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Efficient filtering of SQL tables with generator expressions.

Project description

This module allows you to access a (DB API 2) SQL table using nothing but Python to build the query:

import re
import sqlite3
from simpleql.table import Table

conn = sqlite3.connect(":memory:")
curs = conn.cursor()
curs.execute("CREATE TABLE test (a integer, b char(1))")
curs.executemany("INSERT INTO test (a, b) VALUES (?, ?)", ([1,'a'], [2,'b'], [3,'c']))
conn.commit()

table = Table(conn, "test", verbose=1)
for row in table:
    print row

This will print:

SELECT a, b FROM test;
{'a': 1, 'b': u'a'}
{'a': 2, 'b': u'b'}
{'a': 3, 'b': u'c'}

Note that each row in the table is a dictionary. We can filter this using a generator expression:

aspan = (1, 3)
for row in (t for t in table if min(aspan) < t['a'] < max(aspan)):
    print row

This will print:

SELECT a, b FROM test WHERE (1<a) AND (a<3);
{'a': 2, 'b': u'b'}

As you can see, the query string is built from a generator expression. You can also use list comprehensions. Regular expressions are supported by the use of the re.search method:

filtered = [t for t in table if re.search('a', t['b'])]
print filtered

Which outputs:

SELECT a, b FROM test WHERE b LIKE "%a%";
[{'a': 1, 'b': u'a'}]

Note that since the module has to analyse the source code it doesn’t work on the interactive shell.

The advantage of this approach over the similar recipe is that if the (efficient) query builder fails when it encounters a complex filter the data will still be filtered (unefficiently) by the generator expression.

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