CODEWITHSUNDEEP
pythonsyntax
Oblomov
Oblomov

Reputation: 9635

Python syntax: x = SomeClass(some_input)(x)

I am trying to understand the following python snippet:

 x = SomeObject
 x = SomeClass(some_input)(x)

Can anyone shed some light on what is going on here?

Upvotes: 1

Views: 84

Answers (2)

Rafael Aguilar
Rafael Aguilar

Reputation: 3279

Extending others response will try to give you a more general answer, on how Python will solve that statement

x = SomeClass(some_input)(x)

First Python's interpreter will try solve the statement after the = and then assign it to x, so this leave us with_

SomeClass(some_input)(x)

In this statement there are two main elements the SomeClass instation and the call for the result instance that should be a callable)

SomeClass(some_input):  (x)
#---instantiation----:--call--

It will solve first the left-side to determine what should be called, in the left-side We have a Class instantiation, it means that Python interpreter will call the SomeClass.__init__(...) method with the arguments (some_input). This involves that if some_input is a call of a method, instantiation, formula or so itself, it will try to solve that first.

SomeClass(some_input) # => <new object>

This <new_object> is the one who will be called with the arguments (x) and this is possible because the SomeClass can (and should in this example) be callable just with implementing the __call__ method as you would do with __init__, as example:

class Add:
    def __init__(self, num1, num2):
        self.num1 = num1
        self.num2 = num2
        print "Sum of", self.num1,"and",self.num2, "is:"

    def __call__(self):
        return (self.num1 + self.num2)

add = Add(1,2)
print add()

+More info about objects being callables

note: The reference <new_object>, as it not bound with any variable, will be lost after the last statement.

Upvotes: 1

Leon
Leon

Reputation: 32484

It can be simplified to the following (assuming that the auxiliary variables y and z are not used by surrounding code):

y = SomeObject
z = SomeClass(some_input)
x = z(y)

Upvotes: 3

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