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psycopg2 is the most widely used PostgreSQL adapter for Python. It implements the DB-API 2.0 specification and gives you a clean, Pythonic interface to open connections, execute SQL, and retrieve results. Whether you are building a small script or a production-grade API, psycopg2 is the de-facto standard for talking to PostgreSQL from Python.

​
Installing psycopg2

Install the package from PyPI before writing any connection code: 
pip install psycopg2
If you run into binary dependency errors on Linux, install the standalone binary wheel instead: 
pip install psycopg2-binary

​
Creating a Connection

Use psycopg2.connect() and pass your database credentials as keyword arguments. The example below uses the test_bd database running on localhost:
connection.py
import psycopg2  # Esto es para poder conectarnos a Postgre

conexion = psycopg2.connect(
    user='postgres',
    password='admin',
    host='127.0.0.1',
    port='5432',
    database='test_bd'
)
Never hardcode passwords in production code. Store sensitive credentials in environment variables and read them with os.getenv('DB_PASSWORD'). This keeps secrets out of version control and makes your code portable across environments.

​
Creating a Cursor and Executing a SELECT Query

Once you have a connection object, create a cursor — the object you use to send SQL statements to the database and retrieve results:
cursor_fetchall.py
import psycopg2

conexion = psycopg2.connect(
    user='postgres',
    password='admin',
    host='127.0.0.1',
    port='5432',
    database='test_bd'
)

cursor = conexion.cursor()
sentencia = 'SELECT * FROM persona'
cursor.execute(sentencia)       # Execute the statement
registros = cursor.fetchall()   # Retrieve all rows as a list of tuples
print(registros)

cursor.close()
conexion.close()
cursor.execute() sends the SQL to the server. The results are not automatically downloaded — you must call a fetch method to pull them into Python.

​
fetchone() vs fetchall()

psycopg2 offers two primary fetch methods. Choose the one that fits the expected result size: 
import psycopg2

conexion = psycopg2.connect(
    user='postgres',
    password='admin',
    host='127.0.0.1',
    port='5432',
    database='test_bd'
)
try:
    with conexion:
        with conexion.cursor() as cursor:
            sentencia = 'SELECT * FROM persona WHERE id_persona = %s'  # Placeholder
            id_persona = input('Digite un numero para el id_persona: ')
            cursor.execute(sentencia, (id_persona,))
            registros = cursor.fetchone()  # Returns a single tuple, or None
            print(registros)
except Exception as e:
    print(f'Ocurrió un error: {e}')
finally:
    conexion.close()

# https://www.psycopg.org/docs/usage.html
MethodReturnsBest used when
fetchone()A single tuple, or NoneYou expect exactly one row (e.g., lookup by primary key)
fetchall()A list of tuplesYou need all matching rows at once

​
Closing the Connection

Always release the connection and cursor when you are done. There are two ways to do this: 
import psycopg2

conexion = psycopg2.connect(
    user='postgres',
    password='admin',
    host='127.0.0.1',
    port='5432',
    database='test_bd'
)

cursor = conexion.cursor()
sentencia = 'SELECT * FROM persona'
cursor.execute(sentencia)
registros = cursor.fetchall()
print(registros)

cursor.close()    # Release the cursor first
conexion.close()  # Then release the connection
Using the with statement as a context manager on the cursor ensures it is closed automatically when the block exits, even if an exception is raised. The outer try / finally guarantees the connection is always closed.
Note that with conexion: (the connection as a context manager) controls the transaction, not the connection lifetime. You still need to call conexion.close() explicitly in the finally block to return the connection to the operating system. See the psycopg2 usage docs for full details.

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