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118 lines
4.0 KiB
Python
118 lines
4.0 KiB
Python
"""
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Using a tuple as a bound variable in "SELECT ... IN (...)" clauses
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in PostgreSQL using psycopg 2
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Some time ago someone asked on the psycopg mailing list how to have a
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bound variable expand to the right SQL for an SELECT IN clause:
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SELECT * FROM atable WHERE afield IN (value1, value2, value3)
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with the values to be used in the IN clause to be passed to the cursor
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.execute() method in a tuple as a bound variable, i.e.:
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in_values = ("value1", "value2", "value3")
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curs.execute("SELECT ... IN %s", (in_values,))
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psycopg 1 does support typecasting from Python to PostgreSQL (and back)
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only for simple types and this problem has no elegant solution (short or
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writing a wrapper class returning the pre-quoted text in an __str__
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method.
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But psycopg 2 offers a simple and elegant solution by partially
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implementing the Object Adaptation from PEP 246. psycopg 2 (still in
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beta and currently labeled as 1.99.9) moves the type-casting logic into
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external adapters and a somehow broken adapt() function.
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While the original adapt() takes 3 arguments, psycopg's one only takes
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1: the bound variable to be adapted. The result is an object supporting
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a not-yet well defined protocol that we can call IPsycopgSQLQuote:
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class IPsycopgSQLQuote:
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def getquoted(self):
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"Returns a quoted string representing the bound variable."
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def getbinary(self):
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"Returns a binary quoted string representing the bound variable."
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def getbuffer(self):
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"Returns the wrapped object itself."
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__str__ = getquoted
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Then one of the functions (usually .getquoted()) is called by psycopg at
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the right time to obtain the right, sql-quoted representation for the
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corresponding bound variable.
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The nice part is that the default, built-in adapters, derived from
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psycopg 1 tyecasting code can be overridden by the programmer, simply
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replacing them in the psycopg.extensions.adapters dictionary.
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Then the solution to the original problem is now obvious: write an
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adapter that adapts tuple objects into the right SQL string, by calling
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recursively adapt() on each element.
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Note: psycopg 2 adapter code is still very young and will probably move
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to a more 'standard' (3 arguments) implementation for the adapt()
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function; as long as that does not slow down too much query execution.
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Psycopg 2 development can be tracked on the psycopg mailing list:
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http://lists.initd.org/mailman/listinfo/psycopg
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and on the psycopg 2 wiki:
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http://wiki.initd.org/Projects/Psycopg2
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"""
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import psycopg
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import psycopg.extensions
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from psycopg.extensions import adapt as psycoadapt
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class AsIs(object):
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"""An adapter that just return the object 'as is'.
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psycopg 1.99.9 has some optimizations that make impossible to call
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adapt() without adding some basic adapters externally. This limitation
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will be lifted in a future release.
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"""
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def __init__(self, obj):
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self.__obj = obj
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def getquoted(self):
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return self.__obj
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class SQL_IN(object):
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"""Adapt a tuple to an SQL quotable object."""
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def __init__(self, seq):
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self._seq = seq
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def getquoted(self):
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# this is the important line: note how every object in the
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# list is adapted and then how getquoted() is called on it
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qobjs = [str(psycoadapt(o).getquoted()) for o in self._seq]
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return '(' + ', '.join(qobjs) + ')'
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__str__ = getquoted
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# add our new adapter class to psycopg list of adapters
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psycopg.extensions.adapters[tuple] = SQL_IN
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psycopg.extensions.adapters[float] = AsIs
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psycopg.extensions.adapters[int] = AsIs
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# usually we would call:
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#
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# conn = psycopg.connect("...")
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# curs = conn.cursor()
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# curs.execute("SELECT ...", (("this", "is", "the", "tuple"),))
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#
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# but we have no connection to a database right now, so we just check
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# the SQL_IN class by calling psycopg's adapt() directly:
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if __name__ == '__main__':
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print "Note how the string will be SQL-quoted, but the number will not:"
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print psycoadapt(("this is an 'sql quoted' str\\ing", 1, 2.0))
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