modpipe: modules as pipelines

What is this?

An opinionated package that loads a module as a callable pipeline.

Installation

pip install modpipe

Why is this?

Very frequently, I interactively write data transformations in Jupyter; then, once it works as (initially) expected, I move it to some module. Recognizing that pattern, this package makes using modules as pipelines (esp. in the context of ETL) easier.

Can I see an example?

Assume you have a set of raw items (probably deserialized objects) that you want to transform into clean ones. You accumulate your transformation functions in ingest_pipeline.py. When the following code runs,

import modpipe

with modpipe.ModPipe.on('ingest_pipeline') as f:
    clean_items = [f(item) for item in raw_items]

it automatically re/loads the ingest_pipeline module, sythesizing a composition ordered by source code line position. That's a mouth full. Concretely, say you have the following in ingest_pipeline.py,

def twice(x):
    return x * 2

def weird_pair(x):
    return x, -x

Calling f -- an instance of ModPipe -- works as if it were defined as,

def f(x):
    return weird_pair(twice(x))

So What?

If you do a painful amount of ETL work, this might strike you as useful. If you don't, you might be saying, "so what?" Unfortunately, I think this doesn't communicate well without a developer-in-motion screencast (XXX: TODO). But, for now, I'll lean on the concluding line in Tim Peter's wonderful PEP20,

Namespaces are one honking great idea -- let's do more of those!

By moving transformational code into modules, you free up notebook (and cognitive space) for subsequent steps. If you spend a little time properly naming the module, it's somewhat easy to navigate. If you partition your transformation code into different logical units, it's somewhat robust. And, if you pepper your pipeline module code with assertions, it documents your expectations for the data.

Everyone wants to write ETL code the same way that they would write other code: carefully and with a good test suite. But, realistically, we don't do that because of various constraints. Plus, it's really tedious code. This package (and, the one I extracted it from, vaquero) recognizes your competing demands and the reality of the task, and works with you.

How is it opinionated?

Auxillary Functions

By default, underscore-prefixed callables do not enter in the pipeline. By assumption, these callables are auxillary functions used by the ones you really want in the linearized pipeline.

def _encode_as_binary(x):  # Not in pipeline by default!
    return bin(x)[2:].rjust(32, '0')

def convert_seeds(seeds):
    return [_encode_as_binary(seed) for seed in seeds]

Assert Your Expectations

As mentioned above, unit-testing ETL code is a pain. And, since dirty data violate expectations near-continously, it seriously impedes progress. Rather than relying upon unit tests for the transformation functions, it's easier to write in-line (in the context of the module) assertions that document your assumptions and guard against inadvertent regressions by failing loudly at re/import time.

The following (contrived) example ensures that your pipeline's sqrt function properly computes the square root and returns the result in the same numeric type as given.

def sqrt(x):
    return type(x)(x ** 0.5)

assert sqrt(1776) == 42, "Uh oh!"  # Fails loudly if bad code!

DAGS are Confusing

Directed Acyclic Graphs (DAGs) are greate when computers construct them for you. But, in lots of contexts, they make it hard to reason about what your code is doing when it fails. For the most part, the pipeline is a linearized composition of the functions in your module. Thus, if the function on line 85 raises an exception when used, you know that only the functions above have already executed. This is a surprisingly useful cognitive device, especially when you step way from your code for six months and visit it again only when it becomes a problem.

But, there are two exception to this simple linearization. Sometimes, it is necessary to either: 1) abort the pipeline early or 2) skip over some of the functions. This package provides a sentinel return value for both cases.

from lxml.html import fromstring
from modpipe import SkipTo, Done

def extract_doc(raw_html):
    if raw_html.strip():
        return {'doc': fromstring(raw_html)}
    else:
        return Done(None)  # Nothing can be done! Abort!

def extract_title(d):
    for title in d['doc'].xpath("//title/text()"):
        d['title'] = title

    if 'error' in d['title'].lower():
        return SkipTo(cleanup, d)  # Skip to cleanup!
    else:
        return d

def extract_headers(d):
     d['headers'] = d['doc'].xpath('//h1/text()')

def cleanup(d):
     del d['doc']

Returning None

In the prior code listing, extract_headers and cleanup did in-place transformations on the passed dict. To cut down on LoC while communicating mutation, neither returned a value. There are pros on cons to this style. But, in any case, modpipe handles it by assuming the given arguments to a function that returns None should be passed to the next function in the pipe. Thus, cleanup receives d.

This begs the question: if you want to return None, how do you do so? In this case, you need to return a Result. For example,

from modpipe import Result

def f(s):
    tok = s.upper().strip()
    return tok if tok else Result(None)  # or Done(None)

Tuples are special

If you return a tuple from a function and that tuple's length matches the arity of the next function in the pipeline, modpipe star-expands it when calling the next function, otherwise, it does f(res).

def f(x):
    return x, -x  # i.e. add(x, -x)

def g(x, y):
    return x + y, x * y  # i.e. h(x + y, x * y)

def h(items):
    return sum(items)

This works for tuples and tuples alone. (That is, if you returned a list, it always passes the whole list as an argument.) You'll note that the call structure doesn't allow for keyword arguments. I've tried working around this but I didn't find anything that wasn't intrusive.

Is there anything else?

Yes. Pipelines in modpipe are very pyspark friendly. Although the Spark team doesn't recommend using RDDs anymore, Spark is useful for writing ETL pipelines. But, python object serialization and deserialization adds a lot of expense to chains of transformations in pyspark proper (i.e. map on RDDs). If you collect your transformations into logical units serialized as modules, it amortizes the pickling-related expenses. It won't be scala speed, but at least you can take advantage of already existing infrastructure in a somewhat more performant manner.

Misc

I think I got this idea from PyMC3. For the major version bump, lots of examples started using modules and I thought it was annoying at first. Then I realized how nice it can be. Since modeling and ETL tend to go hand-to-hand (albeit in a 1:99 ratio), I started writing my ETL code in the same way. I'm sure I'm not the first to do so, but I hadn't seen it before. (It's probably just one of those things that lots of people do naturally without writing up.)

I also wanted to point out bonobo. It's a lot more mature and flexible. According to the docs,

Bonobo is a young rewrite of an old python2.7 tool that ran millions of transformations per day for years on production. Although it may not yet be complete or fully stable (please, allow us to reach 1.0), the basics are there.

Still, for 90% of projects, vaquero (which uses modpipe) suits me better.