QGIS equivalent to arcpy.SearchCursor?

Question

What is the equivalent in QGIS (or other FOSS) to this:

rows = arcpy.SearchCursor(table,
                          fields="ORIG_FID; field1; field2; field3; field4",
                          sort_fields="ORIG_FID A")

for row in rows:

Answer 1

Check out the PyQGIS Cookbook's section on Using Vector Layers.

Hacking together the examples there:

# The expression will filter the features where the field "location_name" contains
# the word "Lake" (case insensitive)
exp = QgsExpression('location_name ILIKE \'%Lake%\'')
request = QgsFeatureRequest(exp)
request.setSubsetOfAttributes(['name','id'],layer.pendingFields())
for feature in layer.getFeatures(request):
    # do whatever you need with the feature

Also, see this question for some other hints: What are QGIS Arcpy Equivalent Functions for Update/Delete Row/Field?

Answer 2

Use .getFeatures(), this returns all layer field values:

Query the provider for features specified in request.

for feature in table.getFeatures():
    if feature['myField'] == someVariable:
        # do something