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Multiple calls to __init__ during object initialization

ID: py/multiple-calls-to-init
Kind: problem
Security severity: 
Severity: warning
Precision: very-high
Tags:
   - reliability
   - correctness
Query suites:
   - python-security-and-quality.qls

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Python, unlike statically typed languages such as Java, allows complete freedom when calling methods during object initialization. However, standard object-oriented principles apply to Python classes using deep inheritance hierarchies. Therefore the developer has responsibility for ensuring that objects are properly initialized when there are multiple __init__ methods that need to be called.

Calling an __init__ method more than once during object initialization risks the object being incorrectly initialized. It is unlikely that the relevant __init__ method is designed to be called more than once.

There are a number of ways that an __init__ method may be be called more than once.

  • There may be more than one explicit call to the method in the hierarchy of __init__ methods.

  • A class using multiple inheritance directly calls the __init__ methods of its base types. One or more of those base types uses super() to pass down the inheritance chain.

Recommendation

Either be careful not to explicitly call an __init__ method more than once, or use super() throughout the inheritance hierarchy.

Alternatively refactor one or more of the classes to use composition rather than inheritance.

Example

In the first example, explicit calls to __init__ are used, but SportsCar erroneously calls both Vehicle.__init__ and Car.__init__. This can be fixed by removing the call to Vehicle.__init__, as shown in FixedSportsCar.

#Calling a method multiple times by using explicit calls when a base inherits from other base
class Vehicle(object):
    
    def __init__(self):
        self.mobile = True
        
class Car(Vehicle):
    
    def __init__(self):
        Vehicle.__init__(self)
        self.car_init()
        
    def car_init(self):
        pass
    
class SportsCar(Car, Vehicle):
    
    # Vehicle.__init__ will get called twice
    def __init__(self):
        Vehicle.__init__(self)
        Car.__init__(self)
        self.sports_car_init()
        
    def sports_car_init(self):
        pass
        
#Fix SportsCar by only calling Car.__init__
class FixedSportsCar(Car, Vehicle):
    
    def __init__(self):
        Car.__init__(self)
        self.sports_car_init()
        
    def sports_car_init(self):
        pass

In the second example, there is a mixture of explicit calls to __init__ and calls using super(). To fix this example, super() should be used throughout.


#Calling a method multiple times by using explicit calls when a base uses super()
class Vehicle(object):
     
    def __init__(self):
        super(Vehicle, self).__init__()
        self.mobile = True
        
class Car(Vehicle):
    
    def __init__(self):
        super(Car, self).__init__()
        self.car_init()
        
    def car_init(self):
        pass
        
class SportsCar(Car, Vehicle):
    
    # Vehicle.__init__ will get called twice
    def __init__(self):
        Vehicle.__init__(self)
        Car.__init__(self)
        self.sports_car_init()
        
    def sports_car_init(self):
        pass
        
#Fix SportsCar by using super()
class FixedSportsCar(Car, Vehicle):
    
    def __init__(self):
        super(SportsCar, self).__init__()
        self.sports_car_init()
        
    def sports_car_init(self):
        pass

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