Psycopg ppygis select query

问题

I'm trying to setup a basic working postgis setup with python ppygis package.

>>> import psycopg2
>>> import ppygis
>>> connection = psycopg2.connect(database='spre', user='postgres')
>>> cursor = connection.cursor()
>>> cursor.execute('CREATE TABLE test (geometry GEOMETRY)')
>>> cursor.execute('INSERT INTO test VALUES(%s)', (ppygis.Point(1.0, 2.0),))
>>> cursor.execute('SELECT * from test')
>>> point = cursor.fetchone()[0]
>>> print point
0101000000000000000000F03F0000000000000040
>>>

I should have got a python object with separate X and Y coordinate. Something like

>>> Point(X: 1.0, Y: 2.0)

What am I doing wrong?

回答1:

You are doing nothing wrong. Following the same steps as the PPyGIS basic example, I get the same hex-encoded EWKB as shown in the question (010100...), which is normally expected. Perhaps this worked with an older version of PPyGIS / Psycopg? It doesn't today.

The package does not appear to properly register itself as an adapter for PostGIS types, so my advice is to not use the package. Besides, you don't need additional packages to use PostGIS from Psycopg2.

Here is the normal approach to read/write points, without any extra packages:

# Assuming PostGIS 2.x, use a typmod
cursor.execute('CREATE TEMP TABLE test (geom geometry(PointZ,4326));')
# Longyearbyen, 78.22°N 15.65°E, altitude 10 m
cursor.execute('''\
    INSERT INTO test (geom)
    VALUES(ST_SetSRID(ST_MakePoint(%s, %s, %s), 4326));
''', (15.65, 78.22, 10.0))
cursor.execute('''\
    SELECT ST_Y(geom) AS latitude, ST_X(geom) AS longitude, ST_Z(geom) AS altitude
    FROM test;
''')
print(cursor.fetchone())  # (78.22, 15.65, 10.0)
cursor.execute('SELECT ST_AsText(geom) FROM test;')
print(cursor.fetchone()[0])  # POINT Z (15.65 78.22 10)
cursor.execute('SELECT ST_AsLatLonText(geom) FROM test;')
print(cursor.fetchone()[0])  # 78°13'12.000"N 15°39'0.000"E

If you want a geometry object on the client-side to do more work with the actual geometry, consider using Shapely, which can be interfaced using WKB data:

from shapely.wkb import loads
cursor.execute('SELECT geom FROM test;')
pt = loads(cursor.fetchone()[0], hex=True)
print(pt)  # POINT Z (15.65 78.22 10)

来源：https://stackoverflow.com/questions/28941640/psycopg-ppygis-select-query

标签

python-2.7

PostGIS

psycopg2

postgresql-9.3