I am pleased to announce that the pygrass library, developed during the Google Summer of Code 2012, is now on GRASS trunk and need to be tested.
Before to continue the announcement, if you prefer the html version of this message please see: https://gist.github.com/gists/3864411
The pygrass library has been developed take into account three main aspects, in the order:
- simplicity, GIS it's an already complex subject, and we want to make the API as simple/natural/intuitive as possible but at the same time trying to respect the GRASS work-flow and nomenclature to make it easier to find information on the GRASS developer manual;
- flexibility even if pygrass want to provide an high level map interaction, internally used some classes that work with the GRASS API at very low level, such as Segment, Bbox, Ilist classes. Every time that a C struct has been used by a class, to leave the user free to implement their own logic, the ctypes pointer of the C strct it's available under the attribute name that start with "c*";
- performance pygrass makes heavy use of the C GRASS functions every time that this is possible.
The pygrass library is divided in three main parts: Raster, Vector, and Modules. Below some small samples, of an interactive python shell, are reported to better understand how users and developers could interact with the pygrass library. All the following examples use the maps present in the North Carolina mapset.
All the raster classes read and write meta-data like: categories and history. The pygrass interface for raster maps is divided in 4 classes that represent different ways to interact with rasters. In order to give greater freedom of implementation to users and developers, each class uses a different C API, providing the tools to fit different needs.
The RasterRow class reads the content of the raster row by row and writes it in a sequential mode: row after row. Read and write the same map at the same time is not supported by the RasterRow class.
The RasterRowIO class implements a row cache that allows users to read and re-read raster rows randomly.
The RasterSegment class divides the map into several tiles, each tile is saved into a file.With this class it is possible to read and write the pixel value randomly at the same time in the same map.
The RasterNumpy class inherits from a numpy.memmap class and allows users to interact with the map as numpy matrix.
All the Raster classes shared common methods to open, read and get raster information.
Do simple operation with the RasterRow class: :
>>> from grass.pygrass.raster import RasterRow
>>> elev = RasterRow('elevation')
>>> elev.exist()
True
>>> elev.name # get the raster name
'elevation'
>>> elev.mapset
'PERMANENT'Now open the raster in a read mode, get raster information and read the rows: :
>>> elev.open('r') # open in read mode
>>> elev.is_open()
True
>>> elev.range
(55.578792572021484, 156.32986450195312)
>>> for row in elev[:5]: # show the first 5 rows
... print(row[:3]) # show the first 3 columns of each row
...
[ 141.99613953 141.27848816 141.37904358]
[ 142.90461731 142.39450073 142.68611145]
[ 143.81854248 143.54707336 143.83972168]
[ 144.56524658 144.58493042 144.86477661]
[ 144.99488831 145.22894287 145.57142639]
>>> for row in elev[:5]:
... print(row[:3] < 144)
...
[1 1 1]
[1 1 1]
[1 1 1]
[0 0 0]
[0 0 0]Open a new raster map, save, rename and remove: :
>>> new = RasterRow('new')
>>> new.exist()
False
>>> new.open('w', 'CELL') # open a new CELL map in write mode
>>> for row in elev:
... new.put_row(row < 144) # write the boolean row
...
>>> new.close()
>>> new.mapset
'user1'
>>> new.name
'new'
>>> new.name = 'new_name' # rename the map changing the attribute value
>>> new.name
'new_name'
>>> new.exist()
True
>>> new.remove() # remove the map
>>> new.exist()
False
>>> elev.close()Almost the same operations are available for the other Raster classes, please see the documentation: http://www.ing.unitn.it/~zambelli/projects/pygrass/raster.html
The pygrass interface defines two classes to work with vector maps, the first class Vector loads a vector map without the GRASS topology, while the VectorTopo class supports the topology. The pygrass library implements the geometry features: Point, Line, Boundary, Area, Isle, each class has its method to return vector information or to compute its properties such as the length of a line, the distance between two points, or between a point and a line, etc.
Here just small samples to better understand how it is now possible to interact with the GRASS vectors and features are reported: :
>>> from grass.pygrass.vector import VectorTopo
>>> municip = VectorTopo('boundary_municp_sqlite')
>>> municip.open()
>>> municip.number_of("areas")
3579
>>> municip.number_of("islands")
2629
>>> municip.number_of('pizzas') # suggest the right vector type if wrong
Traceback (most recent call last):
...
ValueError: vtype not supported, use one of: 'areas', 'dblinks', 'faces',
'holes', 'islands', 'kernels', 'line_points', 'lines', 'nodes', 'updated_lines',
'updated_nodes', 'volumes'Suppose that we want to select all and only the areas that have a surface bigger than 10000 m2: :
>>> big = [area for area in municip.viter('areas')
... if area.alive() and area.area() >= 10000]Then it is possible to sort the areas, with: :
>>> from operator import methodcaller as method
>>> big.sort(key = method('area'), reverse = True) # sort the list
>>> for area in big[:3]:
... print area, area.area()
...
Area(3102) 697521857.848
Area(2682) 320224369.66
Area(2552) 298356117.948The pygrass library implements classes to access the Table attributes, to manages the database connection, to make queries, to add and remove columns.
Read and write the Link connection with the database of a vector map: :
>>> municip.dblinks
DBlinks([[Link(1, boundary_municp, sqlite)]])
>>> link = municip.dblinks[1]
>>> link.number
1
>>> link.name
'boundary_municp'
>>> link.table_name
'boundary_municp_sqlite'
>>> link.driver
'sqlite'
>>> link.database[-30:]
'north_carolina/user1/sqlite.db'
>>> link.key
'cat'
>>> link.name = 'boundary'
>>> link.driver = 'pg'
>>> link.database = 'host=localhost,dbname=grassdb'
>>> link.key = 'gid'From the Link object it is possible to instantiate a Table object where the user could make simple query with the Filters object: :
>>> table = link.table()
>>> table.filters.select('cat', 'COUNTY', 'AREA','PERIMETER').order_by('AREA').limit(3)
Filters('SELECT cat, COUNTY, AREA, PERIMETER FROM boundary_municp_sqlite ORDER BY AREA LIMIT 3;')
>>> cur = table.execute()
>>> for row in cur.fetchall():
... print repr(row)
... # cat, COUNTY, AREA, PERIMETER
(1, u'SURRY', 0.0, 1415.331)
(2, u'SURRY', 0.0, 48286.011)
(3, u'CASWELL', 0.0, 5750.087)In this way it is possible to work with table properties like name/rename the table and add/remove/rename/cast the columns of the table: :
>>> table.name
'boundary_municp_sqlite'
>>> table.columns
Columns([(u'cat', u'integer'), ..., (u'ACRES', u'double precision')])
>>> table.columns.names()
[u'cat', u'OBJECTID', u'AREA', u'PERIMETER', ..., u'ACRES']
>>> table.columns.types()
[u'integer', u'integer', u'double precision', ..., u'double precision']
>>> table.columns.add('n_pizza', 'int4')
>>> table.columns.names()[-1]
u'n_pizzas'
>>> table.columns.rename(u'n_pizzas', u'n_pizzas_per_person')
>>> table.columns.names()[-1]
u'n_pizzas_per_person'
>>> table.columns.cast(u'n_pizzas_per_person', 'float8')
>>> table.columns.items()[-1]
(u'n_pizzas_per_person', u'float8')
>>> table.columns.drop(u'n_pizzas_per_person')For more examples with the Vector class, please see the documentation: http://www.ing.unitn.it/~zambelli/projects/pygrass/vector.html
The pygrass Module class is compatible with the grass.run_command syntax. :
>>> from grass.pygrass.modules import Module
>>> slope_aspect = Module("r.slope.aspect", elevation='elevation',
... slope='slp', aspect='asp',
... format='percent', overwrite=True)But it is possible to create a run able module object, change some attributes and run later: :
>>> slope_aspect = Module("r.slope.aspect", elevation='elev',
... slope='slp', aspect='asp',
... format='percent', overwrite=True, run_=False)
>>> slope_aspect.inputs['elevation']
Parameter <elev> (required:yes, type:raster, multiple:no)
>>> slope_aspect.inputs["elevation"].value = "elevation"
>>> slope_aspect.inputs["format"]
Parameter <format> (required:no, type:string, multiple:no)
>>> print slope_aspect.inputs["format"].__doc__ # get help for the input parameter
format: 'degrees', optional, string
Format for reporting the slope
Values: 'degrees', 'percent'
>>> slope_aspect.inputs["format"].value = 'percents' # manage and check the errors
Traceback (most recent call last):
...
ValueError: The Parameter <format>, must be one of: ['degrees', 'percent']
>>> slope_aspect.inputs["format"].value = 'percent'
>>> slope_aspect.run()Or it is possible to initialize a module, and give the parameters later, like a python function: :
>>> slope_aspect = Module("r.slope.aspect")
>>> slope_aspect(elevation='elevation', slope='slp', aspect='asp',
... format='percent', overwrite=True)Moreover the pygrass Module allows the user to run GRASS modules in a different process and to manage (wait/kill/terminate) the process, and to manage/record the output of the standard error and the standard output, in a way that is easier than using the actual Python API of GRASS. :
>>> slope_aspect = Module('r.slope.aspect')
>>> slope_aspect(elevation='elevation', slope='slp', aspect='asp',
... overwrite=True, finish_=False)
>>> slope_aspect.popen.wait() # *.kill(), *.terminate()
0
>>> out, err = slope_aspect.popen.communicate()
>>> print err
100%
Aspect raster map <asp> complete
Slope raster map <slp> completeFor more examples with the Module class, please see the documentation: http://www.ing.unitn.it/~zambelli/projects/pygrass/modules.html