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coding_style: Moved more elements to official 'API'
author Olivier Delalleau <delallea@iro>
date Fri, 17 Sep 2010 12:05:14 -0400
parents d7192e52653e
children 4f1b9e0a1377
rev   line source
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1 =========================
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2 Coding Style Guidelines
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3 =========================
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4
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5 Main Goals
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6 ==========
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7
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8 * Code should be compatible with Python 2.4 and above (using 2to3 for
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9 conversion to Python 3.x). This may not be possible in the short term
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10 for Theano-dependent code.
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11
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12 * Code should be easy to read, understand and update by developers and
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13 users.
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14
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15 * Code should be well-documented and well-tested.
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16
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17 Python Coding Guidelines
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18 ========================
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19
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20 Official Guidelines
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21 -------------------
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22
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23 Source Material
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24 ~~~~~~~~~~~~~~~
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25
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26 The four main documents describing our Python coding guidelines are:
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27 * `PEP 8 -- Style Guide for Python Code
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28 <http://www.python.org/dev/peps/pep-0008>`_
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29 * `PEP 257 -- Docstring Conventions
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30 <http://www.python.org/dev/peps/pep-0257>`_
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31 * `Numpy Docstring Standard
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32 <http://projects.scipy.org/numpy/wiki/CodingStyleGuidelines#docstring-standard>`_
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33 * `Google Python Style Guide
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34 <http://google-styleguide.googlecode.com/svn/trunk/pyguide.html>`_
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35
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36
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37 However, there are a few points mentioned in those documents that we decided
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38 to do differently:
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39
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40 * Use only one space (not two) after a sentence-ending period in comments.
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41
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42 * You do not need to add an extra blank line before the closing quotes of
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43 a multi-line docstring.
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44
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45 .. code-block:: python
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46
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47 # Good.
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48 """This is a multi-line docstring.
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49
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50 Which means it has more than one line.
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51 """
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52
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53 # Bad.
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54 """This is a multi-line docstring.
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55
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56 Which means it has more than one line.
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57
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58 """
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59
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60 Excerpts
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61 ~~~~~~~~
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62
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63 We emphasize here a few important topics that are found in the official
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64 guidelines:
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65
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66 * Only use ASCII characters in code files.
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67
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68 * Code indent must be done with four blank characters (no tabs).
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69
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70 * Limit lines to 79 characters.
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71
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72 * Naming conventions: ``ClassName``, ``TOP_LEVEL_CONSTANT``,
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73 ``everything_else``.
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74
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75 * Comments should start with a capital letter (unless the first word is a
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76 code identifier) and end with a period (short inline comments may skip
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77 the period at the end).
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78
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79 * Imports should be listed in alphabetical order. It makes it easier to
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80 verify that something is imported, and avoids duplicated imports.
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81
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82 * Use absolute imports only. This is compatible across a wider range of
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83 Python versions, and avoids confusion about what is being
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84 imported.
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85
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86 * Avoid using lists if all you care about is iterating on something. Using
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87 lists:
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88
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89 - uses more memory (and possibly more CPU if the code may break out of
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90 the iteration),
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91 - can lead to ugly code when converted to Python 3 with 2to3,
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92 - can have a different behavior if evaluating elements in the list has
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93 side effects (if you want these side effects, make it explicit by
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94 assigning the list to some variable before iterating on it).
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95
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96 +------------------------+------------------------+
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97 | Iterative version | List version |
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98 +========================+========================+
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99 | .. code-block:: python | .. code-block:: python |
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100 | | |
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101 | my_dict.iterkeys | my_dict.keys |
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102 | my_dict.itervalues | my_dict.values |
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103 | my_dict.iteritems | my_dict.items |
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104 +------------------------+------------------------+
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105 | .. code-block:: python | .. code-block:: python |
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106 | | |
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107 | itertools.ifilter | filter |
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108 | itertools.imap | map |
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109 | itertools.izip | zip |
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110 +------------------------+------------------------+
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111 | .. code-block:: python | .. code-block:: python |
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112 | | |
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113 | xrange | range |
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114 +------------------------+------------------------+
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115
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116 Code example with ``map``:
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117
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118 .. code-block:: python
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119
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120 # Good.
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121 for f_x in imap(f, x):
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122 ...
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123 all_f_x = map(f, x)
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124 map(f, x) # f has some side effect.
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125 # Bad.
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126 for element in map(f, x):
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127 ...
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128 imap(f, x)
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129
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130 * Generally prefer list comprehensions to ``map`` / ``filter``, as the former are
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131 easier to read.
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132
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133 .. code-block:: python
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134
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135 # Good.
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136 non_comments = [line.strip() for line in my_file.readlines()
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137 if not line.startswith('#')]
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138 # Bad.
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139 non_comments = map(str.strip,
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140 ifilter(lambda line: not line.startswith('#'),
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141 my_file.readlines()))
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142
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143 * Use ``in`` on container objects instead of using class-specific methods:
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144 it is easier to read and may allow you to re-use your code with different
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145 container types.
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146
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147 .. code-block:: python
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148
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149 # Good.
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150 has_key = key in my_dict
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151 has_substring = substring in my_string
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152 # Bad.
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153 has_key = my_dict.has_key(key)
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154 has_substring = my_string.find(substring) >= 0
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155
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156 * Do not use mutable arguments as default values. Instead, use a helper
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157 function (conditional expressions are forbidden at this point, see
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158 below).
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159
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160 .. code-block:: python
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161
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162 # Good.
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163 def f(array=None):
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164 array = pylearn.if_none(array, [])
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165 ...
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166 # Bad.
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167 def f(array=[]): # Dangerous if `array` is modified down the road.
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168 ...
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169
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170 * Use a leading underscore '_' in names of internal attributes / methods,
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171 but avoid the double underscore '__' unless you know what you are
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172 doing.
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173
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174
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175 Additional Recommendations
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176 --------------------------
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177
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178 Things you should do even if they are not listed in official guidelines:
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179
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180 * No conditional expression (not supported in Python 2.4). These are
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181 expressions of the form ``x = y if condition else z``.
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182
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183 * Use ``//`` for integer division and ``/ float(...)`` if you want the
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184 floating point operation (for readability and compatibility across all
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185 versions of Python).
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186
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187 .. code-block:: python
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188
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189 # Good.
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190 n_samples_per_split = n_samples // n_splits
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191 mean_x = sum(x) / float(len(x))
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192 # Bad.
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193 n_samples_per_split = n_samples / n_splits
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194 mean_x = sum(x) / len(x)
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195
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196 * Always raise an exception with ``raise MyException(args)`` where ``MyException``
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197 inherits from ``Exception``. This is required for compatibility across
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198 all versions of Python.
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199
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200 .. code-block:: python
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201
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202 # Good.
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203 raise NotImplementedError('The Pylearn team is too lazy.')
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204 # Bad.
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205 raise NotImplementedError, 'The Pylearn team is too lazy.'
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206 raise 'The Pylearn team is too lazy to implement this.'
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207
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208 * Use either ``try ... except`` or ``try ... finally``, but do not mix
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209 ``except`` with ``finally`` (which is not supported in Python 2.4).
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210 You can however embed one into the other to mimic the ``try ... except ...
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211 finally`` behavior.
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212
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213 .. code-block:: python
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214
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215 # Good.
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216 try:
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217 try:
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218 something_that_may_fail()
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219 except SomeError:
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220 do_something_if_it_failed()
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221 finally:
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222 always_do_this_regardless_of_what_happened()
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223 # Bad.
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224 try:
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225 something_that_may_fail()
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226 except SomeError:
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227 do_something_if_it_failed()
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228 finally:
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229 always_do_this_regardless_of_what_happened()
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230
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231 * Do not use the ``all`` and ``any`` builtin functions (they are not supported
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232 in Python 2.4). Instead, import them from ``theano.gof.python25`` (or
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233 use ``numpy.all`` / ``numpy.any`` for array data).
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234
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235 * Do not use the ``hashlib`` module (not supported in Python 2.4). We will
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236 probably provide a wrapper around it to be compatible with all Python
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237 versions.
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238
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239 * Avoid backslashes whenever possible. They make it more
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240 difficult to edit code, and they are ugly (as well as potentially
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241 dangerous if there are trailing white spaces).
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242
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243 .. code-block:: python
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244
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245 # Good.
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246 if (cond_1 and
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247 cond_2 and
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248 cond_3):
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249 ...
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250 # Bad.
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251 if cond_1 and \
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252 cond_2 and \
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253 cond_3:
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254 ...
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255
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256 * When indenting multi-line statements like lists or function arguments,
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257 keep elements of the same level aligned with each other.
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258 The position of the first
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259 element (on the same line or a new line) should be chosen depending on
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260 what is easiest to read (sometimes both can be ok).
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261
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262 .. code-block:: python
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263
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264 # Good.
1150
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265 for my_very_long_variable_name in [my_foo, my_bar, my_love,
1145
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266 my_everything]:
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267 ...
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268 for my_very_long_variable_name in [
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269 my_foo, my_bar, my_love, my_everything]:
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270 ...
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271 # Good iff the list needs to be frequently updated or is easier to
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272 # understand when each element is on its own line.
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273 for my_very_long_variable_name in [
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274 my_foo,
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275 my_bar,
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276 my_love,
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277 my_everything,
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278 ]:
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279 ...
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280 # Good as long as it does not require more than two lines.
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281 for my_very_long_variable_name in [my_foo,
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282 my_bar]:
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283 ...
1145
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284 # Bad.
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285 for my_very_long_variable_name in [my_foo, my_bar, my_love,
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286 my_everything]:
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287 ...
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288 for my_very_long_variable_name in [my_foo,
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289 my_bar,
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290 my_love,
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291 my_everything]:
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292 ...
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293
1159
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294 * Use the ``key`` argument instead of ``cmp`` when sorting (for Python 3
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295 compatibility).
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296
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297 .. code-block:: python
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298
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299 # Good.
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300 my_list.sort(key=abs)
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301 # Bad.
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302 my_list.sort(cmp=lambda x, y: cmp(abs(x), abs(y)))
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303
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304 * Whenever you read / write binary files, specify it in the mode ('rb' for
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305 reading, 'wb' for writing). This is important for cross-platform and
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306 Python 3 compatibility (e.g. when pickling / unpickling objects).
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307
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308 .. code-block:: python
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309
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310 # Good.
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311 cPickle.dump(obj, open('my_obj.pkl', 'wb', protocol=-1))
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312 # Bad.
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313 cPickle.dump(obj, open('my_obj.pkl', 'w', protocol=-1))
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314
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315 * Avoid tuple parameter unpacking as it can lead to very ugly code when
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316 converting to Python 3.
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317
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318 .. code-block:: python
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319
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320 # Good.
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321 def f(x, y_z):
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322 y, z = y_z
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323 ...
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324 # Bad.
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325 def f(x, (y, z)):
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326 ...
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327
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328 * Only use ``cPickle``, not ``pickle`` (except for debugging purpose since
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329 error messages from ``pickle`` are sometimes easier to understand).
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330
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331 * A script's only top-level code should be something like:
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332
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333 .. code-block:: python
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334
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335 if __name__ == '__main__':
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336 sys.exit(main())
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337
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338
1147
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339 The ``logging`` Module vs. the ``warning`` Module
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340 =================================================
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341
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342 The ``logging`` Module
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343 ----------------------
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344
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345 A central logging facility for Python capable of logging messages of various
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346 categories/urgency and choosing with some granularity which messages are
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347 displayed/suppressed, as well as where they are displayed or written. This
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348 includes an ``INFO`` level for innocuous status information, a ``WARNING`` level
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349 for unexpected state that is still recoverable, ``DEBUG`` for detailed
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350 information which is only really of interest when things are going wrong, etc.
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351
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352 In addition to the `library documentation`_, see this helpful tutorial,
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353 `Python Logging 101`_.
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354
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355 .. _library documentation: http://docs.python.org/library/logging.html
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356 .. _Python Logging 101: http://plumberjack.blogspot.com/2009/09/python-logging-101.html
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357
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358 The ``warning`` Module
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359 ----------------------
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360
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361 The ``warning`` module in the standard library and its main interface, the
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362 ``warn()`` function, allows the programmer to issue warnings in situations where
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363 they wish to alert the user to some condition, but the situation is not
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364 urgent enough to throw an exception. By default, a warning issued at a given
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365 line of the code will only be displayed the first time that line is executed.
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366 By default, warnings are written to ``sys.stderr`` but the ``warning`` module
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367 contains flexible facilities for altering the defaults, redirecting, etc.
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368
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369 Which? When?
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370 ------------
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371
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372 It is our feeling that the ``logging`` module's ``WARNING`` level be used to log
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373 warnings more meant for *internal*, *developer* consumption, to log situations
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374 where something unexpected happened that may be indicative of a problem but
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375 is several layers of abstraction below what a user of the library would
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376 care about.
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377
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378 By contrast, the warning module should be used for warnings intended for user
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379 consumption, e.g. alerting them that their version of Pylearn is older than
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380 this plugin requires, so things may not work as expected, or that a given
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381 function/class/method is slated for deprecation in a coming release (early
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382 in the library's lifetime, ``DeprecationWarning`` will likely be the most common
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383 case). The warning message issued through this facility should avoid
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384 referring to Pylearn internals.
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385
1148
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386 Code Sample
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387 ===========
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388
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389 The following code sample illustrates many of the coding guidelines one should
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390 follow in Pylearn.
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391
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392 .. code-block:: python
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393
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394 import os, sys, time
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395