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49 lines
2.7 KiB
XML
49 lines
2.7 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>
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When initializing an instance of the class in the class' <code>__init__</code> method, calls tha are made using the instance may receive an instance of the class that is not
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yet fully initialized. When a method called in an initializer is overridden in a subclass, the subclass method receives the instance
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in a potentially unexpected state. Fields that would be initialized after the call, including potentially in the subclass' <code>__init__</code> method,
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will not be initialized. This may lead to runtime errors, as well as make the code more difficult to maintain, as future changes may not
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be aware of which fields would not be initialized.
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</p>
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</overview>
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<recommendation>
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<p>If possible, refactor the initializer method such that initialization is complete before calling any overridden methods.
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For helper methods used as part of initialization, avoid overriding them, and instead call any additional logic required
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in the subclass' <code>__init__</code> method.
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</p>
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<p>
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If the overridden method does not depend on the instance <code>self</code>, and only on its class, consider making it a <code>@classmethod</code> or <code>@staticmethod</code> instead.
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</p>
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<p>
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If calling an overridden method is absolutely required, consider marking it as an internal method (by using an <code>_</code> prefix) to
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discourage external users of the library from overriding it and observing partially initialized state, and ensure that the fact it is called during initialization
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is mentioned in the documentation.
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</p>
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</recommendation>
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<example>
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<p>In the following case, the <code>__init__</code> method of <code>Super</code> calls the <code>set_up</code> method that is overridden by <code>Sub</code>.
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This results in <code>Sub.set_up</code> being called with a partially initialized instance of <code>Super</code> which may be unexpected. </p>
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<sample src="examples/InitCallsSubclassMethodBad.py" />
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<p>In the following case, the initialization methods are separate between the superclass and the subclass.</p>
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<sample src="examples/InitCallsSubclassMethodGood.py" />
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</example>
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<references>
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<li>CERT Secure Coding: <a href="https://www.securecoding.cert.org/confluence/display/java/MET05-J.+Ensure+that+constructors+do+not+call+overridable+methods">
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Rule MET05-J</a>. Reference discusses Java but is applicable to object oriented programming in many languages.</li>
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<li>StackOverflow: <a href="https://stackoverflow.com/questions/3404301/whats-wrong-with-overridable-method-calls-in-constructors">Overridable method calls in constructors</a>.</li>
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<li>Python documentation: <a href="https://docs.python.org/3/library/functions.html#classmethod">@classmethod</a>.</li>
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</references>
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</qhelp>
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