ModificationOfParameterWithDefault.qhelp

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<qhelp>


<overview>
<p> The default value of a parameter is computed once when the function is
created, not for every invocation. The "pre-computed" value is then used for every 
subsequent call to the function. Consequently, if you modify the default
value for a parameter this "modified" default value is used for the parameter 
in future calls to the function. This means that the function may not behave as 
expected in future calls and also makes the function more difficult to understand.
</p>

</overview>
<recommendation>
<p>If a parameter has a default value, do not modify the default value. When 
you use a mutable object as a default value, you should use a placeholder value 
instead of modifying the default value. This is a particular problem when you 
work with lists and dictionaries but there are standard methods of avoiding 
modifying the default parameter (see References).</p>

</recommendation>
<example>
<p>In the following example, the <code>default</code> parameter is set with a default 
value of an empty list. Other commands in the function then append values to the 
list. The next time the function is called, the list will contain values, which 
may not have been intended.</p>
<sample src="ModificationOfParameterWithDefault.py" />

<p>The recommended workaround is use a placeholder value. That is, define the 
function with a default of <code>default=None</code>, check if the parameter is 
<code>None</code> and then set the parameter to a list.</p>

</example>
<references>

  <li>Effbot: <a href="https://web.archive.org/web/20201112004749/http://effbot.org/zone/default-values.htm">Default Parameter Values in Python</a>.</li>
  <li>Python Language Reference: <a href="http://docs.python.org/3/reference/compound_stmts.html#function-definitions">Function definitions</a>.</li>


</references>
</qhelp>