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Property in old-style class

ID: py/property-in-old-style-class
Kind: problem
Security severity: 
Severity: error
Precision: very-high
Tags:
   - portability
   - correctness
Query suites:
   - python-security-and-quality.qls

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Property descriptors are only supported for the new-style classes that were introduced in Python 2.1. Property descriptors should only be used in new-style classes.

Recommendation

If you want to define properties in a class, then ensure that the class is a new-style class. You can convert an old-style class to a new-style class by inheriting from object.

Example

In the following example all the classes attempt to set a property for x. However, only the third and fourth classes are new-style classes. Consequently, the x property is only available for the NewStyle and InheritNewStyle classes.

If you define the OldStyle class as inheriting from a new-style class, then the x property would be available for both the OldStyle and InheritOldStyle classes.


class OldStyle:

    def __init__(self, x):
        self._x = x

    # Incorrect: 'OldStyle' is not a new-style class and '@property' is not supported
    @property
    def x(self):
        return self._x


class InheritOldStyle(OldStyle):

    def __init__(self, x):
        self._x = x

    # Incorrect: 'InheritOldStyle' is not a new-style class and '@property' is not supported
    @property
    def x(self):
        return self._x


class NewStyle(object):

    def __init__(self, x):
        self._x = x

    # Correct: 'NewStyle' is a new-style class and '@property' is supported
    @property
    def x(self):
        return self._x

class InheritNewStyle(NewStyle):

    def __init__(self, x):
        self._x = x

    # Correct: 'InheritNewStyle' inherits from a new-style class and '@property' is supported
    @property
    def x(self):
        return self._x

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