This rule raises an issue when a property getter, setter or deleter does not have the correct number of arguments.

Why is this an issue?

Creating property accessors and mutators is quite common in Object-Oriented Programming. Python provides two main ways of implementing getters, setters and deleters; either with the @property decorator, or with the property function.

class WithPropertyDecorator(object):
    @property
    def foo(self)
        return self._foo

    @foo.setter
    def foo(self, value):
        self._foo = value

    @foo.deleter
    def foo(self):
        del self._foo

class WithPropertyMethod(object):
    def get_foo(self):
        return self._foo

    def set_foo(self, value):
        self._foo = value

    def del_foo(self):
        del self._foo

    foo = property(get_foo, set_foo, del_foo, "'foo' property.")

The following program illustrates how using the built-in operations will call the custom methods defined above.

with_decorator = WithPropertyDecorator()
with_decorator.foo = 1 # the method defined under @foo.setter will be called.
some_var = with_decorator.foo # the method defined under @foo.getter will be called.
del with_decorator.foo # the method defined under @foo.deleter will be called.


with_method = WithPropertyMethod()
with_method.foo = 1 # the method set_foo will be called.
some_var = with_method.foo # the method get_foo will be called.
del with_method.foo # the method del_foo will be called.

Defining a property this way allows for flexibility when refactoring the implementation of the getters, setters and deleters method, as all the accesses and modifications are done through the Python built-in operators (=,.) and keyword (del).

Property getter, setter and deleter methods are called by the Python interpreter with a specific number of arguments:

• Property getter and deleter methods only require a "self" argument.
• Property setter methods require a "self" argument as well as a value.

Adding any other parameters, or removing these mandatory parameters will throw a TypeError exception at runtime when trying to access or modify the property.

How to fix it

Make sure to specify the correct number of argument for each setter, getter and deleter methods.

Code examples

Noncompliant code example

class A:
    @property
    def foo(self, unexpected, unexpected2):  # Noncompliant: too many parameters.
        return self._foo

    @foo.setter
    def foo(self):  # Noncompliant: a parameter is missing.
        self._foo = 42

    @foo.deleter
    def foo(self, unexpected):  # Noncompliant: too many parameters.
        del self._foo

class B:
    def get_foo(self, unexpected):  # Noncompliant: too many parameters.
        return self._foo

    def set_foo(self, value, unexpected):  # Noncompliant: too many parameters.
        self._foo = value

    def del_foo(self, unexpected):  # Noncompliant: too many parameters.
        del self._foo

    foo = property(get_foo, set_foo, del_foo, "'foo' property.")

Compliant solution

class A:
    @property
    def foo(self):
        return self._foo

    @foo.setter
    def foo(self, value):
        self._foo = value

    @foo.deleter
    def foo(self):
        del self._foo

class B:
    def get_foo(self):
        return self._foo

    def set_foo(self, value):
        self._foo = value

    def del_foo(self):
        del self._foo

    foo = property(get_foo, set_foo, del_foo, "'foo' property.")

Resources

Documentation

• Built-in Functions - property - Python Documentation