On Mon, May 21, 2012 at 5:32 AM, Andy Casey <andycasey@gmail.com> wrote:
> Hi,
>
> I'm just re-raising this problem to anyone on the mailing list, because I
> haven't had any luck on StackOverflow, or any suggested answers from the
> mailing list:
>
> http://stackoverflow.com/questions/10529351/using-a-psycopg2-converter-to-retrieve-bytea-data-from-postgresql
Sorry, I forgot to get back on the question. The default bytea
typecaster (which is the object that can parse the postgres binary
representation and return a buffer object out of it) is
psycopg2.BINARY. We can use it to create a typecaster converting to
array instead:
In [1]: import psycopg2
In [2]: import numpy as np
In [3]: a = np.eye(3)
In [4]: a
Out[4]:
array([[ 1., 0., 0.],
[ 0., 1., 0.],
[ 0., 0., 1.]])
In [5]: cnn = psycopg2.connect('')
In [6]: cur = cnn.cursor()
# The adapter: converts from python to postgres
# note: this only works on numpy version whose array support the
buffer protocol,
# e.g. it works on 1.5.1 but not on 1.0.4 on my tests.
In [12]: def adapt_array(a):
....: return psycopg2.Binary(a)
....:
In [13]: psycopg2.extensions.register_adapter(np.ndarray, adapt_array)
# The typecaster: from postgres to python
In [21]: def typecast_array(data, cur):
....: if data is None: return None
....: buf = psycopg2.BINARY(data, cur)
....: return np.frombuffer(buf)
....:
In [24]: ARRAY = psycopg2.extensions.new_type(psycopg2.BINARY.values,
'ARRAY', typecast_array)
In [25]: psycopg2.extensions.register_type(ARRAY)
# Now it works "as expected"
In [26]: cur = cnn.cursor()
In [27]: cur.execute("select %s", (a,))
In [28]: cur.fetchone()[0]
Out[28]: array([ 1., 0., 0., 0., 1., 0., 0., 0., 1.])
As you know, np.frombuffer(a) loses the array shape, so you will have
to figure out a way to preserve it.
Cheers,
-- Daniele
