Writing simple custom reducer in Pathway

Pathway supports natively aggregation using a wide range of reducers, e.g., sum, count, or max. However, those might not cover all the necessary ways of aggregating values. In this tutorial, you learn how to write reducers implementing custom logic.

For example, let's implement a custom stdev reducer that computes the standard deviation.

import pathway as pw

SHOW_DEBUG = False


class StdDevAccumulator(pw.BaseCustomAccumulator):
    def __init__(self, cnt, sum, sum_sq):
        self.cnt = cnt
        self.sum = sum
        self.sum_sq = sum_sq

    @classmethod
    def from_row(cls, row):
        [val] = row
        if SHOW_DEBUG:
            print("from_row()")
        return cls(1, val, val**2)

    def update(self, other):
        self.cnt += other.cnt
        self.sum += other.sum
        self.sum_sq += other.sum_sq
        if SHOW_DEBUG:
            print("update()")

    def compute_result(self) -> float:
        mean = self.sum / self.cnt
        mean_sq = self.sum_sq / self.cnt
        if SHOW_DEBUG:
            print("compute_result()")
        return mean_sq - mean**2


stddev = pw.reducers.udf_reducer(StdDevAccumulator)

Above, the pw.BaseCustomAccumulator class is used as a base for the StdDevAccumulator, which describes the logic of the underlying accumulator. The accumulator class requires a few methods:

from_row, which constructs an accumulator from the values of a single row of a table (here, a single value since our reducer applies to a single column),
update, which updates one accumulator by another accumulator,
compute_result, which produces the output based on the accumulator state,
retract, is an optional method, which processes negative updates,
neutral, is an optional method, which returns state corresponding to consuming 0 rows.

Now, let's see the reducer in action.

temperature_data = pw.debug.table_from_markdown(
    """
date       |   temperature
2023-06-06 |   28.0
2023-06-07 |   23.1
2023-06-08 |   24.5
2023-06-09 |   26.0
2023-06-10 |   28.3
2023-06-11 |   25.7
"""
)

temperature_statistics = temperature_data.reduce(
    avg=pw.reducers.avg(pw.this.temperature), stddev=stddev(pw.this.temperature)
)

pw.debug.compute_and_print(temperature_statistics)

[2024-04-27T07:03:30]:INFO:Preparing Pathway computation


            | avg                | stddev
^PWSRT42... | 25.933333333333334 | 3.3355555555555156

However, with this logic, our reducer is not smartly processing negative updates: it starts the computation from scratch whenever a negative update is encountered. You can see this in action by enabling debug information and processing table where row removal happens. Let's insert several values at time 0 and then remove one already inserted value and add another at time 2.

SHOW_DEBUG = True
temperature_data_with_updates = pw.debug.table_from_markdown(
    """
date       |   temperature | __time__ | __diff__
2023-06-06 |   28.0        |        0 |        1
2023-06-07 |   23.1        |        0 |        1
2023-06-08 |   24.5        |        0 |        1
2023-06-09 |   26.0        |        0 |        1
2023-06-10 |   28.3        |        0 |        1
2023-06-11 |   25.7        |        0 |        1
2023-06-11 |   25.7        |        2 |       -1
2023-06-11 |   25.9        |        2 |        1
"""
)

temperature_statistics_with_updates = temperature_data_with_updates.reduce(
    avg=pw.reducers.avg(pw.this.temperature), stddev=stddev(pw.this.temperature)
)

pw.debug.compute_and_print(temperature_statistics_with_updates)

[2024-04-27T07:03:30]:INFO:Preparing Pathway computation


from_row()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
update()
compute_result()
compute_result()
            | avg               | stddev
^PWSRT42... | 25.96666666666667 | 3.3255555555555247

It can be alleviated by extending our reducer and providing a method for processing negative updates.

class ImprovedStdDevAccumulator(StdDevAccumulator):
    def retract(self, other):
        self.cnt -= other.cnt
        self.sum -= other.sum
        self.sum_sq -= other.sum_sq
        if SHOW_DEBUG:
            print("retract()")


improved_stddev = pw.reducers.udf_reducer(ImprovedStdDevAccumulator)

And now you can test the improved reducer in action.


temperature_statistics_improved = temperature_data_with_updates.reduce(
    avg=pw.reducers.avg(pw.this.temperature),
    stddev=improved_stddev(pw.this.temperature),
)

pw.debug.compute_and_print(temperature_statistics_improved)

[2024-04-27T07:03:30]:INFO:Preparing Pathway computation


from_row()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
update()
from_row()
retract()
compute_result()
compute_result()
            | avg               | stddev
^PWSRT42... | 25.96666666666667 | 3.3255555555555247

In the example above, 10x calls to update() and 12x calls to from_row() are replaced with 6x calls to update(), 1x call to retract() and 8x calls to from_row().

This comes from the fact that former reducer:

had to call from_row() for each row of the table, wrapping each single value into separate StdDevAccumulator object,
had to call update() for each row of the table except the first consumed,
had to restart from scratch after the update to the table, thus it had to pay the cost twice.

While the latter reducer aggregated the table at time 0 in the same way as former one, but processed the update differently:

had to wrap both delete and insert updates with from_row() calls
called once retract() and once update().