Getting unique values from multiple fields as matched using PyQGIS

Question

There are "Code" and "Color" fields in my layer like this: (Sample 6 rows)

Code	Color
A	Red
B	Blue
A	Red
A	Red
B	Blue
C	Green

All As are Red, all Bs are Blue and so on.

I can get the unique values for each field:

layer = iface.activeLayer()
code_index = layer.fields().indexFromName("Code")
code_unique = layer.uniqueValues(code_index)
color_index = layer.fields().indexFromName("Color")
color_unique = layer.uniqueValues(color_index)
print(code_unique)
print(color_unique)
# OUTPUT
# {'A', 'C', 'B'}
# {'Blue', 'Red', 'Green'}

As you can see, the order of two outputs doesn't match. (When you try, you might get an ordered result, but it depends and is not reliable)

How can I get unique value pairs like [('A', 'Red'), ('B', 'Blue'), ('C', 'Green')] using PyQGIS?

I couldn't find groupby-like method. It can be done by means of groupby(["Code", "Color"]).first() in GeoPandas.

Answer 1

You can use set() which is a Python built-in function.

layer = iface.activeLayer() 
pairs = list(set([(f["Code"], f["Color"]) for f in layer.getFeatures()]))
print(pairs) # [('A', 'Red'), ('C', 'Green'), ('B', 'Blue')]

A set() is an unordered data structure, so it does not preserve the insertion order. You will probably get an unordered but a matched list (('A', 'Red') ('C', 'Green'), ...).

• Kadir Şahbaz: No need list(), if pairs will be used in a loop (like for p in pairs). Thanks.
• Kavyajeet Bora: you can also put into a dict instead of list, that would make it easy to call by keys later

Answer 2

You can use the class Counter() from the collections module.

from collections import Counter
layer = iface.activeLayer()
c = Counter([(feat['Code'],feat['Color']) for feat in layer.getFeatures()])
print(c) # Counter({('A', 'Red'): 3, ('B', 'Blue'): 2, ('C', 'Green'): 1})
combination = [comb for comb in c]
print(combination) # [('A', 'Red'), ('B', 'Blue'), ('C', 'Green')]