Merge pull request #3 from mundialis/v_example_attributes

add examples for fns in vector.py and db.py

.flake8

@@ -9,4 +9,4 @@ exclude = .git
 max-line-length = 80
 
 per-file-ignores =
-   ./v.example.py: F821
+   ./v.example.py: F821, E501

v.example.py

@@ -35,17 +35,31 @@
 
 # %option
 # % key: box
+# % key_desc: width,height
+# % type: double
 # % required: yes
-# % multiple: yes
+# % multiple: no
 # % description: Width and height of boxes in grid
 # %end
 
-# %option
+# %option G_OPT_V_INPUT
 # % key: polygon_aoi
 # % required: no
 # % description: Polygon to extract grid-tiles for
 # %end
 
+# %option G_OPT_V_FIELD
+# %end
+
+# %option G_OPT_DB_COLUMN
+# %end
+
+# %flag
+# % key: p
+# % description: Print attribute values of given column for input vector polygon_aoi
+# %end
+
+
 # import needed libraries
 import atexit
 import os
@@ -55,6 +69,7 @@
 rm_vec = []
 
 
+# cleanup function (can be extended)
 def cleanup():
     """Cleanup fuction (can be extended)"""
     nuldev = open(os.devnull, "w", encoding="utf-8")
@@ -68,6 +83,94 @@ def main():
     """Main function of v.example"""
     global rm_vec
 
+    # print attribute values if requested
+    if flags["p"] and options["polygon_aoi"] and options["column"]:
+        aoi_vector = options["polygon_aoi"]
+        layer = options["layer"]
+        column = options["column"]
+
+        # examples for interfaces in
+        # python/grass/script/vector.py
+        # python/grass/script/db.py
+
+        # get all database connections as dictionary
+        db_connections = grass.vector_db(aoi_vector)
+        if len(db_connections) == 0:
+            grass.fatal(_("No database connections for map %s") % aoi_vector)
+
+        # check if there is an attribute table at the given layer
+        # grass.vector_layer_db() will exit with fatal error if there is
+        # no database connection for the given layer
+        db_connection = grass.vector_layer_db(aoi_vector, layer)
+
+        # get attribute columns as dictionary (default)
+        columns = grass.vector_columns(aoi_vector, layer, getDict=True)
+        if column not in columns.keys():
+            grass.fatal(
+                _("Column %s does not exist in layer %s of vector %s")
+                % (column, layer, aoi_vector)
+            )
+
+        # get attribute columns as list
+        columns = grass.vector_columns(aoi_vector, layer, getDict=False)
+        colidx = -1
+        try:
+            colidx = columns.index(column)
+        except ValueError:
+            grass.fatal(
+                _("Column %s does not exist in layer %s of vector %s")
+                % (column, layer, aoi_vector)
+            )
+
+        if colidx >= 0:
+            grass.verbose(
+                _("Found column %s in vector %s, layer %s")
+                % (column, aoi_vector, layer)
+            )
+
+        # query the database and table directly using information in the
+        # database connection of the vector
+        database = db_connection["database"]
+        table = db_connection["table"]
+        driver = db_connection["driver"]
+
+        table_description = grass.db_describe(
+            table=table, database=database, driver=driver
+        )
+        found = False
+        # TODO: pythonize this for loop
+        for i in range(len(table_description["cols"])):
+            column_description = table_description["cols"][i]
+            if column_description[0] == column:
+                found = True
+                break
+
+        if found is False:
+            grass.fatal(
+                _("Column %s does not exist in layer %s of vector %s")
+                % (column, layer, aoi_vector)
+            )
+
+        # select attribute values with vector_db_select()
+        grass.message(
+            _("Print attribute values using %s") % "vector_db_select()"
+        )
+        column_values = grass.vector_db_select(
+            aoi_vector, int(layer), columns=column
+        )
+        # go over table rows
+        for key in column_values["values"]:
+            # print value of first selected column
+            print(column_values["values"][key][0])
+
+        # select attribute values with SQL statement
+        grass.message(_("Print attribute values using %s") % "db_select()")
+        sql = f"select {column} from {table}"
+        values = grass.db_select(sql=sql, database=database, driver=driver)
+        for value in values:
+            print(value[0])
+
+    # use process ID as suffix for unique names of temporary maps
     pid = os.getpid()
 
     # set grid