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| 55 | <H3><A NAME="SECTION007234000000000000000"></A><A NAME="doctest-exceptions"></A> | |
| 56 | <BR> | |
| 57 | 5.2.3.4 What About Exceptions? | |
| 58 | </H3> | |
| 59 | ||
| 60 | <P> | |
| 61 | No problem, provided that the traceback is the only output produced by | |
| 62 | the example: just paste in the traceback. Since tracebacks contain | |
| 63 | details that are likely to change rapidly (for example, exact file paths | |
| 64 | and line numbers), this is one case where doctest works hard to be | |
| 65 | flexible in what it accepts. | |
| 66 | ||
| 67 | <P> | |
| 68 | Simple example: | |
| 69 | ||
| 70 | <P> | |
| 71 | <div class="verbatim"><pre> | |
| 72 | >>> [1, 2, 3].remove(42) | |
| 73 | Traceback (most recent call last): | |
| 74 | File "<stdin>", line 1, in ? | |
| 75 | ValueError: list.remove(x): x not in list | |
| 76 | </pre></div> | |
| 77 | ||
| 78 | <P> | |
| 79 | That doctest succeeds if <tt class="exception">ValueError</tt> is raised, with the | |
| 80 | "<tt class="samp">list.remove(x): x not in list</tt>" detail as shown. | |
| 81 | ||
| 82 | <P> | |
| 83 | The expected output for an exception must start with a traceback | |
| 84 | header, which may be either of the following two lines, indented the | |
| 85 | same as the first line of the example: | |
| 86 | ||
| 87 | <P> | |
| 88 | <div class="verbatim"><pre> | |
| 89 | Traceback (most recent call last): | |
| 90 | Traceback (innermost last): | |
| 91 | </pre></div> | |
| 92 | ||
| 93 | <P> | |
| 94 | The traceback header is followed by an optional traceback stack, whose | |
| 95 | contents are ignored by doctest. The traceback stack is typically | |
| 96 | omitted, or copied verbatim from an interactive session. | |
| 97 | ||
| 98 | <P> | |
| 99 | The traceback stack is followed by the most interesting part: the | |
| 100 | line(s) containing the exception type and detail. This is usually the | |
| 101 | last line of a traceback, but can extend across multiple lines if the | |
| 102 | exception has a multi-line detail: | |
| 103 | ||
| 104 | <P> | |
| 105 | <div class="verbatim"><pre> | |
| 106 | >>> raise ValueError('multi\n line\ndetail') | |
| 107 | Traceback (most recent call last): | |
| 108 | File "<stdin>", line 1, in ? | |
| 109 | ValueError: multi | |
| 110 | line | |
| 111 | detail | |
| 112 | </pre></div> | |
| 113 | ||
| 114 | <P> | |
| 115 | The last three lines (starting with <tt class="exception">ValueError</tt>) are | |
| 116 | compared against the exception's type and detail, and the rest are | |
| 117 | ignored. | |
| 118 | ||
| 119 | <P> | |
| 120 | Best practice is to omit the traceback stack, unless it adds | |
| 121 | significant documentation value to the example. So the last example | |
| 122 | is probably better as: | |
| 123 | ||
| 124 | <P> | |
| 125 | <div class="verbatim"><pre> | |
| 126 | >>> raise ValueError('multi\n line\ndetail') | |
| 127 | Traceback (most recent call last): | |
| 128 | ... | |
| 129 | ValueError: multi | |
| 130 | line | |
| 131 | detail | |
| 132 | </pre></div> | |
| 133 | ||
| 134 | <P> | |
| 135 | Note that tracebacks are treated very specially. In particular, in the | |
| 136 | rewritten example, the use of "<tt class="samp">...</tt>" is independent of doctest's | |
| 137 | <tt class="constant">ELLIPSIS</tt> option. The ellipsis in that example could be left | |
| 138 | out, or could just as well be three (or three hundred) commas or digits, | |
| 139 | or an indented transcript of a Monty Python skit. | |
| 140 | ||
| 141 | <P> | |
| 142 | Some details you should read once, but won't need to remember: | |
| 143 | ||
| 144 | <P> | |
| 145 | ||
| 146 | <UL> | |
| 147 | <LI>Doctest can't guess whether your expected output came from an | |
| 148 | exception traceback or from ordinary printing. So, e.g., an example | |
| 149 | that expects "<tt class="samp">ValueError: 42 is prime</tt>" will pass whether | |
| 150 | <tt class="exception">ValueError</tt> is actually raised or if the example merely | |
| 151 | prints that traceback text. In practice, ordinary output rarely begins | |
| 152 | with a traceback header line, so this doesn't create real problems. | |
| 153 | ||
| 154 | <P> | |
| 155 | </LI> | |
| 156 | <LI>Each line of the traceback stack (if present) must be indented | |
| 157 | further than the first line of the example, <em>or</em> start with a | |
| 158 | non-alphanumeric character. The first line following the traceback | |
| 159 | header indented the same and starting with an alphanumeric is taken | |
| 160 | to be the start of the exception detail. Of course this does the | |
| 161 | right thing for genuine tracebacks. | |
| 162 | ||
| 163 | <P> | |
| 164 | </LI> | |
| 165 | <LI>When the <tt class="constant">IGNORE_EXCEPTION_DETAIL</tt> doctest option is | |
| 166 | is specified, everything following the leftmost colon is ignored. | |
| 167 | ||
| 168 | <P> | |
| 169 | </LI> | |
| 170 | <LI>The interactive shell omits the traceback header line for some | |
| 171 | <tt class="exception">SyntaxError</tt>s. But doctest uses the traceback header | |
| 172 | line to distinguish exceptions from non-exceptions. So in the rare | |
| 173 | case where you need to test a <tt class="exception">SyntaxError</tt> that omits the | |
| 174 | traceback header, you will need to manually add the traceback header | |
| 175 | line to your test example. | |
| 176 | ||
| 177 | <P> | |
| 178 | </LI> | |
| 179 | <LI>For some <tt class="exception">SyntaxError</tt>s, Python displays the character | |
| 180 | position of the syntax error, using a <code>^</code> marker: | |
| 181 | ||
| 182 | <P> | |
| 183 | <div class="verbatim"><pre> | |
| 184 | >>> 1 1 | |
| 185 | File "<stdin>", line 1 | |
| 186 | 1 1 | |
| 187 | ^ | |
| 188 | SyntaxError: invalid syntax | |
| 189 | </pre></div> | |
| 190 | ||
| 191 | <P> | |
| 192 | Since the lines showing the position of the error come before the | |
| 193 | exception type and detail, they are not checked by doctest. For | |
| 194 | example, the following test would pass, even though it puts the | |
| 195 | <code>^</code> marker in the wrong location: | |
| 196 | ||
| 197 | <P> | |
| 198 | <div class="verbatim"><pre> | |
| 199 | >>> 1 1 | |
| 200 | Traceback (most recent call last): | |
| 201 | File "<stdin>", line 1 | |
| 202 | 1 1 | |
| 203 | ^ | |
| 204 | SyntaxError: invalid syntax | |
| 205 | </pre></div> | |
| 206 | ||
| 207 | <P> | |
| 208 | </LI> | |
| 209 | </UL> | |
| 210 | ||
| 211 | <P> | |
| 212 | ||
| 213 | <span class="versionnote">Changed in version 2.4: | |
| 214 | The ability to handle a multi-line exception detail, | |
| 215 | and the <tt class="constant">IGNORE_EXCEPTION_DETAIL</tt> doctest option, | |
| 216 | were added.</span> | |
| 217 | ||
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