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+\[ [Index](index.md) | [Exercise 8.1](ex8_1.md) | [Exercise 8.3](ex8_3.md) \]
+
+# Exercise 8.2
+
+*Objectives:*
+
+- Using generators to set up processing pipelines
+
+*Files Created:* `ticker.py`
+
+**Note**
+
+For this exercise the `stocksim.py` program should still be
+running in the background.  You're going to use the `follow()`
+function you wrote in the previous exercise.
+
+## (a) Setting up a processing pipeline
+
+A major power of generators is that they allow you to create programs
+that set up processing pipelines--much like pipes on Unix systems.
+Experiment with this concept by performing these steps:
+
+```python
+>>> from follow import follow
+>>> import csv
+>>> lines = follow('Data/stocklog.csv')
+>>> rows = csv.reader(lines)
+>>> for row in rows:
+        print(row)
+
+['BA', '98.35', '6/11/2007', '09:41.07', '0.16', '98.25', '98.35', '98.31', '158148']
+['AA', '39.63', '6/11/2007', '09:41.07', '-0.03', '39.67', '39.63', '39.31', '270224']
+['XOM', '82.45', '6/11/2007', '09:41.07', '-0.23', '82.68', '82.64', '82.41', '748062']
+['PG', '62.95', '6/11/2007', '09:41.08', '-0.12', '62.80', '62.97', '62.61', '454327']
+...
+```
+
+Well, that's interesting.  What you're seeing here is that the output of the
+`follow()` function has been piped into the `csv.reader()` function and we're
+now getting a sequence of split rows.   
+
+## (b) Making more pipeline components
+
+In a file `ticker.py`, define the following class (using your structure code from before) and set up
+a pipeline:
+
+```python
+# ticker.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()
+
+if __name__ == '__main__':
+    from follow import follow
+    import csv
+    lines = follow('Data/stocklog.csv')
+    rows = csv.reader(lines)
+    records = (Ticker.from_row(row) for row in rows)
+    for record in records:
+        print(record)
+```
+
+When you run this, you should see some output like this:
+
+    Ticker('IBM',103.53,'6/11/2007','09:53.59',0.46,102.87,103.53,102.77,541633)
+    Ticker('MSFT',30.21,'6/11/2007','09:54.01',0.16,30.05,30.21,29.95,7562516)
+    Ticker('AA',40.01,'6/11/2007','09:54.01',0.35,39.67,40.15,39.31,576619)
+    Ticker('T',40.1,'6/11/2007','09:54.08',-0.16,40.2,40.19,39.87,1312959)
+
+## (c) Keep going
+
+Oh, you can do better than that.  Let's plug this into your table generation code. Change
+the program to the following:
+
+```python
+# ticker.py
+...
+
+if __name__ == '__main__':
+    from follow import follow
+    import csv
+    from tableformat import create_formatter, print_table
+
+    formatter = create_formatter('text')
+
+    lines = follow('Data/stocklog.csv')
+    rows = csv.reader(lines)
+    records = (Ticker.from_row(row) for row in rows)
+    negative = (rec for rec in records if rec.change < 0)
+    print_table(negative, ['name','price','change'], formatter)
+```
+
+This should produce some output that looks like this:
+
+          name      price     change 
+    ---------- ---------- ---------- 
+             C      53.12      -0.21 
+           UTX      70.04      -0.19 
+           AXP      62.86      -0.18 
+           MMM      85.72      -0.22 
+           MCD      51.38      -0.03 
+           WMT      49.85      -0.23 
+            KO       51.6      -0.07 
+           AIG      71.39      -0.14 
+            PG      63.05      -0.02 
+            HD      37.76      -0.19 
+
+Now, THAT is crazy! And pretty awesome.
+
+**Discussion**
+
+Some lessons learned: You can create various generator functions and
+chain them together to perform processing involving data-flow
+pipelines.  
+
+A good mental model for generator functions might be Lego blocks.
+You can make a collection of small iterator patterns and start
+stacking them together in various ways.  It can be an extremely powerful way to program.
+
+\[ [Solution](soln8_2.md) | [Index](index.md) | [Exercise 8.1](ex8_1.md) | [Exercise 8.3](ex8_3.md) \]
+
+----
+`>>>` 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/)