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Functional over object-oriented style for pipeline-esque code

written by Eric J. Ma on 2022-04-01 | tags: data science pipeline programming style programming software development

Where possible, in my data science projects, I usually go for a functional programming style over an object-oriented style. I'd like to explain why this could be advantageous from the standpoint of a mental model, which is an idea that my friend Jesse Johnson has elaborated on in the context of in biotech organizations.

Types, standards, and pipelines

It all starts with types -- basically, what objects are floating around in a program. When writing data science code, data type transformations are commonplace. Reasoning about the data types is hard enough without introducing another object -- and, therefore, another type. Introducing another type into the program is like introducing another standard -- suddenly, you have one more thing to worry about.

Here's an example. We read in a CSV file and spit out a pandas DataFrame. That pandas DataFrame goes into another function that constructs a PyMC model and returns the model. Next, the model goes into another function that calls on a PyMC sampler and returns an InferenceData object. The InferenceData object then gets parsed back into another DataFrame for multiple uses -- to generate plots, get checked into a database, and more.

If we were to introduce a new object, say, a DataModeller object, we might encapsulate all of those steps in the following way:

class DataModeller:
    def __init__(...):

    def make_model(...):
        self.model = model

    def sample(...):
        with self.model:
            self.idata = pm.sample()

    def process_posterior(...):

Now, the critical trouble I usually run into with classes-for-data-pipelining is not merely about needing to create new objects and hence, new "types" in the project. It's also that the class methods can, in theory, be called without any enforcement of order - and errors will result. It's challenging to reason clearly about the correct order of operations when reading just the code. The atomic class methods are attached to the class at the same level of abstraction as the higher-level class methods, and the object's internal state can be created on-the-fly -- and is implicit, not explicit.

By adopting a functional style, we can make keeping atomic functions and higher-order functions mentally separate easier. We can keep the atomic functions inside a submodule and higher-order functions higher up in the source code hierarchy. And by calling on the higher-order functions, we can easily read off the correct order of operations that need to happen, i.e. the pipeline. Riffing off the above example:

# This is where the atomic functions might live, e.g. `src/models/`
def make_model_1(df: pd.DataFrame) -> pm.Model:
    return model

def make_model_2(df: pd.DataFrame) -> pm.Model:
    return model

def sample(model: pm.Model) -> az.InferenceData:
    """Convenience wrapper around `pm.sample` with defaults for our problem."""
    return idata

def process_posterior(idata: az.InferenceData) -> pd.DataFrame:
    return posterior_df

And now the functions are easily composable to make higher-order functions:

# This is where the higher-order functions live, e.g. in `src/`
from src.models.stuff import make_model_1, make_model_2, sample, process_posterior

model_name_mapping = {
    "model1": make_model_1,
    "model2": make_model_2,

def model(df: pd.DataFrame, model_name: str) -> pd.DataFrame:
    """Some docstring here."""
    model_func: Callable = model_name_mapping.get(model_name)
    model: pm.Model = model_func(df)
    idata: az.InferenceData = sample(model)
    posterior_df: pd.DataFrame = process_posterior(idata)
    return posterior_df

A natural hierarchy appears here! We import lower-level functions from deeper within the library; with higher-level functions, we go closer to the top-level imports.

(In the example above, I also took the liberty of annotating types in-line. They are another form of in-line documentation. They also help with reasoning about the workflow much more efficiently, especially if someone is just getting used to, say, the df -> model -> idata -> posterior_df workflow.)

As you probably can see here, too, using functions enables us to easily see the idiomatic workflow. Our mental model is more easily read off by the next person in the pipelines-as-functions paradigm rather than pipelines-as-objects paradigm.

And as a side benefit, if we think carefully about the data types we pass around, consume, and generate, then we end up with easily reusable functions. That's another awesome side effect!

Objects are fine, just not everywhere

Now, just to be clear, I'm not advocating an abdication of objects altogether. Clearly, a PyMC Model, InferenceData and DataFrames are all objects themselves. For someone who works with data, when do objects make sense?

I think in the context of data pipelines, objects make the most sense as a data container. One clear example is configuration objects (which I usually implement as a dataclass with very-few-to-zero class methods). Another clear example is when 3-4 related dataframes need to be kept together. Also, we may need convenient class methods to dump them to disk at once. Here a dataclass with 3-4 corresponding attributes and a single class method would make a ton of sense.

Pipelines are best thought of as functions and are therefore best implemented as functions. Data (and data containers) are best thought of as objects and are therefore best implemented as objects. Objects as data pipelines, though? In my opinion, doing so only adds confusion. Once again, it's my conviction, having written more than 9 years of Python data science code, that pipelines-as-functions express the developer's mental model of the pipeline more cleanly than pipelines-as-objects.

The twist

Aaaaaand I know there's going to be someone who says, "But you can make objects callable too, right?" Yeah, I know that's doable :). On this point, the flexibility of Python allows for things to be implemented in multiple ways. However, in line with the Zen of Python, it's probably best to adopt one clear way of implementing things. Pipelines as functions, data as objects. This is a much saner way of establishing simple idioms that, subjectively, allows for cleaner reasoning about our code.

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