84 lines
2.6 KiB
Plaintext
84 lines
2.6 KiB
Plaintext
This rule raises an issue when a special method is defined with an unexpected number of parameters.
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== Why is this an issue?
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Python allows developers to customize how code is interpreted by defining special methods (also called magic methods). For example, it is possible to override how the multiplication operator (``++a * b++``) will apply to instances of a class by defining in this class the ``++__mul__++`` and ``++__rmul__++`` methods. Whenever a multiplication operation is performed with this class, the Python interpreter will call one of these methods instead of performing the default multiplication.
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Each special method expects a specific number of parameters. The Python interpreter will call these methods with those parameters. Calls to a special method will throw a ``++TypeError++`` if it is defined with an incorrect number of parameters.
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== How to fix it
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Make sure to use the same signature defined in the Python documentation for each special methods.
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=== Code examples
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==== Noncompliant code example
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[source,python,diff-id=1,diff-type=noncompliant]
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----
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class A:
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def __mul__(self, other, unexpected): # Noncompliant: too many parameters
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return 42
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def __add__(self): # Noncompliant: missing one parameter
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return 42
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A() * 3 # TypeError: __mul__() missing 1 required positional argument: 'unexpected'
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A() + 3 # TypeError: __add__() takes 1 positional argument but 2 were given
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----
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==== Compliant solution
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[source,python,diff-id=1,diff-type=compliant]
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----
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class A:
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def __mul__(self, other):
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return 42
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def __add__(self, other):
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return 42
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A() * 3
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A() + 3
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----
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== Resources
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=== Documentation
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* https://docs.python.org/3/reference/datamodel.html#special-method-names[Special method names] - Python special methods
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* https://docs.python.org/3/library/copy.html[Copy module] - Documentation of ``++__copy__++`` and ``++__deepcopy__++``
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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=== Message
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* Add XXX parameters. Method YYY should have ZZZ parameters.
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* Remove XXX parameters. Method YYY should have ZZZ parameters.
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=== Highlighting
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Primary: The method signature.
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Secondary: The unexpected parameters if there are too many parameters.
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'''
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== Comments And Links
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(visible only on this page)
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=== deprecates: S2733
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=== on 11 Feb 2020, 17:22:41 Nicolas Harraudeau wrote:
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Special methods which are out of scope for this rule: ++__new__++, ++__init__++, ++__call__++
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These methods have no maximum number of parameters and require at minimum a "self" parameter. Missing a "self" parameter is already covered by RSPEC-5720.
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endif::env-github,rspecator-view[]
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