Python sqlite3: UPDATE failure claims "no such column"

The following script evokes the following exception in the UPDATE command. What I think should happen for this simple UPDATE command is that the value of db_2.foo.bar should be doubled from 1 to 2. My guess is that I have some subtle syntax error in the UPDATE statement (or else a bug at the Python or sqlite3 level); however, I've pored over the sqlite3 documentation -- especially the "UPDATE" and "expression" pages -- and don't see that I'm doing anything wrong.

db_1.foo_bar= (1,)
Traceback (most recent call last):
  File "try2.py", line 29, in <module>
    db_2.execute( 'UPDATE foo SET bar = bar + db_1.foo.bar WHERE rowid = db_1.foo.rowid' ) 
sqlite3.OperationalError: no such column: db_1.foo.bar

Any suggestions or workarounds?

import sqlite3

# Create db_1, populate it and close it:
open( 'db_1.sqlite', 'w+' )
db_1 =  sqlite3.connect( 'db_1.sqlite' )
db_1.execute( 'CREATE TABLE foo(bar INTEGER)' )
db_1.execute( 'INSERT INTO foo (bar) VALUES (1)' )
db_1.commit()
db_1.close()

# Create db_2:
open( 'db_2.sqlite', 'w+' )
db_2 =  sqlite3.connect( 'db_2.sqlite' )
db_2.execute( 'CREATE TABLE foo(bar INTEGER)' )

# Attach db_1 to db_2 connection:
db_2.execute( 'ATTACH "db_1.sqlite" AS db_1' )

# Populate db_2 from db_1:
db_2.execute( 'INSERT INTO foo SELECT ALL * FROM db_1.foo' )

# Show that db_1.foo.bar exists:
cur_2 = db_开发者_运维知识库2.cursor()
cur_2.execute( 'SELECT bar from db_1.foo' )
for result in cur_2.fetchall():
    print 'db_1.foo_bar=', result

# However, the following claims that db_1.foo.bar does not exist:
db_2.execute( 'UPDATE foo SET bar = bar + db_1.foo.bar WHERE rowid = db_1.foo.rowid' ) 

db_2.execute( 'DETACH db_1')
db_2.commit()
db_2.close()

To update foo with values from a different table, you can use a nested SELECT expression. Note that foo.rowid refers to the rowid of the outer table, while t.rowid refers to the rowid of the inner table:

cur_2.execute( '''\
    UPDATE foo SET bar = bar +
        IFNULL( (SELECT t.bar 
                 FROM db_1.foo AS t
                 WHERE foo.rowid = t.rowid), 0)''' )  

To test that the proper rowids are indeed being matched together, I modified your code a bit so the rowids of db_1.foo do not match the rowids of db_2.foo:

import sqlite3

# Create db_1, populate it and close it:
open( 'db_1.sqlite', 'w+' )
db_1 =  sqlite3.connect( 'db_1.sqlite' )
db_1.execute( 'CREATE TABLE foo(bar INTEGER)' )
db_1.execute( 'INSERT INTO foo (rowid,bar) VALUES (2,1)' )
db_1.execute( 'INSERT INTO foo (rowid,bar) VALUES (3,2)' )
db_1.commit()
db_1.close()

# Create db_2:
open( 'db_2.sqlite', 'w+' )
db_2 =  sqlite3.connect( 'db_2.sqlite' )
cur_2 = db_2.cursor()
cur_2.execute( 'CREATE TABLE foo(bar INTEGER)' )

# Attach db_1 to db_2 connection:
cur_2.execute( 'ATTACH "db_1.sqlite" AS db_1' )

# Populate db_2 from db_1:
cur_2.execute( 'INSERT INTO foo SELECT * FROM db_1.foo' )

Note the rowids of foo are 1 and 2:

cur_2.execute( 'SELECT rowid,bar from foo' )
for result in cur_2.fetchall():
    print('foo: {0}'.format(result))
    # foo: (1, 1)
    # foo: (2, 2)

Note the rowids of db_1.foo are 2 and 3:

# Show that db_1.foo.bar exists:
cur_2.execute( 'SELECT rowid,bar from db_1.foo' )
for result in cur_2.fetchall():
    print('db_1.foo: {0}'.format(result))
    # db_1.foo: (2, 1)
    # db_1.foo: (3, 2)

cur_2.execute( '''\
    UPDATE foo SET bar = bar +
        IFNULL( (SELECT t.bar 
                 FROM db_1.foo AS t
                 WHERE foo.rowid = t.rowid), 0)''' )  

After the UPDATE, the row with rowid = 1 has not changed, while the row with rowid = 2 has been updated.

cur_2.execute( 'SELECT rowid,bar from foo' )
for result in cur_2.fetchall():
    print('foo after update: {0} '.format(result))
    # foo after update: (1, 1) 
    # foo after update: (2, 3) 

cur_2.execute('DETACH db_1')
db_2.commit()
db_2.close()

I found these pages helpful in constructing this answer: here and here, though any mistakes are of course my own.

Hmmm... the problem may very well be that I am looking at the 3.7.3 documentation, whereas my installed version is 3.6.16. Investigating.