slides/caesar-encipher.html

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  34       .ciphertext {
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  44
  45 # Caesar ciphers
  46
  47 ![centre-aligned Caesar wheel](caesarwheel1.gif)
  48
  49 Letter-by-letter enciphering
  50
  51 ---
  52
  53 # Enciphering and deciphering
  54
  55 ## Arithmetic on letters
  56
  57 Convert .plaintext[letter] → .plaintext[number] →
  58 .ciphertext[number] → .ciphertext[letter]
  59
  60 Functions you will need
  61
  62 ```python
  63 ord()
  64
  65 chr()
  66
  67 %
  68 ```
  69
  70 * What are good test cases?
  71
  72 ---
  73
  74 # The [string module](http://docs.python.org/3.3/library/string.html) is your friend
  75
  76 ```python
  77 import string
  78
  79 string.ascii_letters
  80 string.ascii_lowercase
  81 string.ascii_uppercase
  82 string.digits
  83 string.punctuation
  84 ```
  85
  86 ---
  87 # DRY and YAGNI
  88
  89 Is your code DRY?
  90
  91 ---
  92
  93 # Doctest
  94
  95 * Why document?
  96 * Why test?
  97
  98 ```python
  99 def caesar_encipher_letter(letter, shift):
 100     """Encipher a letter, given a shift amount
 101
 102     >>> caesar_encipher_letter('a', 1)
 103     'b'
 104     """
 105     if letter in string.ascii_letters:
 106     .
 107     .
 108     .
 109 ```
 110
 111 ---
 112
 113 # The magic doctest incantation
 114
 115 ```python
 116 if __name__ == "__main__":
 117     import doctest
 118     doctest.testmod()
 119 ```
 120
 121 ---
 122
 123 # Accents
 124
 125 ```python
 126 >>> caesar_encipher_letter('é', 1)
 127 ```
 128 What does it produce?
 129
 130 What should it produce?
 131
 132 ## Unicode, combining codepoints, and normal forms
 133
 134 Text encodings will bite you when you least expect it.
 135
 136 * urlencoding is the other pain point.
 137
 138 ---
 139
 140 # Five minutes on StackOverflow later...
 141
 142 ```python
 143 def unaccent(text):
 144     """Remove all accents from letters.
 145     It does this by converting the unicode string to decomposed compatibility
 146     form, dropping all the combining accents, then re-encoding the bytes.
 147
 148     >>> unaccent('hello')
 149     'hello'
 150     >>> unaccent('HELLO')
 151     'HELLO'
 152     >>> unaccent('héllo')
 153     'hello'
 154     >>> unaccent('héllö')
 155     'hello'
 156     >>> unaccent('HÉLLÖ')
 157     'HELLO'
 158     """
 159     return unicodedata.normalize('NFKD', text).\
 160         encode('ascii', 'ignore').\
 161         decode('utf-8')
 162 ```
 163
 164 ---
 165
 166 # Doing all the letters
 167
 168 ## Test-first developement
 169
 170 1. Write the tests.
 171     * They will fail. There is no code.
 172 2. Write code until the tests pass.
 173 3. Refactor.
 174
 175 ---
 176
 177 # Doing all the letters
 178
 179 ## Abysmal
 180
 181 ```python
 182 ciphertext = ''
 183 for i in range(len(plaintext)):
 184     ciphertext += caesar_encipher_letter(plaintext[i], key)
 185 ```
 186
 187 ---
 188
 189 # Doing all the letters
 190
 191 ## Bad
 192
 193 ```python
 194 ciphertext = ''
 195 for p in plaintext:
 196     ciphertext += caesar_encipher_letter(p, key)
 197 ```
 198
 199 ...but easily generalisable
 200
 201 ---
 202
 203 # Doing all the letters
 204
 205 ## Good (but unPythonic)
 206
 207 ```python
 208 ciphertext = map(lambda p: caesar_encipher_letter(p, key), plaintext)
 209 ```
 210
 211 ---
 212
 213 # Doing all the letters
 214
 215 ## Best
 216
 217 ```python
 218 ciphertext = [caesar_encipher_letter(p, key) for p in plaintext]
 219 ```
 220 ---
 221
 222 # Not all iterables are equal
 223
 224 ```python
 225 ''.join()
 226 ```
 227
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