Understanding and Resolving the “Attempt to Write a Read-Only Database” Error in Python SQLite

The error message “attempt to write a readonly database” is a common issue encountered by many Python developers when working with SQLite databases. In this article, we’ll delve into the causes of this error, explore its implications on performance and database integrity, and provide practical solutions for resolving it.

What Causes the Error?

When you attempt to append data to an existing SQLite database using the to_sql() method from pandas or SQLAlchemy, a “readonly database” error can occur if the database is not properly flushed or committed. This happens because the database’s file descriptor remains open until it’s explicitly closed, which prevents any subsequent write operations.

Why Does the Error Occur?

In your code snippet, you’re using if_exists='append' to append new data to the existing database. However, if you’re not properly committing or flushing the changes after each write operation, the database file descriptor might not be closed until the end of the program execution. This prevents any subsequent write operations from being executed.

Why Does the Database File Descriptor Remain Open?

The to_sql() method returns a connection object that remains open until it’s explicitly closed using the close() or dispose() methods. If you’re using an iterator to read chunks from your CSV file, each chunk operation will return a new connection object, which can lead to multiple connections being opened and left open.

Implications of the Error

The “attempt to write a readonly database” error can have significant implications on performance and database integrity:

• Performance issues: When the database becomes too large or unwieldy, it can become slower and more difficult to maintain.
• Data integrity issues: If data is not properly flushed or committed, it may be lost or corrupted when the program ends.

Solutions for Resolving the Error

To resolve this error and ensure that your Python SQLite database remains readable and writable throughout its entire lifecycle:

1. Flush Changes

When using if_exists='append' to append data to an existing database, make sure to flush changes by explicitly calling the flush() method on the connection object.

disk_engine = create_engine('sqlite:///C:\\databases\\test.db')

start = dt.datetime.now()
chunksize = 100000
j = 0
index_start = 1

for df in pd.read_csv('C:\my_file.txt',sep='\t', error_bad_lines=False, chunksize=chunksize, iterator=True, encoding='ISO-8859-1'):

    # Flush changes before appending new data
    disk_engine.flush()

    df = df.rename(columns={c: c.replace(' ', '') for c in df.columns}) # Remove spaces from columns

    df.index += index_start

    columns = ['column_1']

    for c in df.columns:
     if c not in columns:
        df = df.drop(c, axis=1)    

    j+=1
    print '{} seconds: completed {} rows'.format((dt.datetime.now() - start).seconds, j*chunksize)

    df.to_sql('data', disk_engine, if_exists='append')
    index_start = df.index[-1] + 1

2. Use a Context Manager

If you’re using Python’s with statement to manage the connection object, ensure that the with block is properly closed before attempting any further write operations.

from contextlib import contextmanager

@contextmanager
def managed_connection(engine):
    try:
        conn = engine.connect()
        yield conn
    finally:
        conn.close()

disk_engine = create_engine('sqlite:///C:\\databases\\test.db')

start = dt.datetime.now()
chunksize = 100000
j = 0
index_start = 1

for df in pd.read_csv('C:\my_file.txt',sep='\t', error_bad_lines=False, chunksize=chunksize, iterator=True, encoding='ISO-8859-1'):

    with managed_connection(disk_engine) as conn:
        # Write data to the database
        df.to_sql('data', conn, if_exists='append')

3. Commit Changes Regularly

To avoid the “attempt to write a readonly database” error, make sure to commit changes regularly by explicitly calling the commit() method on the connection object.

disk_engine = create_engine('sqlite:///C:\\databases\\test.db')

start = dt.datetime.now()
chunksize = 100000
j = 0
index_start = 1

for df in pd.read_csv('C:\my_file.txt',sep='\t', error_bad_lines=False, chunksize=chunksize, iterator=True, encoding='ISO-8859-1'):

    # Flush changes and commit before appending new data
    disk_engine.flush()
    disk_engine.commit()

    df = df.rename(columns={c: c.replace(' ', '') for c in df.columns}) # Remove spaces from columns

    df.index += index_start

    columns = ['column_1']

    for c in df.columns:
     if c not in columns:
        df = df.drop(c, axis=1)    

    j+=1
    print '{} seconds: completed {} rows'.format((dt.datetime.now() - start).seconds, j*chunksize)

    df.to_sql('data', disk_engine, if_exists='append')
    index_start = df.index[-1] + 1

By implementing these strategies, you can ensure that your Python SQLite database remains readable and writable throughout its entire lifecycle, avoiding the “attempt to write a readonly database” error.

Last modified on 2024-05-12