Python sqlite3.Cursor.setoutputsize Method
Last modified April 15, 2025
This guide explores Python's sqlite3.Cursor.setoutputsize method,
which is part of the DB-API 2.0 specification but not implemented in SQLite.
Basic Definitions
The setoutputsize method is defined in Python's DB-API 2.0
specification for database cursors. It's meant to predefine memory allocation.
In SQLite's implementation, this method exists but does nothing. It's provided for API compatibility with other database systems that might use this feature.
Method Signature
The method has the following signature:
setoutputsize(size, column=None). Both parameters are optional.
size specifies the expected output size in bytes. column
can specify a particular column index. The method returns None in all cases.
Basic Usage
This example shows the basic syntax of calling the method, though it has no effect in SQLite.
import sqlite3
with sqlite3.connect(':memory:') as conn:
with conn.cursor() as cur:
# This call does nothing in SQLite
cur.setoutputsize(1024) # 1KB expected output
cur.execute("CREATE TABLE test (data TEXT)")
cur.execute("INSERT INTO test VALUES ('Sample data')")
conn.commit()
cur.execute("SELECT * FROM test")
print(cur.fetchone()) # ('Sample data',)
The setoutputsize call is present but doesn't affect the query
results or memory usage. The method exists for API compatibility only.
With Column Parameter
This example demonstrates using the optional column parameter, which also has no effect in SQLite.
import sqlite3
with sqlite3.connect(':memory:') as conn:
with conn.cursor() as cur:
# Specify output size for column 0
cur.setoutputsize(2048, 0) # No effect
cur.execute("""CREATE TABLE products
(id INTEGER, name TEXT, price REAL)""")
data = [(1, 'Laptop', 999.99),
(2, 'Phone', 699.99)]
cur.executemany("INSERT INTO products VALUES (?, ?, ?)", data)
cur.execute("SELECT * FROM products")
for row in cur:
print(row)
The column index parameter (0 in this case) would specify which column to pre-allocate memory for in databases that support this feature.
In Transaction
This example shows the method being called within a transaction block.
import sqlite3
with sqlite3.connect('transactions.db') as conn:
conn.isolation_level = 'EXCLUSIVE'
with conn:
cur = conn.cursor()
cur.setoutputsize(4096) # Still no effect
cur.execute("CREATE TABLE IF NOT EXISTS logs (message TEXT)")
cur.execute("INSERT INTO logs VALUES ('Transaction started')")
# The transaction commits automatically when exiting 'with' block
Even within an explicit transaction, the method has no effect on SQLite's behavior. The transaction proceeds normally regardless of this call.
With Large Data
This example attempts to use the method with potentially large data.
import sqlite3
with sqlite3.connect(':memory:') as conn:
with conn.cursor() as cur:
# Try to pre-allocate for large BLOB
cur.setoutputsize(10*1024*1024) # 10MB
# Create table with BLOB column
cur.execute("CREATE TABLE blobs (id INTEGER, data BLOB)")
# Insert 5MB of data
data = b'\x01' * 5*1024*1024
cur.execute("INSERT INTO blobs VALUES (1, ?)", (data,))
cur.execute("SELECT data FROM blobs WHERE id = 1")
retrieved = cur.fetchone()[0]
print(f"Retrieved {len(retrieved)//1024}KB of data")
SQLite handles the large BLOB data without needing pre-allocation hints. The method call doesn't affect memory usage or performance in this scenario.
Multiple Calls
This example shows multiple calls to the method with different parameters.
import sqlite3
with sqlite3.connect('example.db') as conn:
with conn.cursor() as cur:
# Multiple calls with different sizes
cur.setoutputsize(1024)
cur.setoutputsize(2048, 0)
cur.setoutputsize(4096, 1)
cur.execute("""CREATE TABLE IF NOT EXISTS measurements
(timestamp TEXT, value REAL, unit TEXT)""")
import time
current_time = time.strftime('%Y-%m-%d %H:%M:%S')
cur.execute("INSERT INTO measurements VALUES (?, ?, ?)",
(current_time, 23.5, 'Celsius'))
cur.execute("SELECT * FROM measurements")
print(cur.fetchall())
Multiple calls to the method are allowed but have no cumulative effect. Each call is effectively ignored by the SQLite driver.
With Custom Row Factory
This example combines the method with a custom row factory.
import sqlite3
def dict_factory(cursor, row):
return {col[0]: row[idx] for idx, col in enumerate(cursor.description)}
with sqlite3.connect(':memory:') as conn:
conn.row_factory = dict_factory
with conn.cursor() as cur:
cur.setoutputsize(512) # No effect on row factory
cur.execute("CREATE TABLE books (title TEXT, author TEXT, year INTEGER)")
cur.execute("INSERT INTO books VALUES (?, ?, ?)",
('Python Cookbook', 'David Beazley', 2013))
cur.execute("SELECT * FROM books")
print(cur.fetchone()) # {'title': 'Python Cookbook', ...}
The custom row factory works normally regardless of the setoutputsize
call. The method doesn't interact with row factory behavior in any way.
Error Cases
This example shows that the method doesn't raise errors with invalid parameters.
import sqlite3
with sqlite3.connect(':memory:') as conn:
with conn.cursor() as cur:
# These calls are all accepted but do nothing
cur.setoutputsize(-100) # Negative size
cur.setoutputsize(0)
cur.setoutputsize(1.5) # Float size
cur.setoutputsize(100, -1) # Invalid column
cur.setoutputsize(100, 1000) # Out of bounds column
cur.execute("SELECT 1")
print(cur.fetchone()) # (1,)
The SQLite implementation accepts any parameters without validation since the method doesn't actually perform any operations. This differs from some other database systems.
Best Practices
- Understand it's a no-op: The method exists for API compatibility only
- Don't rely on it: SQLite manages memory automatically
- Document usage: If used, comment why it's present
- Consider alternatives: For performance, optimize queries instead
- Test thoroughly: If switching databases, behavior may differ
Source References
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