Calling a method that have return value in Python

Question

I'm new to python and came into this piece of code. From my previous programming knowledge, I'm assuming this method should return something(results). But why do some statements like self.children[0].query(rect, results) don't assign the return value to any variable while recursively calling the method?

def query(self, rect, results=None):

    if results is None:
        rect = normalize_rect(rect)
        results = set()

    if len(self.children) > 0:
        if rect[0] <= self.center[0]:
            if rect[1] <= self.center[1]:
                self.children[0].query(rect, results)
            if rect[3] > self.center[1]:
                self.children[1].query(rect, results)
        if rect[2] > self.center[0]:
            if rect[1] <= self.center[1]:
                self.children[2].query(rect, results)
            if rect[3] > self.center[1]:
                self.children[3].query(rect, results)

    for node in self.nodes:
        if (node.rect[2] > rect[0] and node.rect[0] <= rect[2] and
                node.rect[3] > rect[1] and node.rect[1] <= rect[3]):
            results.add(node.item)

    return results

Chris Martin · Accepted Answer · 2016-09-26 05:45:48Z

2

You're right that the query function does return results, but it also modifies results on this line:

results.add(node.item)

A parameter used in this was is sometimes described as an "output parameter".

query is not a pure function. In those places where query is called without using its return value, query is being called for its side effects.

edited Sep 26, 2016 at 5:45

answered Sep 26, 2016 at 5:40

Chris Martin

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