ex8_3.md

[ Index | Exercise 8.2 | Exercise 8.4 ]

Exercise 8.3

Objectives:

• Using coroutines to set up processing pipelines

Files Created: cofollow.py, coticker.py

Note

For this exercise the stocksim.py program should still be running in the background.

In Exercise 8.2 you wrote some code that used generators to set up a processing pipeline. A key aspect of that program was the idea of data flowing between generator functions. A very similar kind of dataflow can be set up using coroutines. The only difference is that with a coroutine, you send data into different processing elements as opposed to pulling data out with a for-loop.

(a) A coroutine example

Getting started with coroutines can be a little tricky. Here is an example program that performs the same task as Exercise 8.2, but with coroutines. Take this program and copy it into a file called cofollow.py.

# cofollow.py
import os
import time

# Data source
def follow(filename,target):
    with open(filename,'r') as f:
        f.seek(0,os.SEEK_END)
        while True:
            line = f.readline()
            if line != '':
                target.send(line)
            else:
                time.sleep(0.1)

# Decorator for coroutine functions
from functools import wraps

def consumer(func):
    @wraps(func)
    def start(*args,**kwargs):
        f = func(*args,**kwargs)
        f.send(None)
        return f
    return start

# Sample coroutine
@consumer
def printer():
    while True:
        item = yield     # Receive an item sent to me
        print(item)

# Example use
if __name__ == '__main__':
    follow('Data/stocklog.csv',printer())

Run this program and make sure produces output.. Make sure you understand how the different pieces are hooked together.

(b) Build some pipeline components

In a file coticker.py, build a series of pipeline components that carry out the same tasks as the ticker.py program in Exercise 8.2. Here is the implementation of the various pieces.

# coticker.py
from structure import Structure

class Ticker(Structure):
    name = String()
    price =Float()
    date = String()
    time = String()
    change = Float()
    open = Float()
    high = Float()
    low = Float()
    volume = Integer()

from cofollow import consumer, follow
from tableformat import create_formatter
import csv

# This one is tricky. See solution for notes about it
@consumer
def to_csv(target):
    def producer():
        while True:
            yield line

    reader = csv.reader(producer())
    while True:
        line = yield
        target.send(next(reader))

@consumer
def create_ticker(target):
    while True:
        row = yield
        target.send(Ticker.from_row(row))

@consumer
def negchange(target):
    while True:
        record = yield
        if record.change < 0:
            target.send(record)

@consumer
def ticker(fmt, fields):
    formatter = create_formatter(fmt)
    formatter.headings(fields)
    while True:
        rec = yield
        row = [getattr(rec, name) for name in fields]
        formatter.row(row)

Your challenge: Write the main program that hooks all of these components together to generate the same stock ticker as in the previous exercise.

[ Solution | Index | Exercise 8.2 | Exercise 8.4 ]

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