qgis_count_distinct_values_in_vector_layer_list_of_fields.py

# QGIS 3 (compatible with 2.18) python code to count features
# for each unique value of chosen fields
#
# Copy paste in your python console
# You need to change the "fields" variable below
# And select the layer in QGIS legend before running it
#
# The output memory layer attribute table would be like
# fieldname	fieldvalue	count
# menace	EN	19355
# menace	LC	129845
# menace	NA	15068
# menace		122127
# categorie	Gymnospermes	248
# categorie	Vers	1
# categorie	Poissons	649
# categorie	Autres	7
# categorie	Angiospermes	212331
# categorie	Arachnides	2

layer = iface.activeLayer()

# List of field names from which to gather stats
fields = ('categorie', 'menace')

# import needed classes
from qgis.core import QgsVectorLayer, QgsFeature, QgsField, QgsFeatureRequest
try:
    # QGIS 3
    from PyQt5.QtCore import QVariant
except:
    # QGIS 2
    from PyQt4.QtCore import QVariant
    
# Create dictionnary with the list of unique values for each field
c = dict([(f,{}) for f in fields])
for k in c.keys():
    try:
        # QGIS 3
        idx = layer.fields().indexFromName(k)
    except:
        # QGIS 2
        idx = layer.fieldNameIndex(k)
        
    c[k] = dict([(a,0) for a in layer.uniqueValues(idx)])

# Loop through the active layer
# And sum up counters from feature
# it is very important to set the NoGeometry flag
# and add the subset of interesting fields
# to improve performance
request = QgsFeatureRequest().setFlags(QgsFeatureRequest.NoGeometry).setSubsetOfAttributes(fields, layer.fields() )
for feat in layer.getFeatures(request):
    for k in c.keys():
        a = feat[k]
        c[k][a]+= 1

# create memory layer to store data
vl = QgsVectorLayer(
    "None", 
    "fields_unique_values_count", 
    "memory"
)
pr = vl.dataProvider()
pr.addAttributes([
    QgsField("fieldname", QVariant.String),
    QgsField("fieldvalue",  QVariant.String),
    QgsField("count", QVariant.String)
])
vl.updateFields()

# Add layer to QGIS canvas
try:
    # QGIS 3
    QgsProject.instance().addMapLayer(vl)
except:
    # QGIS 2
    QgsMapLayerRegistry.instance().addMapLayer(vl)

# add features to the memory layer
for k,v in c.items():
    for i,j in v.items():
        #print(k,i,j)
        fet = QgsFeature()
        fet.setAttributes([k, i, j])
        pr.addFeatures([fet])