Connecting to and Querying Databases in Python Applications

Python connects and querys the database and needs to select the appropriate driver and follow the standard process. 1. Select the driver according to the database type, such as SQLite uses sqlite3, MySQL uses mysql-connector-python or PyMySQL, PostgreSQL uses psycopg2; 2. To connect to the database, you need to ensure that the service is available and the connection parameters are correctly configured. The remote database also needs to open the firewall port. It is recommended to use try-except to handle exceptions; 3. When executing the query, use parameterized statements to prevent SQL injection, execute SQL through cursors and obtain the results with fetch method, and call commit to submit transactions after the write operation; 4. After the operation is completed, the cursor and connection should be closed and the connection should be released, which can be handled automatically in conjunction with the context manager; 5. It is recommended to encapsulate connection logic, print and debug SQL statements, and small projects should be given priority to use SQLite, and complex business recommendation ORM tools to improve efficiency. Mastering these steps can effectively deal with common database operation problems.

Python provides a variety of ways to connect and query databases, suitable for a variety of scenarios from local small applications to enterprise-level systems. Whether you want to read SQLite for data analysis, or operate MySQL or PostgreSQL to build a web application backend, mastering basic database connections and query methods is the key.

Select the right database driver

Different types of databases require different libraries in Python to support connections. for example:

• SQLite : The standard library comes with sqlite3 , no additional installation is required
• MySQL : Commonly used mysql-connector-python or PyMySQL
• PostgreSQL : It is recommended to use psycopg2 or asyncpg (asynchronous)
• SQL Server : You can use pyodbc or pymssql

Choosing the right driver is the first step. If you are not sure which one to use, you can find clues from official documentation or project dependencies. For example, Django supports SQLite by default, but the production environment is usually replaced by PostgreSQL.

Basic steps to connect to a database

Taking SQLite as an example, connecting to a database is very simple:

 import sqlite3
conn = sqlite3.connect('example.db')

If it is a remote database, such as MySQL, the code will be a little more complicated:

 import mysql.connector
conn = mysql.connector.connect(
    host='localhost',
    user='root',
    password='yourpassword',
    database='testdb'
)

No matter which database you are connected, the following points should be paid attention to when connecting:

• Make sure the database service is started and accessible
• Check whether the username, password, and host address are correct
• If it is a remote database, confirm that the firewall allows communication to the corresponding port

When the connection fails, most drivers will throw exceptions. Remember to use try-except to wrap the connection logic for error handling.

Execute query and get results

After the connection is successful, you can create a cursor object to execute SQL statements:

 cursor = conn.cursor()
cursor.execute("SELECT * FROM users WHERE age > %s", (18,))
results = cursor.fetchall()

Pay attention to a few details:

• Use parameterized queries such as %s to prevent SQL injection
• After query, use fetchall() , fetchone() or fetchmany(n) to get the data
• After the operation is completed, you must call cursor.close() and conn.close() to close the resource.

If writing operations are involved (INSERT, UPDATE, DELETE), don't forget to submit the transaction:

 conn.commit()

Otherwise the changes will not take effect.

Some practical suggestions for database operations

• For frequently used connections, they can be encapsulated into a class or function to avoid duplicate code
• Automatically handle connections and cursor closures using context manager (with)
• During the development process, you can print executed SQL statements for easy debugging
• Small projects can be directly used with SQLite, making it more convenient to deploy
• Complex business recommendations to improve development efficiency with ORM (such as SQLAlchemy, Django ORM)

Basically that's it. Although database operations don't seem difficult, many problems lie in details, such as parameter format, connection configuration, transaction control, etc. Practice a few more times and be familiar with common error checking methods, you can easily deal with most scenarios.

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