Python sqlite3 misinterpreting parameter

Question

I'm trying to put a column name into a SQL query thusly

import sqlite3
db = sqlite3.connect('path/to/my/database.sqlite')
cur = db.cursor()
def get_val(field):
 return cur.execute('''
 SELECT x, y, :field FROM literal_table
 WHERE 
 :valid_field_name > 0 AND
 (some other conditions)
 GROUP BY x, y''',
 {'field': field}).fetchall()
get_val('valid_field_name')

but when I execute the code, instead of grabbing the value from the column, python is returning

[(x1, y1, u'valid_field_name'), (x2, y3, u'valid_field_name'), ...]

I know that you can't encode table names, but there are examples in the docs of column names being parametrized. I've hacked together a solution with string formatting (the code's for private data processing, so SQL injection isn't a problem) but I need to understand what's going on!

I'm using Python 2.7.3

• MGP: I don't see the problem, the code is doing what is supposed to do. Be aware that you are returning the result of cur.execute sql query, and that is a tuple.
• CL.: What examples in the docs?
• Nat Knight: It would appear I read the docs too quickly; select ? is present, but not select ? from ....
• Nat Knight: Also, thanks @ManuelGutierrez. I've added .fetchall() and edited the question.

Answer 1

SQLite replaces parameters with the supplied values. In your case, this would be the same as:

SELECT x, y, 'valid_field_name' FROM literal_table
WHERE 'valid_field_name' > 0 AND (some other conditions)
GROUP BY x, y

In other words, strings are strings; it is not possible to have parameters as column names.