[GRASS-user] Accessing GRASS databases from Python

Joshua Arnott josh at snorfalorpagus.net
Thu Jan 20 13:52:23 EST 2011

Hi grass-users,

I'd like to find out what is the best way of accessing a pre-connected 
database in GRASS from a Python script. I can see a few different 
possible approaches to this, but I'm not sure which to take. Ultimately, 
I want to be able to do two different tasks:

1) Change data for a row, or rows, in one column based on the values of 
other columns in the same row(s). Similar to v.db.update qcolumn=???, 
but I'd like to do the calculations in Python so I have a bit more 
freedom (accessing existing functions, etc).

2) Calculate statistics based on the content of all, or some (based on 
an SQL where statement), of the rows from a column in a table. For (a 
very simplistic) example, calculate the mean from a list of values in a 
'temperature' column.

What is the best way to get this information into a manageable form in 

I'm running GRASS 6.4.1RC1 on OS X, as compiled by: 

If I could guarantee the database would be in a certain format (DBF, 
SQLite, etc) I could just use Pythons respective libraries, but that 
doesn't seem very portable. I've had a look at the database functions in 
the grass.script.db python library, but haven't had any luck getting the 
db_select function to work. In the default install, the function didn't 
even exist in db.py. I've added it, based on the current source, but all 
I get out is:

 >>> grass.script.db.db_select('Population_Density','select * from 
Sorry, <output> is not a valid parameter

Any wisdom would be appreciated.


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