Python is a very popular programming language that comes with many pitfalls. This presentation describes some of these pitfalls, especially when they could trick unsuspecting object-oriented developers. It proposes solutions to these pitfalls, in particular regarding inheritance, which is easily broken because of the implementation choice of Python for explicit delegation, its method resolution order, and its use of the C3 algorithm. It discusses some advantages of using Python, especially regarding meta-classes.

1. Yann-Gaël Guéhéneuc
This work is licensed under a Creative
Commons Attribution-NonCommercial-
ShareAlike 3.0 Unported License
Yann-Gaël Guéhéneuc
Python Pitfalls
yann-gael.gueheneuc@concordia.ca
Version 0.9
2024/01/14

2. 2/64
Outline
 All attributes are dynamic
 Everything is a method
 Inheritance is just a suggestion
 Metaclasses always come last

3. 3/64
Outline
 All attributes are dynamic
 Everything is a method
 Inheritance is just a suggestion
 Metaclasses always come last

4. 4/64
All Attributes Are Dynamic
 Python allows the dynamic creation of
attributes

5. 5/64
All Attributes Are Dynamic
class A:
pass
a = A()
print()
print("A.attr = "1"")
A.attr = "1"
print(f"A.attr = {A.attr} (id = {id(A.attr)})")
print(f"a.attr = {a.attr} (id = {id(a.attr)})")
print()
print("a.attr = "2"")
a.attr = "2"
print(f"A.attr = {A.attr} (id = {id(A.attr)})")
print(f"a.attr = {a.attr} (id = {id(a.attr)})")

6. 6/64
All Attributes Are Dynamic
class A:
pass
a = A()
print()
print("A.attr = "1"")
A.attr = "1"
print(f"A.attr = {A.attr} (id = {id(A.attr)})")
print(f"a.attr = {a.attr} (id = {id(a.attr)})")
print()
print("a.attr = "2"")
a.attr = "2"
print(f"A.attr = {A.attr} (id = {id(A.attr)})")
print(f"a.attr = {a.attr} (id = {id(a.attr)})")
A.attr = "1"
A.attr = 1 (id = 140736437823448)
a.attr = 1 (id = 140736437823448)
a.attr = "2"
A.attr = 1 (id = 140736437823448)
a.attr = 2 (id = 140736437823496)

7. 7/64
All Attributes Are Dynamic
 Python automagically ⚙ assign the value
of a class attribute to the instance attribute of
the same name

8. 8/64
All Attributes Are Dynamic
class B:
pass
b = B()
print()
print("b.attr = "1"")
b.attr = "1"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
print(f"B.attr = {B.attr} (id = {id(B.attr)})")
print()
print("b.attr = "1"")
B.attr = "2"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
print(f"B.attr = {B.attr} (id = {id(B.attr)})")

9. 9/64
class B:
pass
b = B()
print()
print("b.attr = "1"")
b.attr = "1"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
try:
print(f"B.attr = {B.attr} (id = {id(B.attr)})")
except:
print("AttributeError: type object 'B' has no attribute 'attr'")
print()
print("b.attr = "1"")
B.attr = "2"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
print(f"B.attr = {B.attr} (id = {id(B.attr)})")

10. 10/64
class B:
pass
b = B()
print()
print("b.attr = "1"")
b.attr = "1"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
try:
print(f"B.attr = {B.attr} (id = {id(B.attr)})")
except:
print("AttributeError: type object 'B' has no attribute 'attr'")
print()
print("b.attr = "1"")
B.attr = "2"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
print(f"B.attr = {B.attr} (id = {id(B.attr)})")
b.attr = "1"
b.attr = 1 (id = 140736437823448)
<What error can it be?>
b.attr = "1"
b.attr = 1 (id = 140736437823448)
B.attr = 2 (id = 140736437823496)

11. 11/64
class B:
pass
b = B()
print()
print("b.attr = "1"")
b.attr = "1"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
try:
print(f"B.attr = {B.attr} (id = {id(B.attr)})")
except:
print("AttributeError: type object 'B' has no attribute 'attr'")
print()
print("b.attr = "1"")
B.attr = "2"
print(f"b.attr = {b.attr} (id = {id(b.attr)})")
print(f"B.attr = {B.attr} (id = {id(B.attr)})")
b.attr = "1"
b.attr = 1 (id = 140736437823448)
AttributeError: type object 'B' has no attribute 'attr'
b.attr = "1"
b.attr = 1 (id = 140736437823448)
B.attr = 2 (id = 140736437823496)

12. 12/64
All Attributes Are Dynamic
 Even popular questions with popular
answers on StackOverflow confuses
class and instance variables!
https://stackoverflow.com/questions/6760685/what-is-the-best-way-of-implementing-singleton-in-python

13. 13/64
All Attributes Are Dynamic
 Read/Write accesses on classes behave as
expected in any other language
 Write accesses on instances behave
differently and shadow the class variable!
https://stackoverflow.com/questions/3434581/how-do-i-set-and-access-attributes-of-a-class
A.a_var1 = "New value for a_var1"
print(f"A.a_var1 = {A.a_var1} (id = {id(A.a_var1)})")
print(f"C.a_var1 = {C.a_var1} (id = {id(C.a_var1)})")
print(f"a1.a_var1 = {a1.a_var1} (id = {id(a1.a_var1)})")
print(f"a2.a_var1 = {a2.a_var1} (id = {id(a2.a_var1)})")
a1.a_var1 = "Another value for a_var1"
print(f"A.a_var1 = {A.a_var1} (id = {id(A.a_var1)})")
print(f"C.a_var1 = {C.a_var1} (id = {id(C.a_var1)})")
print(f"a1.a_var1 = {a1.a_var1} (id = {id(a1.a_var1)})")
print(f"a2.a_var1 = {a2.a_var1} (id = {id(a2.a_var1)})")
A.a_var1 = New value for a_var1 (id = 2238584427760)
C.a_var1 = New value for a_var1 (id = 2238584427760)
a1.a_var1 = New value for a_var1 (id = 2238584427760)
a2.a_var1 = New value for a_var1 (id = 2238584427760)
A.a_var1 = New value for a_var1 (id = 2238584427760)
C.a_var1 = New value for a_var1 (id = 2238584427760)
a1.a_var1 = Another value for a_var1 (id = 2238584286432)
a2.a_var1 = New value for a_var1 (id = 2238584427760)

14. 14/64
All Attributes Are Dynamic
 Read/Write accesses on classes behave as
expected in any other language
 Write accesses on instances behave
differently and shadow the class variable!
https://stackoverflow.com/questions/3434581/how-do-i-set-and-access-attributes-of-a-class
A.a_var1 = "New value for a_var1"
print(f"A.a_var1 = {A.a_var1} (id = {id(A.a_var1)})")
print(f"C.a_var1 = {C.a_var1} (id = {id(C.a_var1)})")
print(f"a1.a_var1 = {a1.a_var1} (id = {id(a1.a_var1)})")
print(f"a2.a_var1 = {a2.a_var1} (id = {id(a2.a_var1)})")
a1.a_var1 = "Another value for a_var1"
print(f"A.a_var1 = {A.a_var1} (id = {id(A.a_var1)})")
print(f"C.a_var1 = {C.a_var1} (id = {id(C.a_var1)})")
print(f"a1.a_var1 = {a1.a_var1} (id = {id(a1.a_var1)})")
print(f"a2.a_var1 = {a2.a_var1} (id = {id(a2.a_var1)})")
A.a_var1 = New value for a_var1 (id = 2238584427760)
C.a_var1 = New value for a_var1 (id = 2238584427760)
a1.a_var1 = New value for a_var1 (id = 2238584427760)
a2.a_var1 = New value for a_var1 (id = 2238584427760)
A.a_var1 = New value for a_var1 (id = 2238584427760)
C.a_var1 = New value for a_var1 (id = 2238584427760)
a1.a_var1 = Another value for a_var1 (id = 2238584286432)
a2.a_var1 = New value for a_var1 (id = 2238584427760)
Same name, but now
an instance variable!

15. 15/64
Outline
 All attributes are dynamic
 Everything is a method
 Inheritance is just a suggestion
 Metaclasses always come last

16. 16/64
Everything Is A Method
 Python includes
– Instance methods
– Class methods
– Static methods

17. 17/64
Everything Is A Method
class A:
def instanceMethod(self):
print(f"A.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"A.classMethod({cls})")
@staticmethod
def staticMethod():
print("A.staticMethod()")
print("On A")
A.instanceMethod(A())
A.classMethod()
A.staticMethod()
print("On a = A()")
a = A()
a.instanceMethod()
a.classMethod()
a.staticMethod()

18. 18/64
Everything Is A Method
class A:
def instanceMethod(self):
print(f"A.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"A.classMethod({cls})")
@staticmethod
def staticMethod():
print("A.staticMethod()")
print("On A")
A.instanceMethod(A())
A.classMethod()
A.staticMethod()
print("On a = A()")
a = A()
a.instanceMethod()
a.classMethod()
a.staticMethod()
On A
A.instanceMethod(<__main__.A object at 0x...>)
A.classMethod(<class '__main__.A'>)
A.staticMethod()
On a = A()
A.instanceMethod(<__main__.A object at 0x...>)
A.classMethod(<class '__main__.A'>)
A.staticMethod()

19. 19/64
Everything Is A Method
class A:
def instanceMethod(self):
print(f"A.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"A.classMethod({cls})")
@staticmethod
def staticMethod():
print("A.staticMethod()")
print("On A")
A.instanceMethod(A())
A.classMethod()
A.staticMethod()
print("On a = A()")
a = A()
a.instanceMethod()
a.classMethod()
a.staticMethod()
On A
A.instanceMethod(<__main__.A object at 0x...>)
A.classMethod(<class '__main__.A'>)
A.staticMethod()
On a = A()
A.instanceMethod(<__main__.A object at 0x...>)
A.classMethod(<class '__main__.A'>)
A.staticMethod()

20. 20/64
class A:
def instanceMethod(self):
print(f"A.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"A.classMethod({cls})")
@staticmethod
def staticMethod():
print("A.staticMethod()")
class B(A):
def instanceMethod(self):
print(f"B.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"B.classMethod({cls})")
@staticmethod
def staticMethod():
print("B.staticMethod()")
print("On B")
B.instanceMethod(B())
B.instanceMethod(A())
B.classMethod()
B.staticMethod()
print("On b = B()")
b = B()
b.instanceMethod()
b.classMethod()
b.staticMethod()

21. 21/64
class A:
def instanceMethod(self):
print(f"A.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"A.classMethod({cls})")
@staticmethod
def staticMethod():
print("A.staticMethod()")
class B(A):
def instanceMethod(self):
print(f"B.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"B.classMethod({cls})")
@staticmethod
def staticMethod():
print("B.staticMethod()")
print("On B")
B.instanceMethod(B())
B.instanceMethod(A())
B.classMethod()
B.staticMethod()
print("On b = B()")
b = B()
b.instanceMethod()
b.classMethod()
b.staticMethod()
On B
B.instanceMethod(<__main__.B object at 0x...>)
B.instanceMethod(<__main__.A object at 0x...>)
B.classMethod(<class '__main__.B'>)
B.staticMethod()
On b = B()
B.instanceMethod(<__main__.B object at 0x...>)
B.classMethod(<class '__main__.B'>)
B.staticMethod()

22. 22/64
class A:
def instanceMethod(self):
print(f"A.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"A.classMethod({cls})")
@staticmethod
def staticMethod():
print("A.staticMethod()")
class B(A):
def instanceMethod(self):
print(f"B.instanceMethod({self})")
@classmethod
def classMethod(cls):
print(f"B.classMethod({cls})")
@staticmethod
def staticMethod():
print("B.staticMethod()")
print("On B")
B.instanceMethod(B())
B.instanceMethod(A())
B.classMethod()
B.staticMethod()
print("On b = B()")
b = B()
b.instanceMethod()
b.classMethod()
b.staticMethod()
On B
B.instanceMethod(<__main__.B object at 0x...>)
B.instanceMethod(<__main__.A object at 0x...>)
B.classMethod(<class '__main__.B'>)
B.staticMethod()
On b = B()
B.instanceMethod(<__main__.B object at 0x...>)
B.classMethod(<class '__main__.B'>)
B.staticMethod()

23. 23/64
Everything Is A Method
 All methods are overloadable
Class methods are methods
Therefore, class methods are overloadable
 Same goes for static methods!
https://en.wikipedia.org/wiki/Syllogism

24. 24/64
Everything Is A Method
 The decorations @classmethod and
@staticmethod are decorators
 The decorations @classmethod and
@staticmethod are about bindings
– Not about the receiver / call site
– Not the object model (i.e., metaclasses)

25. 25/64
Everything Is A Method
class C(A):
def instanceMethod(self):
print(f"C.instanceMethod({self})")
super().instanceMethod()
def classMethod(self):
print(f"C.classMethod({self})")
super().classMethod()
def staticMethod(self):
print("C.staticMethod()")
super().staticMethod()
print("On C")
C.instanceMethod(C())
C.instanceMethod(A())
C.classMethod(C())
C.staticMethod(C())
print("On c = C()")
c = C()
c.instanceMethod()
c.classMethod()
c.staticMethod()

26. 26/64
Everything Is A Method
class C(A):
def instanceMethod(self):
print(f"C.instanceMethod({self})")
super().instanceMethod()
def classMethod(self):
print(f"C.classMethod({self})")
super().classMethod()
def staticMethod(self):
print("C.staticMethod()")
super().staticMethod()
print("On C")
C.instanceMethod(C())
C.instanceMethod(A())
C.classMethod(C())
C.staticMethod(C())
print("On c = C()")
c = C()
c.instanceMethod()
c.classMethod()
c.staticMethod()
No more decorations
All instance methods

27. 27/64
Everything Is A Method
class C(A):
def instanceMethod(self):
print(f"C.instanceMethod({self})")
try:
super().instanceMethod()
except:
print("TypeError: super(type, obj): obj must be an instance or subtype of type")
def classMethod(self):
print(f"C.classMethod({self})")
super().classMethod()
def staticMethod(self):
print("C.staticMethod()")
super().staticMethod()
print("On C")
C.instanceMethod(C())
C.instanceMethod(A())
C.classMethod(C())
C.staticMethod(C())
print("On c = C()")
c = C()
c.instanceMethod()
c.classMethod()
c.staticMethod()

28. 28/64
Everything Is A Method
class C(A):
def instanceMethod(self):
print(f"C.instanceMethod({self})")
try:
super().instanceMethod()
except:
print("TypeError: super(type, obj): obj must be an instance or subtype of type")
def classMethod(self):
print(f"C.classMethod({self})")
super().classMethod()
def staticMethod(self):
print("C.staticMethod()")
super().staticMethod()
print("On C")
C.instanceMethod(C())
C.instanceMethod(A())
C.classMethod(C())
C.staticMethod(C())
print("On c = C()")
c = C()
c.instanceMethod()
c.classMethod()
c.staticMethod()
On C
C.instanceMethod(<__main__.C object at 0x...>)
A.instanceMethod(<__main__.C object at 0x...>)
C.instanceMethod(<__main__.A object at 0x...>)
<What error can it be?>
C.classMethod(<__main__.C object at 0x...>)
A.classMethod(<class '__main__.C'>)
C.staticMethod()
A.staticMethod()
On c = C()
C.instanceMethod(<__main__.C object at 0x...>)
A.instanceMethod(<__main__.C object at 0x...>)
C.classMethod(<__main__.C object at 0x...>)
A.classMethod(<class '__main__.C'>)
C.staticMethod()
A.staticMethod()

29. 29/64
Everything Is A Method
class C(A):
def instanceMethod(self):
print(f"C.instanceMethod({self})")
try:
super().instanceMethod()
except:
print("TypeError: super(type, obj): obj must be an instance or subtype of type")
def classMethod(self):
print(f"C.classMethod({self})")
super().classMethod()
def staticMethod(self):
print("C.staticMethod()")
super().staticMethod()
print("On C")
C.instanceMethod(C())
C.instanceMethod(A())
C.classMethod(C())
C.staticMethod(C())
print("On c = C()")
c = C()
c.instanceMethod()
c.classMethod()
c.staticMethod()
On C
C.instanceMethod(<__main__.C object at 0x...>)
A.instanceMethod(<__main__.C object at 0x...>)
C.instanceMethod(<__main__.A object at 0x...>)
TypeError: super(type, obj): obj must be an…
C.classMethod(<__main__.C object at 0x...>)
A.classMethod(<class '__main__.C'>)
C.staticMethod()
A.staticMethod()
On c = C()
C.instanceMethod(<__main__.C object at 0x...>)
A.instanceMethod(<__main__.C object at 0x...>)
C.classMethod(<__main__.C object at 0x...>)
A.classMethod(<class '__main__.C'>)
C.staticMethod()
A.staticMethod()

30. 30/64
Everything Is A Method
 Python 3 super() is equivalent to Python 2
super(__class__, <firstarg>)
– “where __class__ is the class [in which] the
method was defined, and <firstarg> is the first
parameter of the method (normally self for
instance methods, and cls for class methods).”
 Contravariance on <firstarg>
– Obviously! ᦓ
https://peps.python.org/pep-3135/

31. 31/64
Everything Is A Method
 Contravariance on <firstarg>
class C(A):
...
class D(C):
pass
print("On C")
C.instanceMethod(C())
C.instanceMethod(D())
C.classMethod(C())
C.staticMethod(C())
print("On d = D()")
d = D()
d.instanceMethod()
d.classMethod()
d.staticMethod()

32. 32/64
Everything Is A Method
 Contravariance on <firstarg>
class C(A):
...
class D(C):
pass
print("On C")
C.instanceMethod(C())
C.instanceMethod(D())
C.classMethod(C())
C.staticMethod(C())
print("On d = D()")
d = D()
d.instanceMethod()
d.classMethod()
d.staticMethod()
On C
C.instanceMethod(<__main__.C object at 0x...>)
A.instanceMethod(<__main__.C object at 0x...>)
C.instanceMethod(<__main__.D object at 0x...>)
A.instanceMethod(<__main__.D object at 0x...>)
C.classMethod(<__main__.C object at 0x...>)
A.classMethod(<class '__main__.C'>)
C.staticMethod()
A.staticMethod()
On d = D()
C.instanceMethod(<__main__.D object at 0x...>)
A.instanceMethod(<__main__.D object at 0x...>)
C.classMethod(<__main__.D object at 0x...>)
A.classMethod(<class '__main__.D'>)
C.staticMethod()
A.staticMethod()

33. 33/64
Outline
 All attributes are dynamic
 Everything is a method
 Inheritance is just a suggestion
 Metaclasses always come last

34. 34/64
Inheritance Is Just A Suggestion
 Java
– “[The] super keyword is used to access methods of the parent class
while this is used to access methods of the current class.”
 Smalltalk
– “self is used when an object wishes to refer to itself, and super is
used to refer to the superclass of the object.”
 C++
– “this is a keyword that refers to the current instance of the class.”
– There is no super keyword in (standard) C++
 Python
– “self is a reference to the object instance […]. super allows you to
access attributes (methods, members, etc.) of an ancestor type.”
https://www.geeksforgeeks.org/super-and-this-keywords-in-java/
https://courses.cs.washington.edu/courses/cse505/99au/oo/smalltalk-concepts.html
https://www.javatpoint.com/cpp-this-pointer
https://stackoverflow.com/questions/72705781/difference-between-self-and-super

35. 35/64
Inheritance Is Just A Suggestion
 Single inheritance
– Java
– Smalltalk
 super refers to the (direct) superclass of a
class so an object can access the methods
and fields of the superclass of its class

36. 36/64
Inheritance Is Just A Suggestion
 Multiple inheritance
– C++
– Python
 Two different approaches
– C++ Ἢ
– Python ⏏

37. 37/64
Inheritance Is Just
A Suggestion
 C++ Ἢ
– virtual keyword in
base clause
– Base initialisation in the
member initializer list
class Object {
public:
Object(int c) {
printf("Objectn");
a = 0;
}
int a;
};
class Object1 : public virtual Object {
public:
Object1(int c) : Object(c) {
printf("Object1n");
a1 = 0;
a = 1;
}
int a1;
};
class Object2 : public virtual Object {
public:
Object2(int c) : Object(c) {
printf("Object2n");
a2= 0;
a = 2;
}
int a2;
};
class Object4 : public Object1, public Object2 { public:
Object4(int c) : Object(c), Object1(c), Object2(c) {
printf("Object4n");
a4 = 0;
a = 4;
}
int a4;
};
https://cplusplus.com/forum/general/1414/
https://cplusplus.com/forum/general/1420/

38. 38/64
Inheritance Is Just
A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
https://dl.acm.org/doi/10.1145/236337.236343
https://www.python.org/download/releases/2.3/mro/
class A:
def __init__(self):
print("A")
super().__init__()
def output(self):
print("A.output()")
class B(A):
def __init__(self):
print("B")
super().__init__()
def output(self):
print("B.output()")
super().output()
class C(B):
def __init__(self):
print("C")
super().__init__()
def output(self):
print("C.output()")
super().output()
class D(A):
def __init__(self):
print("D")
super().__init__()
def output(self):
print("D.output()")
super().output()
class E(C, D):
def __init__(self):
print("E")
super().__init__()
def output(self):
print("E.output()")
super().output()

39. 39/64
Inheritance Is Just
A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
https://dl.acm.org/doi/10.1145/236337.236343
https://www.python.org/download/releases/2.3/mro/
class A:
def __init__(self):
print("A")
super().__init__()
def output(self):
print("A.output()")
class B(A):
def __init__(self):
print("B")
super().__init__()
def output(self):
print("B.output()")
super().output()
class C(B):
def __init__(self):
print("C")
super().__init__()
def output(self):
print("C.output()")
super().output()
class D(A):
def __init__(self):
print("D")
super().__init__()
def output(self):
print("D.output()")
super().output()
class E(C, D):
def __init__(self):
print("E")
super().__init__()
def output(self):
print("E.output()")
super().output()
e = E()
e.output()

40. 40/64
Inheritance Is Just
A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
https://dl.acm.org/doi/10.1145/236337.236343
https://www.python.org/download/releases/2.3/mro/
class A:
def __init__(self):
print("A")
super().__init__()
def output(self):
print("A.output()")
class B(A):
def __init__(self):
print("B")
super().__init__()
def output(self):
print("B.output()")
super().output()
class C(B):
def __init__(self):
print("C")
super().__init__()
def output(self):
print("C.output()")
super().output()
class D(A):
def __init__(self):
print("D")
super().__init__()
def output(self):
print("D.output()")
super().output()
class E(C, D):
def __init__(self):
print("E")
super().__init__()
def output(self):
print("E.output()")
super().output()
e = E()
e.output()
E
C
B
D
A
E.output()
C.output()
B.output()
D.output()
A.output()

41. 41/64
Inheritance Is Just A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
print(E.mro())
print(D.mro())
print(C.mro())
print(B.mro())
print(A.mro())
[<class '__main__.E'>, <class ‘….C'>, <class ‘….B'>, <class ‘….D'>, <class ‘….A'>, <class 'object'>]
[<class '__main__.D'>, <class '__main__.A'>, <class 'object'>]
[<class '__main__.C'>, <class '__main__.B'>, <class '__main__.A'>, <class 'object'>]
[<class '__main__.B'>, <class '__main__.A'>, <class 'object'>]
[<class '__main__.A'>, <class 'object'>]

42. 42/64
Inheritance Is Just
A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
class A:
def __init__(self):
print("A")
super().__init__()
def output(self):
print("A.output()")
class B(A):
def __init__(self):
print("B")
super().__init__()
def output(self):
print("B.output()")
super().output()
class C(B):
def __init__(self):
print("C")
super().__init__()
def output(self):
print("C.output()")
super().output()
class D(A):
def __init__(self):
print("D")
super().__init__()
def output(self):
print("D.output()")
super().output()
class E(C, D):
def __init__(self):
print("E")
super().__init__()
def output(self):
print("E.output()")
super().output()

43. 43/64
Inheritance Is Just
A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
e = E()
e.output()
class A:
def __init__(self):
print("A")
super().__init__()
def output(self):
print("A.output()")
class B(A):
def __init__(self):
print("B")
super().__init__()
def output(self):
print("B.output()")
super().output()
class C(B):
def __init__(self):
print("C")
super().__init__()
def output(self):
print("C.output()")
super().output()
class D(A):
def __init__(self):
print("D")
super().__init__()
def output(self):
print("D.output()")
super().output()
class E(C, D):
def __init__(self):
print("E")
super().__init__()
def output(self):
print("E.output()")
super().output()

44. 44/64
Inheritance Is Just
A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
e = E()
e.output()
class A:
def __init__(self):
print("A")
super().__init__()
def output(self):
print("A.output()")
class B(A):
def __init__(self):
print("B")
super().__init__()
def output(self):
print("B.output()")
super().output()
class C(B):
def __init__(self):
print("C")
super().__init__()
def output(self):
print("C.output()")
super().output()
class D(A):
def __init__(self):
print("D")
super().__init__()
def output(self):
print("D.output()")
super().output()
class E(C, D):
def __init__(self):
print("E")
super().__init__()
def output(self):
print("E.output()")
super().output()
E
C
B
D
A
E.output()
C.output()
B.output()
D.output()
A.output()

45. 45/64
Inheritance Is Just A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
https://news.ycombinator.com/item?id=24255334
class A(object):
def __init__(self):
self.x = 1
class B(object):
def __init__(self):
self.y = 2
class C(A, B):
pass
print(C().y)

46. 46/64
Inheritance Is Just A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
https://news.ycombinator.com/item?id=24255334
class A(object):
def __init__(self):
self.x = 1
class B(object):
def __init__(self):
self.y = 2
class C(A, B):
pass
print(C().y)
Traceback (most recent call last):
File “…Inheritance2.py", line 14, in <module>
print(C().y)
^^^^^
AttributeError: 'C' object has no attribute 'y'

47. 47/64
Inheritance Is Just A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
https://news.ycombinator.com/item?id=24255334
class A(object):
def __init__(self):
super().__init__()
self.x = 1
class B(object):
def __init__(self):
self.y = 2
class C(A, B):
pass
print(C().y)

48. 48/64
Inheritance Is Just A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
https://news.ycombinator.com/item?id=24255334
class A(object):
def __init__(self):
super().__init__()
self.x = 1
class B(object):
def __init__(self):
self.y = 2
class C(A, B):
pass
print(C().y)
2

49. 49/64
Inheritance Is Just A Suggestion
 Python ⏏
– Method Resolution Order
• C3 algorithm
 Breaks (at least) two principles
– Principle of least astonishment / surprise
– Principle of locality

50. 50/64
Inheritance Is Just A Suggestion
 Principle of least astonishment / surprise
– “Transparency is a passive quality. A program is
transparent when it is possible to form a simple
mental model of its behavior that is actually
predictive for all or most cases, because you
can see through the machinery to what is
actually going on.” – Eric Raymond
(Emphasis mine)
https://wiki.c2.com/?PrincipleOfLeastAstonishment

51. 51/64
Inheritance Is Just A Suggestion
 Principle of locality
– “[A]n error is local in time if it is discovered very
soon after it is created; an error is local in
space if it is identified very close (at) the site
where the error actually resides.”
(Emphasis mine)
https://beza1e1.tuxen.de/articles/principle_of_locality.html
https://wiki.c2.com/?CeeVsAdaStudy

52. 52/64
Inheritance Is Just A Suggestion
https://stackoverflow.com/questions/3277367/how-does-pythons-super-work-with-multiple-inheritance

53. 53/64
Inheritance Is Just A Suggestion
https://stackoverflow.com/questions/3277367/how-does-pythons-super-work-with-multiple-inheritance
Inheritance in Python involves explicit
delegations and an obscure algorithm

54. 54/64
Inheritance Is Just A Suggestion
https://stackoverflow.com/questions/3277367/how-does-pythons-super-work-with-multiple-inheritance
Inheritance in Python involves explicit
delegations and an obscure algorithm
Exercise utmost caution

55. 55/64
Outline
 All attributes are dynamic
 Everything is a method
 Inheritance is just a suggestion
 Metaclasses always come last

56. 56/64
Metaclasses Always Come Last
https://blog.invisivel.net/2012/04/10/pythons-object-model-explained/

57. 57/64
Metaclasses
Always
Come
Last

58. 58/64
Metaclasses Always Come Last
 Caveats
– Cannot have both a class and an instance
__new__() method in the same class
– The class Object defines a static (à la Python)
__new__() method that hides any __new__()
method from a metaclass

59. 59/64
Metaclasses Always Come Last
 Very promising…
…But limited by dynamicity of Python ⠦
 Workaround with __call__()

60. 60/64
Metaclasses Always Come Last
 Class creation
– __new__() instantiates a
class
– __init__() initialises
variables
– __prepare__() defines
the class namespace
passed to the metaclass
__new__ and __init__
methods
 Instance creation
– __call__() invoked
after the __new__ and
__init__
– Only because classes
are callable objects
• Instance of a class with a
__call__ method
• Anything with a non-null
tp_call (C struct)
https://elfi-y.medium.com/python-metaclass-7cb56510845
https://stackoverflow.com/questions/111234/what-is-a-callable

61. 61/64
Metaclasses Always Come Last
 Using __call__() for reference counting
class ReferenceCountingMetaClass(type):
def __init__(self, name, bases, namespace):
self._instances = 0
def __call__(self):
newInstance = super().__call__()
self._instances = self._instances + 1
return newInstance
def getNumberOfInstances(self):
return self._instances

62. 62/64
Metaclasses Always Come Last
 Using __call__() for reference counting
class ReferenceCountingMetaClass(type):
def __init__(self, name, bases, namespace):
self._instances = 0
def __call__(self):
newInstance = super().__call__()
self._instances = self._instances + 1
return newInstance
def getNumberOfInstances(self):
return self._instances
Override the __call__ metaclass instance method
Define the get…() metaclass instance method

63. 63/64
Metaclasses Always Come Last
 Using __call__() for reference counting
class C(metaclass=ReferenceCountingMetaClass):
pass
class D():
pass
x = C()
print(C.getNumberOfInstances())
y = C()
print(C.getNumberOfInstances())
z = C()
print(C.getNumberOfInstances())
x = D()
y = D()

64. 64/64
Metaclasses Always Come Last
 Using __call__() for reference counting
class C(metaclass=ReferenceCountingMetaClass):
pass
class D():
pass
x = C()
print(C.getNumberOfInstances())
y = C()
print(C.getNumberOfInstances())
z = C()
print(C.getNumberOfInstances())
x = D()
y = D()
1
2
3