In python's sqlite3 module, will using cursor.fetchone() as the condition for a while loop fetch a row?

Question

I am using python's sqlite3 module.

I want to iterate through the rows in my table until none are left. I want to do this one at a time instead of using fetchall because the table is quite large. I read in the python sqlite3 documentation that curs.fetchone() returns None when no rows are left in the cursor's selection.

So I had the idea to run my code in a while fetchone(): loop, with the 'actual' fetchone() call occuring inside the loop, like so:

while c.fetchone():
    row = c.fetchone()
    print(foo(row))

I can't tell if this loop is actually fetching a row once in the while statement and once in the body, or if it is only in the body. Which one is it?

Answer 1

It will execute fetchone twice so it is not a good idea. Since sqlite3.Cursor is iterable you do something like this:

for row in c:
    print(foo(row))

Alternatively you can use infinite while loop and fetch data in batches using fetchmany:

# Lets take  1000 rows at the time
batch_size = 1000
while True:
    rows = c.fetchmany(batch_size)
    if not rows: break
    for row in rows:
        print(foo(row))