How to use variables in SQL statement in Python?

Question

I have the following Python code:

cursor.execute("INSERT INTO table VALUES var1, var2, var3,")

where var1 is an integer, var2 and var3 are strings.

How can I write the variable names without Python including them as part of the query text?

Answer 1

cursor.execute("INSERT INTO table VALUES (%s, %s, %s)", (var1, var2, var3))

Note that the parameters are passed as a tuple.

The database API does proper escaping and quoting of variables. Be careful not to use the string formatting operator (%), because

1. it does not do any escaping or quoting.
2. it is prone to Uncontrolled string format attacks e.g. SQL injection.

Answer 2

Different implementations of the Python DB-API are allowed to use different placeholders, so you'll need to find out which one you're using -- it could be (e.g. with MySQLdb):

cursor.execute("INSERT INTO table VALUES (%s, %s, %s)", (var1, var2, var3))

or (e.g. with sqlite3 from the Python standard library):

cursor.execute("INSERT INTO table VALUES (?, ?, ?)", (var1, var2, var3))

or others yet (after VALUES you could have (:1, :2, :3) , or "named styles" (:fee, :fie, :fo) or (%(fee)s, %(fie)s, %(fo)s) where you pass a dict instead of a map as the second argument to execute). Check the paramstyle string constant in the DB API module you're using, and look for paramstyle at http://www.python.org/dev/peps/pep-0249/ to see what all the parameter-passing styles are!

Answer 3

Many ways. DON'T use the most obvious one (%s with %) in real code, it's open to attacks.

Here copy-paste'd from pydoc of sqlite3:

# Never do this -- insecure!
symbol = 'RHAT'
c.execute("SELECT * FROM stocks WHERE symbol = '%s'" % symbol)

# Do this instead
t = ('RHAT',)
c.execute('SELECT * FROM stocks WHERE symbol=?', t)
print c.fetchone()

# Larger example that inserts many records at a time
purchases = [('2006-03-28', 'BUY', 'IBM', 1000, 45.00),
             ('2006-04-05', 'BUY', 'MSFT', 1000, 72.00),
             ('2006-04-06', 'SELL', 'IBM', 500, 53.00),
            ]
c.executemany('INSERT INTO stocks VALUES (?,?,?,?,?)', purchases)

More examples if you need:

# Multiple values single statement/execution
c.execute('SELECT * FROM stocks WHERE symbol=? OR symbol=?', ('RHAT', 'MSO'))
print c.fetchall()
c.execute('SELECT * FROM stocks WHERE symbol IN (?, ?)', ('RHAT', 'MSO'))
print c.fetchall()
# This also works, though ones above are better as a habit as it's inline with syntax of executemany().. but your choice.
c.execute('SELECT * FROM stocks WHERE symbol=? OR symbol=?', 'RHAT', 'MSO')
print c.fetchall()
# Insert a single item
c.execute('INSERT INTO stocks VALUES (?,?,?,?,?)', ('2006-03-28', 'BUY', 'IBM', 1000, 45.00))

Answer 4

http://www.amk.ca/python/writing/DB-API.html

Be careful when you simply append values of variables to your statements: Imagine a user naming himself ';DROP TABLE Users;' -- That's why you need to use SQL escaping, which Python provides for you when you use cursor.execute in a decent manner. Example in the URL is:

cursor.execute("insert into Attendees values (?, ?, ?)", (name, seminar, paid))

Answer 5

The syntax for providing a single value can be confusing for inexperienced Python users.

Given the query

INSERT INTO mytable (fruit) VALUES (%s)

Generally*, the value passed to cursor.execute must wrapped in a sequence such as a tuple or list even though the value itself is a singleton, so we must provide a single element tuple, like this: (value,).

cursor.execute("""INSERT INTO mytable (fruit) VALUES (%s)""", ('apple',))

Passing a single string

cursor.execute("""INSERT INTO mytable (fruit) VALUES (%s)""", ('apple'))

will result in an error which varies by the DB-API connector, for example

• psycopg2:

  TypeError: not all arguments converted during string formatting

• sqlite3

  sqlite3.ProgrammingError: Incorrect number of bindings supplied. The current statement uses 1, and there are 5 supplied

• mysql.connector

  mysql.connector.errors.ProgrammingError: 1064 (42000): You have an error in your SQL syntax;

---

* The pymysql connector handles a single string parameter without erroring. However it's better to wrap the string in a tuple even if it's a single because

• you won't need to change the code if you switch connector package
• you keep a consistent mental model of the query parameters being a sequence of objects rather than a single object.