Most Pythonic way of initializing instance variables

Question

I'm currently on some heavy data analytics projects, and am trying to create a Python wrapper class to help streamline a lot of the mundane preprocessing steps involved when cleaning data, partitioning it into test / validation sets, standardizing it, etc. The idea ultimately is to transform raw data into easily consumable processed matrices for machine learning algorithms to input for training and testing purposes. Ideally, I'm working towards the point where

data = DataModel(AbstractDataModel)
processed_data = data.execute_pipeline(**kwargs)

So in many cases I'll start off with a self.df, which is a pandas dataframe object for my instance. But one method may be called standardize_data() and will ultimately return a standardized dataframe called self.std_df.

My IDE has been complaining heavily about me initializing variables outside of __init__. So to try to soothe PyCharm, I've been using the following code inside my constructor:

class AbstractDataModel(ABC):

    @abstractmethod
    def __init__(self, input_path, ...,  **kwargs):

        self.df_train, self.df_test, self.train_ID, self.test_ID, self.primary_key, ... (many more variables) = None, None, None, None, None, ...

Later on, these properties are being initialized and set. I'll admit that I'm coming from heavy-duty Java Spring projects, so I'm still used to verbosely declaring variables. Is there a more Pythonic way of declaring my instance properties here? I know I must be violating DRY with all the None values.

I've researched on SO, and came across this similar question, but the answer that is provided is more about setting instance variables through argv, so it isn't a direct solution in my context.

@Alexander I've attempted this solution. However, Pycharm complains that it needs more values to unpack, and at runtime, I get a TypeError: 'NoneType' object is not iterable error. — Yu Chen
– Yu Chen, Commented Aug 6, 2017 at 20:53
@Alexander, that would result in an error (NoneType not iterable), maybe you meant var1 = var2 = ... = varN = None — AChampion
– AChampion, Commented Aug 6, 2017 at 20:53
My mistake, self.var1 = self.var2 = ... = self.var_n = None — Alexander
– Alexander, Commented Aug 6, 2017 at 20:54
As an aside, when you have lots of variables inside a class instance, you may want to alphabetize them and declare each individually. I find that it helps me to maintain code. — Alexander
– Alexander, Commented Aug 6, 2017 at 20:58

Artyer · Accepted Answer · 2017-08-06 20:52:49Z

2

Use chained assignment:

self.df_train = self.df_test = self.train_ID = self.test_ID = self.primary_key = ... = None

Or set up abstract properties that default to None (So you don't have to set them)

answered Aug 6, 2017 at 20:52

Artyer

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3 Comments

Yu Chen Over a year ago

This is such a good idea, and so simple! Definitely Pythonic. Speaking of the abstract properties that default to None, could you clarify how that is different than what I'm doing in __init__? Since my constructor is an abstract method and all my properties are being set to None within this method?

Alexander Over a year ago

None holds its own location in memory (id(None)). Anything that has a value equal to None will point to this memory location. Both methods are equivalent in their functionality, although one is much simpler.

Yu Chen Over a year ago

Got it. So self.df_train is self.df_test will evaluate to true until I assign it values.

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