Escape SQL "LIKE" value for Postgres with psycopg2

Does psycopg2 have a function for escaping the value of a LIKE operand for Postgres?

For example I may want to match strings that start with the string "20% of all", so I want to write something like this:

sql = '... WHERE ... LIKE %(myvalue)s'
cursor.fetchall(sql, { 'myvalue': escape_sql_like('20% of all') + '%' }

Is there an existing escape_sql_like function that I could plug in here?

(Similar question to How to quote a string value explicitly (Python DB API/Psycopg2), but I couldn't find an answer there.)

Answers


Yeah, this is a real mess. Both MySQL and PostgreSQL use backslash-escapes for this by default. This is a terrible pain if you're also escaping the string again with backslashes instead of using parameterisation, and it's also incorrect according to ANSI SQL:1992, which says there are by default no extra escape characters on top of normal string escaping, and hence no way to include a literal % or _.

I would presume the simple backslash-replace method also goes wrong if you turn off the backslash-escapes (which are themselves non-compliant with ANSI SQL), using NO_BACKSLASH_ESCAPE sql_mode in MySQL or standard_conforming_strings conf in PostgreSQL (which the PostgreSQL devs have been threatening to do for a couple of versions now).

The only real solution is to use the little-known LIKE...ESCAPE syntax to specify an explicit escape character for the LIKE-pattern. This gets used instead of the backslash-escape in MySQL and PostgreSQL, making them conform to what everyone else does and giving a guaranteed way to include the out-of-band characters. For example with the = sign as an escape:

# look for term anywhere within title
term= term.replace('=', '==').replace('%', '=%').replace('_', '=_')
sql= "SELECT * FROM things WHERE description LIKE %(like)s ESCAPE '='"
cursor.execute(sql, dict(like= '%'+term+'%'))

This works on PostgreSQL, MySQL, and ANSI SQL-compliant databases (modulo the paramstyle of course which changes on different db modules).

There may still be a problem with MS SQL Server/Sybase, which apparently also allows [a-z]-style character groups in LIKE expressions. In this case you would want to also escape the literal [ character with .replace('[', '=['). However according to ANSI SQL escaping a character that doesn't need escaping is invalid! (Argh!) So though it will probably still work across real DBMSs, you'd still not be ANSI-compliant. sigh...


You can also look at this problem from a different angle. What do you want? You want a query that for any string argument executes a LIKE by appending a '%' to the argument. A nice way to express that, without resorting to functions and psycopg2 extensions could be:

sql = "... WHERE ... LIKE %(myvalue)s||'%'"
cursor.execute(sql, { 'myvalue': '20% of all'})

Instead of escaping the percent character, you could instead make use of PostgreSQL's regex implementation.

For example, the following query against the system catalogs will provide a list of active queries which are not from the autovacuuming sub-system:

SELECT procpid, current_query FROM pg_stat_activity
WHERE (CURRENT_TIMESTAMP - query_start) >= '%s minute'::interval
AND current_query !~ '^autovacuum' ORDER BY (CURRENT_TIMESTAMP - query_start) DESC;

Since this query syntax doesn't utilize the 'LIKE' keyword, you're able to do what you want... and not muddy the waters with respect to python and psycopg2.


I wonder if all of the above is really needed. I am using psycopg2 and was simply able to use:

data_dict['like'] = psycopg2.Binary('%'+ match_string +'%')
cursor.execute("SELECT * FROM some_table WHERE description ILIKE %(like)s;", data_dict)

Having failed to find a built-in function so far, the one I wrote is pretty simple:

def escape_sql_like(s):
    return s.replace('\\', '\\\\').replace('%', '\\%').replace('_', '\\_')

You can create a Like class subclassing str and register an adapter for it to have it converted in the right like syntax (e.g. using the escape_sql_like() you wrote).


I made some modifications to the code above to do the following:

def escape_sql_like(SQL):
    return SQL.replace("'%", 'PERCENTLEFT').replace("%'", 'PERCENTRIGHT')

def reescape_sql_like(SQL):
    return SQL.replace('PERCENTLEFT', "'%").replace('PERCENTRIGHT', "%'")

SQL = "SELECT blah LIKE '%OUCH%' FROM blah_tbl ... "
SQL = escape_sql_like(SQL)
tmpData = (LastDate,)
SQL = cur.mogrify(SQL, tmpData)
SQL = reescape_sql_like(SQL)
cur.execute(SQL)

I found a better hack. Just append '%' to your search query_text.

con, queryset_list = psycopg2.connect(**self.config), None
cur = con.cursor(cursor_factory=RealDictCursor)
query = "SELECT * "
query += " FROM questions WHERE  body LIKE %s OR title LIKE %s  "
query += " ORDER BY questions.created_at"
cur.execute(query, ('%'+self.q+'%', '%'+self.q+'%'))

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