Python developers can 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.


The python interpreter will always call these methods with the same number of parameters. Every call to a special method will fail if it is defined with an unexpected number of parameters.


This rule raises an issue when a special method is defined with an unexpected number of parameters.


== Noncompliant Code Example

[source,python]
----
class A:
    def __mul__(self, other, unexpected):  # Noncompliant. Too many parameters
        return 42

    def __add__(self):  # Noncompliant. Missing one parameter
        return 42

A() * 3  # TypeError: __mul__() missing 1 required positional argument: 'unexpected'
A() + 3  # TypeError: __add__() takes 1 positional argument but 2 were given
----


== Compliant Solution

[source,python]
----
class A:
    def __mul__(self, other):
        return 42

    def __add__(self, other):
        return 42

A() * 3
A() + 3
----


== See

* Python Documentation - https://docs.python.org/3/reference/datamodel.html#special-method-names[Special method names]
* Python Documentation - https://docs.python.org/3/library/copy.html[copy module]


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