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Exercises/ex8_3.md

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+\[ [Index](index.md) | [Exercise 8.2](ex8_2.md) | [Exercise 8.4](ex8_4.md) \]
+
+# 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.
+
+\[ [Solution](soln8_3.md) | [Index](index.md) | [Exercise 8.2](ex8_2.md) | [Exercise 8.4](ex8_4.md) \]
+
+----
+`>>>` Advanced Python Mastery  
+`...` A course by [dabeaz](https://www.dabeaz.com)  
+`...` Copyright 2007-2023  
+
+![](https://i.creativecommons.org/l/by-sa/4.0/88x31.png). This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)