etl-pipes 0.2.0

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Pypelines

Description

Pypelines is a Python library for creating flow-based programming pipelines.

Primitives

Implemented

• Pipeline
  • used to combine pipes into a single sequential pipeline, used to implement "&&" operator
• Parallel
  • used to combine pipes into a single parallel pipeline
• Maybe
  • used to implement a pattern Chain of Responsibility and "||" operator
• MapReduce
  • used to implement MapReduce pattern (chunking, mapping chunks, reducing to a single result)

Planned

• Void
  • used to implement ";" operator
• Stream
  • used to implement Observable + Observer pattern
  • can be implemented with asyncio, threading, multiprocessing

Examples

Basic usage

A trivial example of a pipeline with two simple pipes. Here two operations of summing and raising to a power are combined into one pipeline. The result of the first pipe is passed to the second pipe as an argument.

import pytest

from etl_pipes.pipes.base_pipe import as_pipe
from etl_pipes.pipes.pipeline.pipeline import Pipeline

@pytest.mark.asyncio
async def test_if_as_base_pipe_works() -> None:
    @as_pipe
    def sum_(a: int, b: int) -> int:
        return a + b

    @as_pipe
    def pow_(a: int) -> int:
        r = 1
        for _ in range(a):
            r *= a
        return r

    pipeline = Pipeline([sum_, pow_])

    result = await pipeline(2, 2)

    assert result == (2 + 2) ** (2 + 2)

An example of a parallel execution of pipes. Currently, it works only with asyncio.

import asyncio
import time
from pathlib import Path

import pytest

from etl_pipes.pipes.base_pipe import as_pipe
from etl_pipes.pipes.parallel import Parallel


@pytest.mark.asyncio
async def test_parallel_log_to_console_and_log_to_file() -> None:
    @as_pipe
    async def log_to_console(data: str) -> int:
        await asyncio.sleep(0.5)
        print(data)
        return 1

    @as_pipe
    async def log_to_file(data: str) -> Path:
        test_file = Path("/tmp/pipe-test.txt")
        if test_file.exists():
            test_file.unlink()

        with test_file.open("a") as f:
            f.write(data)
        await asyncio.sleep(0.5)
        return test_file

    parallel = Parallel(
        [
            log_to_console,
            log_to_file,
        ],
        broadcast=True,  # sends same arguments to all pipes
    )

    start_time_ms = int(time.time() * 1000)

    exit_code, log_path = await parallel("test")

    end_time_ms = int(time.time() * 1000)
    limit = 1000
    diff = end_time_ms - start_time_ms
    assert diff < limit

    assert exit_code == 1
    assert log_path == Path("/tmp/pipe-test.txt")
    assert log_path.exists()
    assert log_path.read_text() == "test"

Maybe pipe example. It is an implementation of pattern Chain of Responsibility.

If the current pipe fails with Nothing exception, the next pipe is executed with the same arguments.

If no pipe can handle the exception, UnhandledNothingError is raised.

import pytest

from etl_pipes.pipes.base_pipe import as_pipe
from etl_pipes.pipes.maybe import Maybe, Nothing, UnhandledNothingError


@as_pipe
async def successful_pipe() -> str:
    return "Success"


@as_pipe
async def failing_pipe() -> str:
    if True:
        raise Nothing()
    return "Failure"



@pytest.mark.asyncio
async def test_maybe_with_fallback_pipe() -> None:
    maybe_pipe = Maybe(failing_pipe).otherwise(successful_pipe)
    result = await maybe_pipe()
    assert result == "Success"


@pytest.mark.asyncio
async def test_maybe_with_all_failing_pipes() -> None:
    maybe_pipe = Maybe(failing_pipe).otherwise(failing_pipe)
    with pytest.raises(UnhandledNothingError):
        await maybe_pipe()

Advanced usage

More sophisticated example from sample ToDo web application.

We want to get a ToDo item from server.

• we check if we have access to this ToDo item.
• we have access -> we try to get it from cache.
• it is not in cache -> we get it from database and cache it.

from fastapi import Depends, FastAPI
from sqlalchemy.orm import Session

from etl_pipes.pipes.maybe import Maybe
from etl_pipes.pipes.parallel import Parallel
from etl_pipes.pipes.pipeline.pipeline import Pipeline
from tests.web_api.auth import AuthToken, CheckAccessForTodo, Ops
from tests.web_api.cache.todo_cache import CacheTodoDTO, GetTodoAndItemsFromCache
from tests.web_api.db import models
from tests.web_api.db.connection import get_db
from tests.web_api.db.read import ReadManyFromDb, ReadOneFromDb
from tests.web_api.domain_types import TodoId
from tests.web_api.dto import (
    GetTodoDto,
)

from tests.web_api.mapping.todo import MapDbTodoAndDbItemsToDto

app = FastAPI()

@app.get("/todos/{todo_id}")
async def read_todo(
    token: AuthToken, todo_id: TodoId, db: Session = Depends(get_db)
) -> GetTodoDto:
    pipeline = Pipeline(
        [
            CheckAccessForTodo(token, todo_id, ops=[Ops.Read]).void(),  # changes return type to None
            Maybe(
                GetTodoAndItemsFromCache(todo_id=todo_id),
            ).otherwise(
                Pipeline(
                    [
                        Parallel(
                            [
                                ReadOneFromDb(
                                    db=db, model=models.Todo, filter={"id": todo_id}
                                ),
                                ReadManyFromDb(
                                    filter={"todo_id": todo_id},
                                    model=models.Item,
                                    db=db,
                                ),
                            ]
                        ),
                        MapDbTodoAndDbItemsToDto(),
                        CacheTodoDTO(),
                    ]
                )
            ),
        ]
    )
    return await pipeline()  # type: ignore[no-any-return]

Critical features

• Add support for Pipeline State
• Add support for Pipeline Context
• Write a mypy plugin to support type checking for pipes instead of doing it in a runtime
• Add PassedArgs class

State

State is a way to pass data between pipes.

It should be a more convenient way to pass data between pipes than passing an argument each time. Something like mutable dependency injection.

Context

Context is a way to pass data between pipes.

Literally read-only State.

Type checking

Currently, mypy does not support type checking for pipes, and Pipeline or Parallel pipes' return type is Any.

It causes some problems, when narrowing from Any to some type (look at web test case).

PassedArgs

PassedArgs is a class that contains all positional and keyword arguments that will be passed to the next pipe.

Now we use tuple for it, and we can only pass positional arguments. It is not convenient to use it in some cases.

Non-critical features

• Add support for ProcessPool and ThreadPool for Parallel pipe
• Implement Parallel pipe as ABC or Protocol and make AsyncioParallel a subclass of it
• Think about getting rid of square brackets in Parallel and Pipeline and rename them to par and seq respectively, overall interface improvement
• Fix broken endpoints in web app test case
• Setup PyPi publishing
• Create Void (or _) pipe instead of using .void() method
• Create more test cases for MapReduce pipe