Summing values in one field using PyQGIS

Question

In QGIS 2.18 there is a shapefile with three fields accordingly

id	Type	Value
1	a	5
2	b	NULL
3	c	1
4	d	6
5	e	NULL

I want to get the total sum of all values in the field "Value". How can I achieve that in the Python Console?

The field "Value" maintains two types of data int and NULL.

I was trying with a while-loop but it does not work

layer = qgis.utils.iface.activeLayer()

if not layer.isValid():
    raise Exception('Layer is not valid')

features = layer.getFeatures()

for feat in features:
    attr = feat.attributes()[2]
    
    if attr == NULL:
        continue

    else:
        total = 0
        index = 0
        while index < layer.featureCount():
            total = total + int(attr)
            index = index + 1
return(total)

How to achieve one number that gives the total sum of a whole field exclusive NULL-values?

After executing @ahmadhanb's solution I received a correct number but somehow with a strange output type, when print (type(sum(total))) it gives me <type 'NoneType'>. What could be a problem?

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References:

• Find Neighbor Polygons in a Layer
• Summing values from different intersecting/overlapping points in QGIS
• Sum new values to existing attribute values in PyQGIS?
• SQL query in Python console to sum column based on values of other two columns

Answer 1

In programming, there are several ways of solving a problem.

That's also the case for summing values of a field in QGIS. But there is one that is the recommended way, truly leveraging the PyQGIS API.

Aggregating field values using PyQGIS

To get the sum of a field you can use:

total_sum = layer.aggregate(QgsAggregateCalculator.Sum, "my_field_name")

Simpler and faster.

Note: Since the result is a tuple (result_value, boolean_result), you just call total_sum[0] and you are done.

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Read the docs to know the list of aggregate types you can use.

You can even pass filter expressions to the aggregates. Really powerful.

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Note: this is available since QGIS 2.16.

Answer 2

In fact your code provides a double number to the actual value. For example, if the total number is 50, your code provides 100. I think the correct code should be something like this:

layer = qgis.utils.iface.activeLayer()

if not layer.isValid():
    raise Exception('Layer is not valid')

features = layer.getFeatures()

total = []
for feat in features:
    attr = feat.attributes()[2]

    if attr == NULL:
        continue

    else:
        total.append((int(attr)))
print(sum(total))

Add a total empty list outside the loop, and if the attr == NULL, it will skip the null values, and if there is a value in the attr it will be appended to the list. Then you can print the sum of the list outside the loop to get one value only.

Here is the screenshot of the output:

Answer 3

In QGIS 3, QgsVectorLayerUtils.getValues() returns all values from a specified field name or expression.

You can use to sum values in one field in that way:

layer = iface.activeLayer()
v = QgsVectorLayerUtils.getValues(layer, 'field_name')[0]
v = list(filter(None, v))
s = sum(v)

NOTE: NULL is None is False (both have different objects in the memory). NULL == None is True (both have the same value). Therefore, we can use None here.