Polars integration

This section explains how to use beavers with polars.

ETF value calculation example

In this example we want to calculate the value of ETFs.

Starting with a data frame of individual share prices:

import polars as pl

PRICE_SCHEMA = pl.Schema(
    [
        ("ticker", pl.String()),
        ("price", pl.Float64()),
    ]
)

price_table = pl.DataFrame(
    [
        {"ticker": "AAPL", "price": 174.79},
        {"ticker": "GOOGL", "price": 130.25},
        {"ticker": "MSFT", "price": 317.01},
        {"ticker": "F", "price": 12.43},
        {"ticker": "GM", "price": 35.28},
    ],
    schema=PRICE_SCHEMA,
)

ticker	price
AAPL	174.79
GOOGL	130.25
MSFT	317.01
F	12.43
GM	35.28

And another data frame containing the composition of each ETF:

ETF_COMPOSITION_SCHEMA = pl.Schema(
    [
        ("etf", pl.String()),
        ("ticker", pl.String()),
        ("quantity", pl.Float64()),
    ]
)


etf_composition_table = pl.DataFrame(
    [
        {"etf": "TECH", "ticker": "AAPL", "quantity": 2.0},
        {"etf": "TECH", "ticker": "GOOGL", "quantity": 2.0},
        {"etf": "TECH", "ticker": "MSFT", "quantity": 1.0},
        {"etf": "CARS", "ticker": "F", "quantity": 3.0},
        {"etf": "CARS", "ticker": "GM", "quantity": 2.0},
    ],
    schema=ETF_COMPOSITION_SCHEMA,
)

etf	ticker	quantity
TECH	AAPL	2.0
TECH	GOOGL	2.0
TECH	MSFT	1.0
CARS	F	3.0
CARS	GM	1.0

In a few line of polars we can derive the value of each ETF:

ETF_VALUE_SCHEMA = pl.Schema(
    [
        ("etf", pl.String()),
        ("value", pl.Float64()),
    ]
)


def calculate_etf_value(
    etf_composition: pl.DataFrame, price: pl.DataFrame
) -> pl.DataFrame:
    return (
        etf_composition.join(price, on=["ticker"])
        .select(pl.col("etf"), (pl.col("price") * pl.col("quantity")).alias("value"))
        .group_by("etf", maintain_order=True)
        .agg(pl.col("value").sum())
        .cast(ETF_VALUE_SCHEMA)
    )


etf_value_table = calculate_etf_value(
    etf_composition=etf_composition_table, price=price_table
)

etf	value
TECH	927.09
CARS	72.57

ETF value calculation DAG

Once the business logic of the calculation is writen and tested it can be added into a Dag. We'll be using the Dag pl helper which makes it easier to deal with polars data frame in beavers.

First we define two source streams, made of polars.DataFrame:

from beavers import Dag

dag = Dag()
price_source = dag.pl.source_df(schema=PRICE_SCHEMA, name="price")
etf_composition_source = dag.pl.source_df(
    schema=ETF_COMPOSITION_SCHEMA, name="etf_composition"
)

Then we keep track of the latest value for each source stream:

price_state = dag.pl.last_by_keys(price_source, ["ticker"])
etf_composition_state = dag.pl.last_by_keys(
    etf_composition_source,
    ["etf", "ticker"],
)

Lastly we put together the share prices and ETF composition:

etf_value_state = dag.state(calculate_etf_value).map(
    etf_composition_state,
    price_state,
)

And that's it:

price_source.set_stream(price_table)
etf_composition_source.set_stream(etf_composition_table)
dag.execute()
polars.testing.assert_frame_equal(etf_value_state.get_value(), etf_value_table)

Taming updates

This simple dag does the job of calculating the ETF value in real time. But there is one issue. The value of every ETF would update every time either price or etf_composition update. Even if the updates comes on a ticker that is not relevant to the ETFs we are tracking.

In the example below, when the price of GameStop updates, we recalculate the value of every ETF. Even though their value hasn't changed:

new_price_updates = pl.DataFrame(
    [{"ticker": "GME", "price": 123.0}],
    PRICE_SCHEMA,
)
price_source.set_stream(new_price_updates)
dag.execute()
assert len(etf_value_state.get_value()) == 2
assert etf_value_state.get_cycle_id() == dag.get_cycle_id()

To tame updates we need to identify which ETF needs updating.

ETF values can update because their composition has changed:

updated_because_of_composition = dag.pl.get_series(
    etf_composition_source,
    "etf",
)

Or because one of their component has updated:

def get_etf_to_update_because_of_price(
    etf_composition_state: pl.DataFrame, price_update: pl.DataFrame
) -> pl.Series:
    updated_tickers = price_update["ticker"].unique()
    return etf_composition_state.filter(pl.col("ticker").is_in(updated_tickers))[
        "etf"
    ].unique()


updated_because_of_price = dag.stream(
    get_etf_to_update_because_of_price, empty=pl.Series(name="etf", dtype=pl.String())
).map(etf_composition_state, price_source)

We can then put it back together and only calculate updates for relevant ETFs:

stale_etfs = dag.pl.concat_series(
    updated_because_of_price, updated_because_of_composition
)


def get_composition_for_etfs(
    etf_composition_state: pl.DataFrame,
    etfs: pl.Series,
) -> pl.DataFrame:
    return etf_composition_state.filter(pl.col("etf").is_in(etfs))


stale_etf_compositions = dag.pl.stream(
    get_composition_for_etfs, ETF_COMPOSITION_SCHEMA
).map(etf_composition_state, stale_etfs)

updated_etf = dag.pl.stream(calculate_etf_value, ETF_VALUE_SCHEMA).map(
    stale_etf_compositions, price_state
)

And see that only the value "TECH" ETF updates when a tech stock update:

price_source.set_stream(
    pl.DataFrame(
        [{"ticker": "MSFT", "price": 317.05}],
        schema=PRICE_SCHEMA,
    )
)
dag.execute()
assert len(updated_etf.get_value()) == 1

etf	value
TECH	927.13