Value given when variable is empty

Question

Suppose I had the following Player base class:

from abc import ABC, abstractmethod

class Player(ABC):

    def __init__(self, name, player_type):
        self.name = name
        self.player_type = player_type

Wizard:

from Player import Player

class Wizard(Player):

    def __init__(self, name, player_type = "Wizard"):
        super().__init__(self,name)

Main:

from Player import Player
from Wizard import Wizard


def main():

    gandalf = Wizard("Gandalf")
    print(gandalf.name)

    # Will print gandalf because the parameter assignment was shifted
    # because self was passed from the child to base class.
    print(gandalf.player_type)

if __name__ == "__main__":
    main()

I'm aware from this question, you shouldn't pass self from the subclass to the base class. That being said, suppose you made the mistake of doing so, the line print(gandalf.name) prints <Wizard.Wizard object at 0x049F20B0> because name was never assigned but what exactly does this value mean?

@StephenRauch - Where does it come from? I understand why it prints that value, but how did come up with that value? — , May 28 '18 at 19:44
Your call to `super().__init__(self,name)` in `Wizard.__init__()` would fail because you need to pass `player_type` as well. — AGN Gazer, May 28 '18 at 19:44
It's the string representation of the object itself: the result of `repr(gandalf)`. — chepner, May 28 '18 at 19:45
@AGNGazer - The code will compile and run because I'm looking at it right now, however, if you attempt to print the `name` that value will be printed for the reason I specified above, but I want to understand where that value came from — , May 28 '18 at 19:45
That is the default string representation inherited from `object`. So try `print(gandalf)`. Realize, the `Wizard` object carries a reference to itself. — juanpa.arrivillaga, May 28 '18 at 19:48
@AGNGazer this code would indeed work. The method expects three arguments and is being passed three arguments. The fact that two of them are `self` is irrelevant. — Daniel Roseman, May 28 '18 at 19:49
@AGNGazer - You shouldn't pass `self` from the child class to the base class, doing so can throw the parameter assignments off. The cod will compile and run — , May 28 '18 at 19:49
@Sveta it is hard to understand your question. You are passing `self` in the argument for `name`, so that is the value that is assigned; what is confusing to you? — Daniel Roseman, May 28 '18 at 19:50
@DanielRoseman - Right, it's only until you print the `name` that you realize there was an error. — , May 28 '18 at 19:50
Well... Going to eat my hat... You are right... I missed `self`! — AGN Gazer, May 28 '18 at 19:50
@DanielRoseman - I want to know how that value was generated. What about the Python environment made it go, I'll assign that value when `self` is pass from child to base when it expected another parameter. — , May 28 '18 at 19:52
@chepner explained this in https://stackoverflow.com/questions/50572602/value-given-when-variable-is-empty#comment88154878_50572602. When you `print(gandalf.name)`, `name` is an object that is being converted to a string for the purpose of printing using `repr(name)`. — AGN Gazer, May 28 '18 at 19:54
Note, the variable is not empty. Such a thing doesn't really exist in Python. — juanpa.arrivillaga, May 28 '18 at 19:54

chepner · Answer 1 · 2018-05-28T20:00:00.403

2

super() does not return a class; it returns a proxy object, so super().__init__(self, name) behaves much the same as foo.__init__(self, name), where foo is a "real" instance of Wizard. That being the case, the first parameter self of the call to Player.__init__ has already been assigned, so your explicit self is assigned to the next parameter, name, and your second argument name is assigned to the third parameter player_type.

Put another way, with Player being the next class in the MRO, super().__init__(self, name) is the same as Player.__init__(self, self, name).

Put yet another way, super().__init__ is a bound method, and so the __init__ function's first parameter self has already been supplied.

edited May 28 '18 at 20:00

answered May 28 '18 at 19:54

chepner

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So, I was correct in stating that it shifted the parameter assignments? – May 28 '18 at 19:56
1

Yes, but that's normal for *any* bound method, just like `gandalf = Wizard("Gandalf")` results in an implicit call to `Wizard.__init__(gandalf, "Gandalf")` – chepner May 28 '18 at 19:58
In other words, don't pass `self`, that's done for you automatically, correct? – May 28 '18 at 20:01
You can clean up the edit I made, but keep the comment about the `repr(gandalf)` because that's the real answer to my question, and I'll give you the checkmark. – May 28 '18 at 20:07

Value given when variable is empty

1 Answers1