How to lock a sqlite3 database in Python?

The way to explicitly lock the database is start a transaction as explained in the documentation:

When a database is accessed by multiple connections, and one of the processes modifies the database, the SQLite database is locked until that transaction is committed.

One way to initiate a transaction is use the connection as a context manager:

import sqlite3
con = sqlite3.connect(...)
...
with con:
    # Database is locked here

Also note that some transactions happen implictly by default:

By default, the sqlite3 module opens transactions implicitly before a Data Modification Language (DML) statement (i.e. INSERT/UPDATE/DELETE/REPLACE), and commits transactions implicitly before a non-DML, non-query statement (i. e. anything other than SELECT or the aforementioned).

From the sqlite FAQ, "Can multiple applications or multiple instances of the same application access a single database file at the same time?":

Multiple processes can have the same database open at the same time. Multiple processes can be doing a SELECT at the same time. But only one process can be making changes to the database at any moment in time, however.

Whether or not you use the with connection construct, many processes can read from by only one can write to the database at any given time.