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| 54 | <H3><A NAME="SECTION007236000000000000000"></A><A NAME="doctest-warnings"></A> |
| 55 | <BR> |
| 56 | 5.2.3.6 Warnings |
| 57 | </H3> |
| 58 | |
| 59 | <P> |
| 60 | <tt class="module"><a href="module-doctest.html">doctest</a></tt> is serious about requiring exact matches in expected |
| 61 | output. If even a single character doesn't match, the test fails. This |
| 62 | will probably surprise you a few times, as you learn exactly what Python |
| 63 | does and doesn't guarantee about output. For example, when printing a |
| 64 | dict, Python doesn't guarantee that the key-value pairs will be printed |
| 65 | in any particular order, so a test like |
| 66 | |
| 67 | <P> |
| 68 | <div class="verbatim"><pre> |
| 69 | >>> foo() |
| 70 | {"Hermione": "hippogryph", "Harry": "broomstick"} |
| 71 | </pre></div> |
| 72 | |
| 73 | <P> |
| 74 | is vulnerable! One workaround is to do |
| 75 | |
| 76 | <P> |
| 77 | <div class="verbatim"><pre> |
| 78 | >>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"} |
| 79 | True |
| 80 | </pre></div> |
| 81 | |
| 82 | <P> |
| 83 | instead. Another is to do |
| 84 | |
| 85 | <P> |
| 86 | <div class="verbatim"><pre> |
| 87 | >>> d = foo().items() |
| 88 | >>> d.sort() |
| 89 | >>> d |
| 90 | [('Harry', 'broomstick'), ('Hermione', 'hippogryph')] |
| 91 | </pre></div> |
| 92 | |
| 93 | <P> |
| 94 | There are others, but you get the idea. |
| 95 | |
| 96 | <P> |
| 97 | Another bad idea is to print things that embed an object address, like |
| 98 | |
| 99 | <P> |
| 100 | <div class="verbatim"><pre> |
| 101 | >>> id(1.0) # certain to fail some of the time |
| 102 | 7948648 |
| 103 | >>> class C: pass |
| 104 | >>> C() # the default repr() for instances embeds an address |
| 105 | <__main__.C instance at 0x00AC18F0> |
| 106 | </pre></div> |
| 107 | |
| 108 | <P> |
| 109 | The <tt class="constant">ELLIPSIS</tt> directive gives a nice approach for the last |
| 110 | example: |
| 111 | |
| 112 | <P> |
| 113 | <div class="verbatim"><pre> |
| 114 | >>> C() #doctest: +ELLIPSIS |
| 115 | <__main__.C instance at 0x...> |
| 116 | </pre></div> |
| 117 | |
| 118 | <P> |
| 119 | Floating-point numbers are also subject to small output variations across |
| 120 | platforms, because Python defers to the platform C library for float |
| 121 | formatting, and C libraries vary widely in quality here. |
| 122 | |
| 123 | <P> |
| 124 | <div class="verbatim"><pre> |
| 125 | >>> 1./7 # risky |
| 126 | 0.14285714285714285 |
| 127 | >>> print 1./7 # safer |
| 128 | 0.142857142857 |
| 129 | >>> print round(1./7, 6) # much safer |
| 130 | 0.142857 |
| 131 | </pre></div> |
| 132 | |
| 133 | <P> |
| 134 | Numbers of the form <code>I/2.**J</code> are safe across all platforms, and I |
| 135 | often contrive doctest examples to produce numbers of that form: |
| 136 | |
| 137 | <P> |
| 138 | <div class="verbatim"><pre> |
| 139 | >>> 3./4 # utterly safe |
| 140 | 0.75 |
| 141 | </pre></div> |
| 142 | |
| 143 | <P> |
| 144 | Simple fractions are also easier for people to understand, and that makes |
| 145 | for better documentation. |
| 146 | |
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