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Python & SQL Bible

Chapter 17: Python Meets SQL

17.5 Handling Transactions in Python

Sure thing! Here's an explanation of handling transactions in Python.

Database transactions provide a way to process a set of database operations in a single unit. If all operations are successful, the changes are committed to the database. If any operation fails, none of the changes are applied.

In a transaction, if a group of interdependent operations are part of a transaction, either all of the operations are executed, or none of them are. This property of transactions is often summarized as ACID (Atomicity, Consistency, Isolation, and Durability).

Python DB-API provides a way to handle transactions. After starting a transaction, you can commit it if no errors are encountered. If there are any errors, you can rollback the transaction to the state before the transaction was started.

Example:

Here is an example of handling transactions in Python using SQLite:

import sqlite3

try:
    # connect to the database
    conn = sqlite3.connect('test.db')

    # create a cursor
    cur = conn.cursor()

    # start a transaction
    cur.execute("BEGIN TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (1, 'Paul', 32, 'California', 20000.00)")
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (2, 'Allen', 25, 'Texas', 15000.00)")

    # commit the transaction
    conn.commit()

    print("Records inserted successfully")

except sqlite3.Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into sqlite table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("the sqlite connection is closed")

In this code:

  • We first connect to the database using sqlite3.connect() and create a cursor object.
  • We start a transaction with cur.execute("BEGIN TRANSACTION").
  • We execute some SQL queries to insert data into the COMPANY table.
  • If all the operations are successful, we commit the transaction using conn.commit().
  • If any error occurs during any operation, we rollback the transaction using conn.rollback(). This ensures that our database remains in a consistent state.
  • In the end, we close the database connection using conn.close().

Remember, it's important to handle exceptions when working with transactions to make sure that an error in a single operation doesn't leave your database in an inconsistent state.

This approach of handling transactions is common across other databases like MySQL and PostgreSQL with slight modifications depending on the specific database driver methods.

Now, while we've already covered handling transactions manually in Python, it's worth mentioning that Python's DB-API also supports a simplified transaction model for those who only execute single commands.

By default, the sqlite3 module opens a transaction automatically before any data-modifying operation (like INSERT, UPDATE, DELETE, etc.), and commits transactions automatically when the cursor is executed.

However, if you're executing more than one command as part of a transaction, it's generally a good idea to manage transactions manually as shown in the previous example, as it gives you finer control over when a transaction is committed or rolled back.

Additionally, while SQLite and PostgreSQL follow the SQL standard for transactions (BEGIN TRANSACTION, COMMIT, and ROLLBACK), MySQL uses slightly different commands. Instead of "BEGIN TRANSACTION", you would use "START TRANSACTION" in MySQL.

Here's an example of how you'd handle transactions in Python with MySQL using the mysql-connector-python module:

import mysql.connector
from mysql.connector import Error

try:
    # connect to the MySQL server
    conn = mysql.connector.connect(user='username', password='password',
                                   host='127.0.0.1', database='testdb')

    # create a new cursor
    cur = conn.cursor()

    # start a new transaction
    cur.execute("START TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (1, 'John Doe', 70000)")
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (2, 'Jane Doe', 80000)")

    # commit the transaction
    conn.commit()

    print("Data inserted successfully")

except Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into MySQL table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("MySQL connection is closed")

This example is similar to the SQLite one, but with the notable difference of using "START TRANSACTION" to begin a transaction in MySQL.

In general, the principle of transaction management remains the same across different SQL databases, although the specific commands and methods may differ slightly. It's important to consult the documentation for your specific database and database driver when working with transactions in Python.

17.5 Handling Transactions in Python

Sure thing! Here's an explanation of handling transactions in Python.

Database transactions provide a way to process a set of database operations in a single unit. If all operations are successful, the changes are committed to the database. If any operation fails, none of the changes are applied.

In a transaction, if a group of interdependent operations are part of a transaction, either all of the operations are executed, or none of them are. This property of transactions is often summarized as ACID (Atomicity, Consistency, Isolation, and Durability).

Python DB-API provides a way to handle transactions. After starting a transaction, you can commit it if no errors are encountered. If there are any errors, you can rollback the transaction to the state before the transaction was started.

Example:

Here is an example of handling transactions in Python using SQLite:

import sqlite3

try:
    # connect to the database
    conn = sqlite3.connect('test.db')

    # create a cursor
    cur = conn.cursor()

    # start a transaction
    cur.execute("BEGIN TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (1, 'Paul', 32, 'California', 20000.00)")
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (2, 'Allen', 25, 'Texas', 15000.00)")

    # commit the transaction
    conn.commit()

    print("Records inserted successfully")

except sqlite3.Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into sqlite table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("the sqlite connection is closed")

In this code:

  • We first connect to the database using sqlite3.connect() and create a cursor object.
  • We start a transaction with cur.execute("BEGIN TRANSACTION").
  • We execute some SQL queries to insert data into the COMPANY table.
  • If all the operations are successful, we commit the transaction using conn.commit().
  • If any error occurs during any operation, we rollback the transaction using conn.rollback(). This ensures that our database remains in a consistent state.
  • In the end, we close the database connection using conn.close().

Remember, it's important to handle exceptions when working with transactions to make sure that an error in a single operation doesn't leave your database in an inconsistent state.

This approach of handling transactions is common across other databases like MySQL and PostgreSQL with slight modifications depending on the specific database driver methods.

Now, while we've already covered handling transactions manually in Python, it's worth mentioning that Python's DB-API also supports a simplified transaction model for those who only execute single commands.

By default, the sqlite3 module opens a transaction automatically before any data-modifying operation (like INSERT, UPDATE, DELETE, etc.), and commits transactions automatically when the cursor is executed.

However, if you're executing more than one command as part of a transaction, it's generally a good idea to manage transactions manually as shown in the previous example, as it gives you finer control over when a transaction is committed or rolled back.

Additionally, while SQLite and PostgreSQL follow the SQL standard for transactions (BEGIN TRANSACTION, COMMIT, and ROLLBACK), MySQL uses slightly different commands. Instead of "BEGIN TRANSACTION", you would use "START TRANSACTION" in MySQL.

Here's an example of how you'd handle transactions in Python with MySQL using the mysql-connector-python module:

import mysql.connector
from mysql.connector import Error

try:
    # connect to the MySQL server
    conn = mysql.connector.connect(user='username', password='password',
                                   host='127.0.0.1', database='testdb')

    # create a new cursor
    cur = conn.cursor()

    # start a new transaction
    cur.execute("START TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (1, 'John Doe', 70000)")
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (2, 'Jane Doe', 80000)")

    # commit the transaction
    conn.commit()

    print("Data inserted successfully")

except Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into MySQL table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("MySQL connection is closed")

This example is similar to the SQLite one, but with the notable difference of using "START TRANSACTION" to begin a transaction in MySQL.

In general, the principle of transaction management remains the same across different SQL databases, although the specific commands and methods may differ slightly. It's important to consult the documentation for your specific database and database driver when working with transactions in Python.

17.5 Handling Transactions in Python

Sure thing! Here's an explanation of handling transactions in Python.

Database transactions provide a way to process a set of database operations in a single unit. If all operations are successful, the changes are committed to the database. If any operation fails, none of the changes are applied.

In a transaction, if a group of interdependent operations are part of a transaction, either all of the operations are executed, or none of them are. This property of transactions is often summarized as ACID (Atomicity, Consistency, Isolation, and Durability).

Python DB-API provides a way to handle transactions. After starting a transaction, you can commit it if no errors are encountered. If there are any errors, you can rollback the transaction to the state before the transaction was started.

Example:

Here is an example of handling transactions in Python using SQLite:

import sqlite3

try:
    # connect to the database
    conn = sqlite3.connect('test.db')

    # create a cursor
    cur = conn.cursor()

    # start a transaction
    cur.execute("BEGIN TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (1, 'Paul', 32, 'California', 20000.00)")
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (2, 'Allen', 25, 'Texas', 15000.00)")

    # commit the transaction
    conn.commit()

    print("Records inserted successfully")

except sqlite3.Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into sqlite table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("the sqlite connection is closed")

In this code:

  • We first connect to the database using sqlite3.connect() and create a cursor object.
  • We start a transaction with cur.execute("BEGIN TRANSACTION").
  • We execute some SQL queries to insert data into the COMPANY table.
  • If all the operations are successful, we commit the transaction using conn.commit().
  • If any error occurs during any operation, we rollback the transaction using conn.rollback(). This ensures that our database remains in a consistent state.
  • In the end, we close the database connection using conn.close().

Remember, it's important to handle exceptions when working with transactions to make sure that an error in a single operation doesn't leave your database in an inconsistent state.

This approach of handling transactions is common across other databases like MySQL and PostgreSQL with slight modifications depending on the specific database driver methods.

Now, while we've already covered handling transactions manually in Python, it's worth mentioning that Python's DB-API also supports a simplified transaction model for those who only execute single commands.

By default, the sqlite3 module opens a transaction automatically before any data-modifying operation (like INSERT, UPDATE, DELETE, etc.), and commits transactions automatically when the cursor is executed.

However, if you're executing more than one command as part of a transaction, it's generally a good idea to manage transactions manually as shown in the previous example, as it gives you finer control over when a transaction is committed or rolled back.

Additionally, while SQLite and PostgreSQL follow the SQL standard for transactions (BEGIN TRANSACTION, COMMIT, and ROLLBACK), MySQL uses slightly different commands. Instead of "BEGIN TRANSACTION", you would use "START TRANSACTION" in MySQL.

Here's an example of how you'd handle transactions in Python with MySQL using the mysql-connector-python module:

import mysql.connector
from mysql.connector import Error

try:
    # connect to the MySQL server
    conn = mysql.connector.connect(user='username', password='password',
                                   host='127.0.0.1', database='testdb')

    # create a new cursor
    cur = conn.cursor()

    # start a new transaction
    cur.execute("START TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (1, 'John Doe', 70000)")
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (2, 'Jane Doe', 80000)")

    # commit the transaction
    conn.commit()

    print("Data inserted successfully")

except Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into MySQL table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("MySQL connection is closed")

This example is similar to the SQLite one, but with the notable difference of using "START TRANSACTION" to begin a transaction in MySQL.

In general, the principle of transaction management remains the same across different SQL databases, although the specific commands and methods may differ slightly. It's important to consult the documentation for your specific database and database driver when working with transactions in Python.

17.5 Handling Transactions in Python

Sure thing! Here's an explanation of handling transactions in Python.

Database transactions provide a way to process a set of database operations in a single unit. If all operations are successful, the changes are committed to the database. If any operation fails, none of the changes are applied.

In a transaction, if a group of interdependent operations are part of a transaction, either all of the operations are executed, or none of them are. This property of transactions is often summarized as ACID (Atomicity, Consistency, Isolation, and Durability).

Python DB-API provides a way to handle transactions. After starting a transaction, you can commit it if no errors are encountered. If there are any errors, you can rollback the transaction to the state before the transaction was started.

Example:

Here is an example of handling transactions in Python using SQLite:

import sqlite3

try:
    # connect to the database
    conn = sqlite3.connect('test.db')

    # create a cursor
    cur = conn.cursor()

    # start a transaction
    cur.execute("BEGIN TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (1, 'Paul', 32, 'California', 20000.00)")
    cur.execute("INSERT INTO COMPANY (ID, NAME, AGE, ADDRESS, SALARY) \\
          VALUES (2, 'Allen', 25, 'Texas', 15000.00)")

    # commit the transaction
    conn.commit()

    print("Records inserted successfully")

except sqlite3.Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into sqlite table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("the sqlite connection is closed")

In this code:

  • We first connect to the database using sqlite3.connect() and create a cursor object.
  • We start a transaction with cur.execute("BEGIN TRANSACTION").
  • We execute some SQL queries to insert data into the COMPANY table.
  • If all the operations are successful, we commit the transaction using conn.commit().
  • If any error occurs during any operation, we rollback the transaction using conn.rollback(). This ensures that our database remains in a consistent state.
  • In the end, we close the database connection using conn.close().

Remember, it's important to handle exceptions when working with transactions to make sure that an error in a single operation doesn't leave your database in an inconsistent state.

This approach of handling transactions is common across other databases like MySQL and PostgreSQL with slight modifications depending on the specific database driver methods.

Now, while we've already covered handling transactions manually in Python, it's worth mentioning that Python's DB-API also supports a simplified transaction model for those who only execute single commands.

By default, the sqlite3 module opens a transaction automatically before any data-modifying operation (like INSERT, UPDATE, DELETE, etc.), and commits transactions automatically when the cursor is executed.

However, if you're executing more than one command as part of a transaction, it's generally a good idea to manage transactions manually as shown in the previous example, as it gives you finer control over when a transaction is committed or rolled back.

Additionally, while SQLite and PostgreSQL follow the SQL standard for transactions (BEGIN TRANSACTION, COMMIT, and ROLLBACK), MySQL uses slightly different commands. Instead of "BEGIN TRANSACTION", you would use "START TRANSACTION" in MySQL.

Here's an example of how you'd handle transactions in Python with MySQL using the mysql-connector-python module:

import mysql.connector
from mysql.connector import Error

try:
    # connect to the MySQL server
    conn = mysql.connector.connect(user='username', password='password',
                                   host='127.0.0.1', database='testdb')

    # create a new cursor
    cur = conn.cursor()

    # start a new transaction
    cur.execute("START TRANSACTION")

    # execute some SQL queries
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (1, 'John Doe', 70000)")
    cur.execute("INSERT INTO employees (id, name, salary) VALUES (2, 'Jane Doe', 80000)")

    # commit the transaction
    conn.commit()

    print("Data inserted successfully")

except Error as error:
    # rollback the transaction in case of error
    conn.rollback()
    print("Failed to insert data into MySQL table", error)

finally:
    # close the connection
    if conn:
        conn.close()
        print("MySQL connection is closed")

This example is similar to the SQLite one, but with the notable difference of using "START TRANSACTION" to begin a transaction in MySQL.

In general, the principle of transaction management remains the same across different SQL databases, although the specific commands and methods may differ slightly. It's important to consult the documentation for your specific database and database driver when working with transactions in Python.