SQLite parameter substitution problem

Using SQLite3 with Python 2.5, I'm trying to iterate through a list and pull the weight of an item from the database based on the item's name.

I tried using the "?" parameter substitution suggested to prevent SQL injections but it doesn't work. For example, when I use:

for item in self.inventory_names:
    self.cursor.execute("SELECT weight FROM Equipment WHERE name = ?", item)
    self.cursor.close()

I get the error:

sqlite3.ProgrammingError: Incorrect number of bindings supplied. The current statement uses 1, and there are 8 supplied.

I believe this is somehow caused by the initial creation of the database; the module I made that actually creates the DB does have 8 bindings.

cursor.execute("""CREATE TABLE Equipment 
    (id INTEGER PRIMARY KEY, 
    name TEXT,
    price INTEGER, 
    weight REAL, 
    info TEXT, 
    ammo_cap INTEGER, 
    availability_west TEXT,
    availability_east TEXT)""")

However, when I use the less-secure "%s" substitution for each item name, it works just fine. Like so:

for item in self.inventory_names:
    self.cursor.execute("SELECT weight FROM Equipment WHERE name = '%s'" % item)
    self.cursor.close()

I can't figure out why it thinks I have 8 bindins when I'm only calling one. How can I fix it?

Solution 1:

The Cursor.execute() method expects a sequence as second parameter. You are supplying a string which happens to be 8 characters long.

Use the following form instead:

self.cursor.execute("SELECT weight FROM Equipment WHERE name = ?", [item])

Python library reference: sqlite3 Cursor Objects.

Solution 2:

I have spent half a day trying to figure out why something like this would give me an error:

cursor.execute("SELECT * from ? WHERE name = ?", (table_name, name))

only to find out that table names cannot be parametrized. Hope this will help other people save some time.

Solution 3:

The argument of cursor.execute that represents the values you need inserted in the database should be a tuple (sequence). However consider this example and see what's happening:

>>> ('jason')
'jason'

>>> ('jason',)
('jason',)

The first example evaluates to a string instead; so the correct way of representing single valued tuple is as in the second evaluation. Anyhow, the code below to fix your error.

self.cursor.execute("SELECT weight FROM Equipment WHERE name = ?", (item,))

Also giving the cursor.execute value arguments as strings,(which is what you are doing) results in the first evaluation in the example and results into the error you are getting.

Solution 4:

The sqlite3 module supports two kinds of placeholders for parameters:

qmark style

Use one or more ? to mark the position of each parameter, and supply a list or tuple of parameters. E.g.:

curs.execute("SELECT weight FROM Equipment WHERE name = ? AND price = ?",
             ['lead', 24])

named style

Use :par placeholders for each named parameter, and supply a dict. E.g.:

curs.execute("SELECT weight FROM Equipment WHERE name = :name AND price = :price",
             {name: 'lead', price: 24})

Advantages of named style parameters is that you don't need to worry about the order of parameters, and each :par can be used multiple times in large/complex SQL queries.