Python SQLite SQL length limits: cap statement text separately from bound values

Operation contract

The connection lowers its SQL-text ceiling to 120 bytes. A long owned statement is rejected before execution, while a short parameterized statement succeeds. The program reads back the actual limit because SQLite may clamp a requested value to its compiled maximum.

Failure boundary

This limit does not replace a bound-value size check, a row-count limit, or query time control. The fixture passes owned SQL; received values must still use parameters. The setting belongs to one connection and should be applied to each connection in a pool.

Working program

import sqlite3

connection = sqlite3.connect(":memory:")
try:
    connection.setlimit(sqlite3.SQLITE_LIMIT_SQL_LENGTH, 120)
    print("limit", connection.getlimit(sqlite3.SQLITE_LIMIT_SQL_LENGTH))
    long_statement = "SELECT 47 " + " " * 160
    try:
        connection.execute(long_statement)
    except sqlite3.Error:
        print("long_sql_rejected", True)
    print("bound_value", connection.execute("SELECT ?", (73,)).fetchone()[0])
finally:
    connection.close()

Output

limit 120
long_sql_rejected True
bound_value 73

Costs and limits

Checking statement length is cheap relative to parsing the SQL. It says nothing about a parameter's bytes; add a separate application limit when large values matter.

Common Mistakes

• Do not count bound data as if it were SQL text.
• Do not build received data into SQL merely to put it under this limit.
• A connection limit is not inherited by a newly opened connection.

Connected lessons

• Python SQLite progress handler: interrupt one expensive statement
• Python SQLite authorizer: deny a statement on the connection that executes it
• Python JSON Lines: cap line bytes and validate the whole batch before returning it

Test this contract.