSON file
data_to_write = {'name': 'John', 'age': 30, 'city': 'New York'}

with open('output.json', 'w') as json_file:
    json.dump(data_to_write, json_file)