Nested function in Python class

Question

i have a little "basic understanding" Python problem. So let me explain my problem.
At first a very simple code snippet.

class Revert:
    __sentence = ""

    def __init__(self, sentence: str):
        self.__sentence = sentence

    def get_sentence(self):
        return self.__sentence

    def revert_sentence(self):
        return self.__sentence[::-1]


if __name__ == '__main__':
    print(Revert("Stackoverflow").get_sentence())
    print(Revert("Stackoverflow").revert_sentence())

So this show normal function calling of python functions. But how can i transform this code so i can call the revert function like this:

    print(Revert("Stackoverflow").get_sentence().revert_sentence())

Maybe I'm miss the forest through the trees. But I didn't get it how to do this. I already tried to solve the problem with innermethods but this didn't work for me

...
    def get_sentence(self):
        def revert_sentence():
            self.revert_sentence()

        return self.__sentence
...

Many thanks in advance

You must understand what an object of a class is. And how you call a function on an object instance. If you want to chain the functions you must return the Object from the first function not a member of the Object(or class). — Devesh
– Devesh, Commented Mar 31, 2022 at 16:36

trincot · Accepted Answer · 2022-03-31 16:53:32Z

2

Implement __str__ to return the actual string. Then in the existing methods, return the object. This way you can chain. But when print is applied to it, that __str__ method will kick in:

class Revert:
    __sentence = ""

    def __init__(self, sentence: str):
        self.__sentence = sentence

    def get_sentence(self):
        return self

    def revert_sentence(self):
        return Revert(self.__sentence[::-1])

    # Some more such methods ...
    def upper(self):
        return Revert(self.__sentence.upper())

    def first(self, count):
        return Revert(self.__sentence[:count])

    def dotted(self):
        return Revert(".".join(self.__sentence))

    # For getting a string
    def __str__(self):
        return self.__sentence


print(Revert("Stackoverflow").get_sentence().revert_sentence())

print(Revert("Stackoverflow")
         .revert_sentence()
         .first(8)
         .upper()
         .revert_sentence()
         .first(4)
         .dotted())   # "O.V.E.R"

Note that now the .get_sentence() method is not really doing much, and you can always strip it from a chain.

edited Mar 31, 2022 at 16:53

answered Mar 31, 2022 at 16:36

trincot

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3 Comments

pho Over a year ago

This is more a comment for Mo7art than for you, but it doesn't really make sense to have a member function that just returns the object. You already need to have the object before you call .get_sentence(). Maybe this is a simplified example for Stack Overflow, and the actual .get_sentence() actually operates on the self object and returns the result, which would make more sense.

Mo7art Over a year ago

@PranavHosangadi Yes this was a very simplified code sample, in the original code I do something in the here called .get_sentence() function. Also, there should be many of the kind of functions like get_sentence() would that work there too ? @trincot The problem is, that I didn't call the print method (only here in the example). I would like to work with the returned Object in other functions.

trincot Over a year ago

Sure, more functions would work too. I added a few more to my answer.

Devesh · Accepted Answer · 2022-03-31 16:48:20Z

Here You go:

class Revert:
    __sentence = ""

    def __init__(self, sentence: str):
        self.__sentence = sentence

    def get_sentence(self):
        return self.__sentence

    def revert_sentence(self):
        # It's important to know that you are making changes in the same instance of the object
        self.__sentence = self.__sentence[::-1]
        return self
    
    def pseudo_revert(self):
        # Return a new object with reverted string, but this instance still has original string intact.
        return Revert(self.__sentence[::-1])


if __name__ == '__main__':
    r1 = Revert("Stackoverflow")
    r2 = Revert("Stackoverflow")

    print(r1.get_sentence()) # Stackoverflow
    print(r1.revert_sentence().get_sentence()) # wolfrevokcatS
    print(r1.get_sentence()) # wolfrevokcatS
    
    print(r2.get_sentence()) # Stackoverflow
    print(r2.pseudo_revert().get_sentence()) # wolfrevokcatS
    print(r2.get_sentence()) # Stackoverflow

Hope this helps you understand the object, instance of an object, and method of object distinctly.

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