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45 lines
1.9 KiB
XML
45 lines
1.9 KiB
XML
<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p> The default value of a parameter is computed once when the function is
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created, not for every invocation. The "pre-computed" value is then used for every
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subsequent call to the function. Consequently, if you modify the default
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value for a parameter this "modified" default value is used for the parameter
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in future calls to the function. This means that the function may not behave as
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expected in future calls and also makes the function more difficult to understand.
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</p>
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</overview>
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<recommendation>
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<p>If a parameter has a default value, do not modify the default value. When
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you use a mutable object as a default value, you should use a placeholder value
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instead of modifying the default value. This is a particular problem when you
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work with lists and dictionaries but there are standard methods of avoiding
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modifying the default parameter (see References).</p>
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</recommendation>
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<example>
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<p>In the following example, the <code>default</code> parameter is set with a default
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value of an empty list. Other commands in the function then append values to the
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list. The next time the function is called, the list will contain values, which
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may not have been intended.</p>
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<sample src="ModificationOfParameterWithDefault.py" />
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<p>The recommended workaround is use a placeholder value. That is, define the
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function with a default of <code>default=None</code>, check if the parameter is
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<code>None</code> and then set the parameter to a list.</p>
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</example>
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<references>
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<li>Effbot: <a href="https://web.archive.org/web/20201112004749/http://effbot.org/zone/default-values.htm">Default Parameter Values in Python</a>.</li>
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<li>Python Language Reference: <a href="http://docs.python.org/2/reference/compound_stmts.html#function-definitions">Function definitions</a>.</li>
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</references>
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</qhelp>
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