python-mastery/Exercises/ex8_3.md

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# 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 link:ex8_2.html[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
link:ex8_2.html[Exercise 8.2], but with coroutines.  Take this program
and copy it into a file called `cofollow.py`.

```python
# 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 link:ex8_2.html[Exercise 8.2].  Here is the implementation of the
various pieces.

```python
# 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.

\[ [Solution](soln8_3.md) | [Index](index.md) | [Exercise 8.2](ex8_2.md) | [Exercise 8.4](ex8_4.md) \]

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