what does this operator means in django `reduce(operator.and_, query_list)`

Question

I am reading this questions

Constructing Django filter queries dynamically with args and kwargs

I am not able to get what does this operator do

fi

Answer 1

The Python docs describe reduce as:

Applying a function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value

So it allows us to easily and quickly perform a single function (that takes two parameters, i.e. add a to b) numerous times over a set of data with one simple function call.

In the case of this Django problem, we want to construct a single QuerySet object by taking a list of individual query (Q) objects and cumulatively performing an AND or OR operation on them to get a single query

For example, for the queries [Q(1), Q(2), Q(3)] we want to OR Q(1) with Q(2) first, and then OR the result of that operation with Q3 to give us our final query

We are used to seeing operations in python (such as addition or subtraction) written like A + B (or in the case of or'ing: A | B) but python also provides functions to perform these (i.e. or(a, b), and and(a, b) which would do the same thing). These are useful in situations such as these, as they can be applied as parameters to other methods. These functions such as 'add', 'subtract' etc. are in the operator module.

So using the operators module, we see that reduce(operator.and_, query_list) is taking each Q object in our list, and "and'ing" it with the ongoing results of the previous objects in the list.

Answer 2

filter is a regular method of Django Model Manager, so there is nothing to explain.

reduce is a built-in function similar to the code below:

def reduce(func, items):
    result = items.pop()
    for item in items:
        result = func(result, item)

    return result

Where func is a user defined function.

operator.or_ is a python standard library function that wraps the or operator. It is similar to this code:

def or_(a, b):
    return a | b

For example:

reduce(operator.or_, [False, False, True])

Will return True.

In your example context, the or and the and operators are overloaded and therefore it should return a new query combined of smaller parts all concatenated by or or and operator.