Psycopg2 - AttributeError: 'NoneType' object has no attribute 'fetchall'

Question

I have a Python script to list PostgreSQL schemas using psycopg2.

#!/usr/bin/env python

import yaml
import psycopg2

def load_config(file_name):
    with open(file_name, 'r') as stream:
        config = yaml.load(stream)
    return config

config = load_config('config.yml')['database']
conn = psycopg2.connect(host=config['host'], port=config['port'], dbname=config['name'], user=config['user'], password=config['password'])
cursor = conn.cursor()

print('conn = %s' % conn)
print('cursor = %s' % cursor)

sql_list_schemas = """
SELECT *
FROM information_schema.schemata
WHERE schema_name <> 'information_schema'
  AND schema_name !~ E'^pg_';
"""
list_schemas = cursor.execute(sql_list_schemas)
print('list_schemas = %s' % list_schemas)
print('list_schemas.fetchall() = %s' % list_schemas.fetchall())

When I run the script, I got:

conn = <connection object at 0x7f0e12eef050; dsn: 'user=test password=xxxxxxxxxxxxx host=127.0.0.1 port=5432 dbname=test', closed: 0>
cursor = <cursor object at 0x7f0e1326c148; closed: 0>
list_schemas = None
Traceback (most recent call last):
  File "./postgres_db_schema.py", line 26, in <module>
    print('list_schemas.fetchall() = %s' % list_schemas.fetchall())
AttributeError: 'NoneType' object has no attribute 'fetchall'

I thought the SQL query is OK. When I execute the query using another PostgreSQL client, it returned some rows. Why did the cursor.execute return None? Is there something wrong in my script?

Answer 1

.execute() just executes the query and does not return anything. It is up to you how you are going to fetch the results (ex: iterator, fetchall(), fetchone() etc.)

>>> cursor.execute(sql_list_schemas)
>>> list_schemas = cursor.fetchall()

--

Similarly,

>>> cursor.execute(sql_list_schemas)

>>> first_row = cursor.fetchone()
>>> second_row = cursor.fetchone()

>>> remaining_rows = cursor.fetchall()

--

PEP-2049 states that return values are not defined for .execute() method so database connectors may return something different than None. (ex: a boolean flag indicating whether sql command has been successfully executed or not)

However, one thing for sure, cursor.execute() never returns the result set.