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.

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.

8. Refactoring:
Improving design
without changing
behavior

9. My analogies
for refactoring:

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 ============

27. Make it
better
over time.

28. Tube of
Toothpaste

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.')

34. Change:
Validate phone number

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=====

37. Make the
change easy.

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=====

42. Make the
easy change.

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.

50. The nuclear
option:
rewrite

51. With code:
1. Reach Parity
2. Switch
3. Improve

52. A story about a
failed rewrite.

53. Let’s say we’re
switching
languages.

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=====

59. Rewrite the
code.

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=====

65. Reaching parity
reduces risk.

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

74. Improving
code pays
dividends.

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

77. Thank you!
Slides:
http://bit.ly/twotrains
www.kevinlondon.com
@kevin_london