String substitution VS proper mysql escaping

Cameron Simpson cs at zip.com.au
Wed Aug 18 06:31:22 CEST 2010


On 17Aug2010 20:15, Νίκος <nikos.the.gr33k at gmail.com> wrote:
| ===============================
| cursor.execute( ''' SELECT host, hits, date FROM visitors WHERE page =
| '%s' ORDER BY date DESC ''' % (page) )
| ===============================
| 
| Someone told me NOT to do string substitution ("%") on SQL statements
| and to let MySQLdb do it
| for me, with proper escaping like the following
| 
| ===============================
| cursor.execute('''SELECT host, hits, date FROM visitors WHERE page=%s
|         ORDER BY date DESC''', (page,))
| ===============================
| 
| The difference is that if some external source can control "page",
| and
| they put in a value like
|         100 ; DELETE FROM visitors; SELECT * FROM visitors
| i will be losing my database table data.

That other difference is that the mysql dialect support knows how to
correctly escape a string for insertion into an SQL statement. You may
not, or may forget to pre-escape the string, etc. Using the MySQLdb
stuff do it for you is reliable and robust.

| a) I wanted to ask what is proper escaping mean and why after variable
| page syntax has a comma

Because this:

  (page)

means the same thing as:

  page

i.e. the argument to the "%" operator is just the string in page.

This:

  (page,)

is a _tuple_ containing a single element, the page variable.
A bit like:

  [page]

which is a list containing a single element. The trailing comma is
needed to tell python you want to use a tuple, not the bare string.

The "%" operator has special knowledge that is it is passed as string instead
of a list or tuple or other sequence then it should act _as_ _if_ it had been
passed a single element tuple containing the string.

Otherwise, because a string _is_ a sequence the "%" might want to treat
the string "foo" as the sequence:

  ("f", "o", "o")

Run these three loops to see the difference:

  for s in "foo":
    print s
  for s in ("foo"):
    print s
  for s in ("foo",):
    print s

Cheers,
-- 
Cameron Simpson <cs at zip.com.au> DoD#743
http://www.cskk.ezoshosting.com/cs/

I couldn't think of anything else to do with it, so I put it on the web.



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