How to translate ESRI cursor example to pyQgis?

Question

For example, how to translate this code into pyqgis?

    rows = gp.UpdateCursor("D:/St_Johns/data.mdb/roads")
    row = rows.Next()

    while row:
       row.buffer_distance = row.road_type * 100
       rows.UpdateRow(row)
       print row
       row = rows.Next()

errata: buffer_distance is a calculate column road_type is a calumn with calculate value

Follow your advice I type this code in to python windows:

>>> from qgis.core import QgsVectorLayer, QgsFeature
>>> layer = QgsVectorLayer(r"D:\fold", "boundingBoxes.shp", "ogr")
>>> road_type_index = layer.fieldNameIndex("road_type")
>>> buffer_distance_index = layer.fieldNameIndex("buffer_dis")
>>> layer.select(layer.pendingAllAttributesList())
>>> layer.startEditing()
True
>>> for feature in layer:
...     newvalue = feature.attributeMap()[road_type_index].toInt()[0] * 100
...     feature.changeAttribute(buffer_distance_index, newvalue)
...     layer.updateFeature(feature)
...

next when I press "enter" the qgis (1.7.4) crash and exit

Answer 1

This should get you started

from qgis.core import QgsVectorLayer, QgsFeature
layer = QgsVectorLayer(r"D:\fold\boundingBoxes.shp", "boundingBoxes", "ogr")

road_type_index = layer.fieldNameIndex("road_type")
buffer_distance_index = layer.fieldNameIndex("buffer_distance")

layer.select(layer.pendingAllAttributesList())

layer.startEditing()
for feature in layer:
   newvalue = feature.attributeMap()[road_type_index].toInt()[0] * 100
   feature.changeAttribute(buffer_distance_index, newvalue)
   layer.updateFeature(feature)

layer.commitChanges()

If in 1.7.4 try this

from qgis.core import QgsVectorLayer, QgsFeature
layer = QgsVectorLayer(r"D:\fold\boundingBoxes.shp", "boundingBoxes", "ogr")

road_type_index = layer.fieldNameIndex("road_type")
buffer_distance_index = layer.fieldNameIndex("buffer_distance")

layer.select(layer.pendingAllAttributesList())

layer.startEditing()
for feature in layer:
   newvalue = feature.attributeMap()[road_type_index].toInt()[0] * 100
   layer.changeAttributeValue(feature.id(),buffer_distance_index,newvalue) 

layer.commitChanges()

Answer 2

Geopandas provides a very intuitive way to update feature attributes. For example, the following is how you would use Geopandas and .itertuples() to update rows as you would an Esri Update Cursor:

import geopandas as gpd

shp = '/path/to/your/shapefile.shp'

# Read shp
data = gpd.read_file(shp)

# Iterate over rows and update buffer_distance field
for row in data.itertuples():
    data.at[row.Index, 'buffer_distance'] = row.road_type *100

# Write to shapefile
data.to_file('/path/to/your/shapefile_updated.shp')