Two Trains and Other Refactoring Analogies
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The document discusses strategies for refactoring legacy code through analogies like cleaning a trash pile or squeezing toothpaste from a tube. It recommends adding tests, refactoring to improve design without changing behavior, and making incremental changes to facilitate switching programming languages or improving code over time. Refactoring pays dividends by preventing technical debt and building technical wealth through continuous improvement that makes future changes easier.
![TAXES = {
'CA': .0875,
'PA': .08,
}
def get_tax(order):
try:
return order.subtotal * TAXES[order.state]
except KeyError:
raise NotImplementedError()](https://image.slidesharecdn.com/twotrains-160830055345/85/Two-Trains-and-Other-Refactoring-Analogies-21-320.jpg)
![TAXES = {
'CA': .0875,
'PA': .08,
'TX': .0825,
}
def get_tax(order):
try:
return order.subtotal * TAXES[order.state]
except KeyError:
raise NotImplementedError()](https://image.slidesharecdn.com/twotrains-160830055345/85/Two-Trains-and-Other-Refactoring-Analogies-25-320.jpg)
![class User:
def __init__(self, data):
self.data = data
def validate(self):
if '@' not in self.data['email']:
raise ValueError('Invalid email provided.')
if len(self.data['first_name']) > 50:
raise ValueError('First name too long.')
if len(self.data['last_name']) > 100:
raise ValueError('Last name too long.')](https://image.slidesharecdn.com/twotrains-160830055345/85/Two-Trains-and-Other-Refactoring-Analogies-33-320.jpg)
![class User:
def __init__(self, data):
self.data = data
def validate(self):
self.validate_email()
self.validate_first_name()
self.validate_last_name()
def validate_email(self):
if '@' not in self.data['email']:
raise ValueError('Invalid email provided.')
def validate_first_name(self):
if len(self.data['first_name']) > 50:
raise ValueError('First name too long.')
def validate_last_name(self):
if len(self.data['last_name']) > 100:
raise ValueError('Last name too long.')](https://image.slidesharecdn.com/twotrains-160830055345/85/Two-Trains-and-Other-Refactoring-Analogies-38-320.jpg)
![class User:
_fields = [
'email',
'first_name',
'last_name'
]
def __init__(self, data):
self.data = data
def validate(self):
for field in self._fields:
method_name = 'validate_{}'.format(field)
validate_field = getattr(self, method_name)
validate_field()
def validate_email(self):
. . .](https://image.slidesharecdn.com/twotrains-160830055345/85/Two-Trains-and-Other-Refactoring-Analogies-40-320.jpg)
![class User:
_fields = [
'email',
'first_name',
'last_name',
'phone_number',
]
PHONE_LENGTH = 10
. . .
def validate_phone_number(self):
if len(self.data['phone_number']) != User.PHONE_LENGTH:
raise ValueError('Phone number is wrong length.')](https://image.slidesharecdn.com/twotrains-160830055345/85/Two-Trains-and-Other-Refactoring-Analogies-45-320.jpg)
![<?php
if (isset($_GET['fahrenheit'])) {
$fahrenheit = $_GET['fahrenheit'];
} else {
die('No degrees in Fahrenheit provided.');
}
$celsius = ($fahrenheit - 32) * 5 / 9;
echo floor($celsius);](https://image.slidesharecdn.com/twotrains-160830055345/85/Two-Trains-and-Other-Refactoring-Analogies-55-320.jpg)
Two Trains and Other Refactoring Analogies
- 1. TWO TRAINSAND OTHER REFACTORING ANALOGIES by Kevin London @kevin_london ChowNow
- 2. Legacy Code
- 3. Formula: 1. No tests 2. Hard to read. 3. Hard to change.
- 4. “Any fool can write code that a computer can understand. Good programmers write code that humans can understand.” Martin Fowler
- 5. It’s easy to create legacy code.
- 6. How can we prevent it?
- 7. 1. Add tests. 2. Refactor.
- 10. The Trash Pile
- 11. “Who will notice if I add this piece of trash?”
- 12. Better to clean as we go.
- 13. An example.
- 14. def get_tax(order): if order.state == 'CA': return order.subtotal * .0875 elif order.state == 'PA': return order.subtotal * .08 else: raise NotImplementedError()
- 15. Need to add in Texas.
- 16. def get_tax(order): if order.state == 'CA': return order.subtotal * .0875 elif order.state == 'PA': return order.subtotal * .08 elif order.state == 'TX': return order.subtotal * .0825 else: raise NotImplementedError()
- 17. def get_tax(order): if order.state == 'CA': return order.subtotal * .0875 elif order.state == 'PA': return order.subtotal * .08 else: raise NotImplementedError()
- 18. Add tests.
- 19. import pytest from taxes import get_tax, Order def test_get_tax_for_ca(): order = Order(subtotal=10, state='CA') assert get_tax(order) == 10 * .0875 def test_get_tax_for_pa(): order = Order(subtotal=10, state='PA') assert get_tax(order) == 10 * 0.08 def test_it_raises_error_for_others(): with pytest.raises(NotImplementedError): order = Order(subtotal=1, state='EQ') assert get_tax(order) == 0
- 20. Refactor.
- 21. TAXES = { 'CA': .0875, 'PA': .08, } def get_tax(order): try: return order.subtotal * TAXES[order.state] except KeyError: raise NotImplementedError()
- 22. ============ test session starts ================= test.py ... ============ 3 passed in 0.01 seconds ============
- 23. def tax_equals_percent(state, percent): subtotal = 10 order = Order(subtotal, state) return get_tax(order) == subtotal * percent . . . def test_get_tax_for_texas(): assert tax_equals_percent(‘TX', .0825) . . .
- 24. ============ test session starts ============ test.py ..F. ===== 1 failed, 3 passed in 0.02 seconds ====
- 25. TAXES = { 'CA': .0875, 'PA': .08, 'TX': .0825, } def get_tax(order): try: return order.subtotal * TAXES[order.state] except KeyError: raise NotImplementedError()
- 26. ============ test session starts ================= test.py .... ============ 4 passed in 0.01 seconds ============
- 29. Easy to get toothpaste from a new tube.
- 30. At the end, need to roll it to get toothpaste
- 31. With Code: Make it easy to change.
- 32. “For each desired change: make the change easy (warning: this may be hard), then make the easy change.” Kent Beck
- 33. class User: def __init__(self, data): self.data = data def validate(self): if '@' not in self.data['email']: raise ValueError('Invalid email provided.') if len(self.data['first_name']) > 50: raise ValueError('First name too long.') if len(self.data['last_name']) > 100: raise ValueError('Last name too long.')
- 35. import pytest def assert_raises_error(data, expected_message): user = User(data=data) with pytest.raises(ValueError) as excinfo: user.validate() assert expected_message in str(excinfo.value) class TestValidateUser: def test_invalid_email_raises_error(self): data = { 'first_name': 'foo', 'email': 'bademail', 'last_name': 'lee', } assert_raises_error(data, 'Invalid email provided') . . .
- 36. =========== test session starts ======= test.py ... ======== 3 passed in 0.01 seconds=====
- 38. class User: def __init__(self, data): self.data = data def validate(self): self.validate_email() self.validate_first_name() self.validate_last_name() def validate_email(self): if '@' not in self.data['email']: raise ValueError('Invalid email provided.') def validate_first_name(self): if len(self.data['first_name']) > 50: raise ValueError('First name too long.') def validate_last_name(self): if len(self.data['last_name']) > 100: raise ValueError('Last name too long.')
- 39. =========== test session starts ======= test.py ... ======== 3 passed in 0.01 seconds=====
- 40. class User: _fields = [ 'email', 'first_name', 'last_name' ] def __init__(self, data): self.data = data def validate(self): for field in self._fields: method_name = 'validate_{}'.format(field) validate_field = getattr(self, method_name) validate_field() def validate_email(self): . . .
- 41. =========== test session starts ======= test.py ... ======== 3 passed in 0.01 seconds=====
- 43. class TestValidateUser: . . . def test_invalid_phone_number_raises_error(self): data = {'phone_number': 'no_phone'} user = User(data) with pytest.raises(ValueError): user.validate_phone_number()
- 44. =========== test session starts ======= test.py ...F == 1 failed, 3 passed in 0.01 seconds==
- 45. class User: _fields = [ 'email', 'first_name', 'last_name', 'phone_number', ] PHONE_LENGTH = 10 . . . def validate_phone_number(self): if len(self.data['phone_number']) != User.PHONE_LENGTH: raise ValueError('Phone number is wrong length.')
- 46. =========== test session starts ======= test.py .... ======== 4 passed in 0.01 seconds======
- 47. By refactoring, we make it easy to change the code.
- 48. The Two Trains
- 49. Need to jump.
- 51. With code: 1. Reach Parity 2. Switch 3. Improve
- 52. A story about a failed rewrite.
- 54. PHP -> Python
- 55. <?php if (isset($_GET['fahrenheit'])) { $fahrenheit = $_GET['fahrenheit']; } else { die('No degrees in Fahrenheit provided.'); } $celsius = ($fahrenheit - 32) * 5 / 9; echo floor($celsius);
- 56. 1. Add tests. 2. Refactor.
- 57. import requests API_URL = 'http://localhost:8888/celsius' def test_api_converts_fahrenheit(): degrees_f = 32 degrees_c = 0 path = '{}?fahrenheit={}'.format(API_URL, degrees_f) response = requests.get(path) assert int(response.content) == degrees_c def test_api_rounds_down(): degrees_f = 100 degrees_c = 37 # rounded down path = '{}?fahrenheit={}'.format(API_URL, degrees_f) response = requests.get(path) assert int(response.content) == degrees_c def test_api_returns_error_if_no_degrees(): response = requests.get(API_URL) assert 'No degrees' in response.content
- 58. =========== test session starts ======= test_php_api.py ... ======== 3 passed in 0.01 seconds=====
- 60. import requests API_URL = 'http://localhost:5000/celsius' . . .
- 61. =========== test session starts ======= test_py_api.py FFF ======== 3 failed in 0.01 seconds=====
- 62. Get the tests passing again
- 63. from math import floor from flask import Flask, request app = Flask(__name__) @app.route('/celsius') def celsius(): try: fahrenheit = int(request.args.get('fahrenheit')) except TypeError: return 'No degrees in Fahrenheit provided.' celsius = (fahrenheit - 32) * 5 / 9; return str(floor(celsius))
- 64. =========== test session starts ======= test_py_api.py ... ======== 3 passed in 0.01 seconds=====
- 66. the smaller the switch, the better.
- 67. IN REVIEW
- 68. Don’t treat code like a pile of trash. Clean it up as you go.
- 69. Remember the tube of toothpaste: Make the change easy, make the easy change.
- 70. If you have to jump across two trains, Get them to the same speed & switch.
- 71. We can change how we think about legacy code.
- 72. Technical debt
- 73. Technical wealth
- 75. References: Refactoring - Martin Fowler & Kent Beck https://www.amazon.com/Refactoring-Improving-Design-Existing- Code/dp/B007WTFWJ6/ref=sr_1_1? s=books&ie=UTF8&qid=1472533198&sr=1-1&keywords=refactori ng+martin+fowler#nav-subnav Working with Legacy Code - Michael Feathers https://www.amazon.com/Working-Effectively-Legacy-Michael- Feathers/dp/0131177052 Forget Technical Debt — Here's How to Build Technical Wealth http://firstround.com/review/forget-technical-debt-heres-how-to- build-technical-wealth/
- 76. Credits: IMG_5163 - Justin Henry, https://flic.kr/p/ebaYR The sky is the limit #2 - Mark Dries, https://flic.kr/p/eeq2eQ The way it should be! - Patrick Dirden, https://flic.kr/p/ 8Yp4mf 0001_zoriah_photojournalist_war_photographer_20120719 _0631 - Zoriah, https://flic.kr/p/dkScaB Freaktography - Ravaged, https://flic.kr/p/nh72Tp Aachen - Aachener Dom Pala d’oro Antependium - Daniel Mennerich, https://flic.kr/p/pBmHyw