138 lines
4.2 KiB
Markdown
138 lines
4.2 KiB
Markdown
\[ [Index](index.md) | [Exercise 8.1](ex8_1.md) | [Exercise 8.3](ex8_3.md) \]
|
|
|
|
# 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
|
|
|
|
. This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)
|