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
Click to see the query in the CodeQL repository
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
References¶
Python Glossary: New-style class.
Python Language Reference: New-style and classic classes, Descriptors.
Python Standard Library: Property.
The History of Python: Inside story on new-style classes.