pyohio magic methods

Slides for my presentation at PyOhio 2019.

Transcript

1. Magic of Python Darshan Markandaiah 1
2. Agenda 2
3. Magic Methods? 3
4. class User: def __init__(self, user_name): self.user_name = user_name 4
5. Groups of Magic Methods 4 Define Custom Behaviour 4 Enable Operator Overloading 4 Emulate Built in types 5
6. Sample Code class Cart: def __init__(self, user_id): self.user_id = user_id # mapping of item_name -> Item object self.cart_items = dict() def add_item(self, name): self.cart_items[name] = Item(name) class Item: def __init__(self, name): self.name = name 6
7. >>> cart_1 = Cart("user_id_1") >>> cart_1.add_item("milk") >>> print(cart_1) <__main__.Cart object at 0x10c7f6ef0> >>> print(cart_1.cart_items) {'milk': <__main__.Item object at 0x10944b940>} 7
8. Define Custom Behavior Define Custom Behavior 8
9. __repr__ Define Custom Behavior 9
10. class Cart: . . def __repr_(self): return f"Cart('{self.user_id}')" . . Define Custom Behavior 10
11. >>> cart_1 = Cart("user_id_1") >>> cart_1.add_item("milk") >>> print(cart_1) Cart('user_id_1') >>> "String Formatting: {0}".format(cart_1) 'String Formatting: Cart of (user_id_1)' Define Custom Behavior 11
12. __bool__ Define Custom Behavior 12
13. class Cart: . . def __bool__(self): return len(self.cart_items) > 0 . . Define Custom Behavior 13
14. >>> cart_1 = Cart("user_id_1") >>> cart_1.add_item("milk") >>> if cart_1: print("Cart contains at least 1 item.") else: print("Cart is empty.") "Cart contains at least 1 item." Define Custom Behavior 14
15. Method Invocation __hash__ hash(object) __str__ str(object) __del__ del object* Define Custom Behavior; *: called when reference count reaches 0 15
16. Enable Operator Overloading Enable Operator Overloading 16
17. __gt__ Enable Operator Overloading 17
18. class Cart: . . def __gt__(self, other): return len(self.cart_items) > len(other.cart_items) . . Enable Operator Overloading 18
19. >>> cart_1 = Cart("user_id_1") >>> cart_2 = Cart("user_id_2") >>> cart_1.add_item("milk") >>> cart_2.add_item("milk") >>> cart_1 > cart_2 False >>> cart_1.add_item("eggs") >>> cart_1 > cart_2 True Enable Operator Overloading 19
20. Other Comparison Methods Method Invocation __ge__ self >= other __lt__ self < other __le__ self <= other __eq__ self == other __ne__ self != other Enable Operator Overloading 20
21. @functools.total_ordering Enable Operator Overloading 21
22. import functools @functools.total_ordering class Cart: . . def __gt__(self, other): return len(self.cart_items) > len(other.cart_items) def __eq__(self, other): return len(self.cart_items) == len(other.cart_items) . . Enable Operator Overloading 22
23. >>> cart_1 = Cart("user_id_1") >>> cart_2 = Cart("user_id_2") >>> cart_1.add_item("milk") >>> cart_2.add_item("milk") >>> cart_1 < cart_2 False >>> cart_1 >= cart_2 True Enable Operator Overloading 23
24. __add__ Enable Operator Overloading 24
25. class Cart: . . def __add__(self, other): new_cart = Cart(self.user_id) for item in self.cart_items: new_cart.add_item(item) for item in other.cart_items: new_cart.add_item(item) return new_cart . . Enable Operator Overloading 25
26. >>> cart_1 = Cart("user_id_1") >>> cart_2 = Cart("user_id_2") >>> cart_1.add_item("milk") >>> cart_2.add_item("eggs") >>> cart_3 = cart_1 + cart_2 >>> for item in cart_3.cart_items: print(item) 'milk' 'eggs' Enable Operator Overloading 26
27. Other arithmatic magic methods Method Invocation __sub__ self - other __mul__ self * other __div__ self / other __pow__ self ** other Enable Operator Overloading 27
28. Reflected numerical magic methods Method Invocation __radd__ other + self __rsub__ other - self __rmul__ other * self __rdiv__ other / self __rpow__ other ** self Enable Operator Overloading 28
29. Unary Operations Method Invocation __abs__ abs(self) __neg__ -self __pos__ +self Enable Operator Overloading 29
30. and others found in the documentation... Enable Operator Overloading 30
31. Emulate Container Types Emulate Container Types 31
32. __len__ Emulate Container Types 32
33. class Cart: . . def __len__(self): return len(self.cart_items) . . Define Custom Behavior 33
34. >>> cart_1 = Cart("user_id_1") >>> cart_1.add_item("milk") >>> cart_1.add_item("eggs") >>> len(cart_1) 2 Define Custom Behavior 34
35. __contains__ Emulate Container Types 35
36. class Cart: . . def __contains__(self, key): return key in self.cart_items . . Emulate Container Types 36
37. >>> cart_1 = Cart("user_id_1") >>> cart_1.add_item("milk") >>> cart_1.add_item("eggs") >>> print("eggs" in cart_1) True >>> print("cheese" in cart_1) False Emulate Container Types 37
38. __iter__ Emulate Container Types 38
39. class Cart: . . def __iter__(self): for item in self.cart_items: yield item . . Emulate Container Types 39
40. >>> cart_1 = Cart("user_id_1") >>> cart_1.add_item("milk") >>> cart_1.add_item("eggs") >>> for item in cart_1: print(item) 'milk' 'eggs' Emulate Container Types 40
41. __getitem__ Emulate Container Types 41
42. class Cart: . . def __getitem__(self, key): return self.cart_items[key] . . Emulate Container Types 42
43. cart_1 = Cart("user_id_1") cart_1.add_item("milk") cart_1.add_item("eggs") cart_1["milk"] Item(milk) Emulate Container Types 43
44. Related Magic Methods Method Name Similar __setitem__ self[key] = value __delitem__ del(self[key]) Emulate Container Types 44
45. and others found in the documentation... 45
46. collections.abc 46
47. Use Case: Checking for interfaces 47
48. >>> from collections import abc >>> isinstance(list(), abc.Sequence) True >>> isinstance(dict(), abc.Mapping) True >>> isinstance(list(), abc.Hashable) False >>> isinstance(str(), abc.Hashable) True 48
49. Use Case: Offer Free Mixin Methods 49
50. from collections import abc class Cart(abc.Mapping): . . def __iter__(self): for key in self.cart_items: yield key def __len__(self): return len(self.cart_items) def __getitem__(self, key): return self.cart_items[key] . . 50
51. >>> cart_1 = Cart("user_id_1") >>> cart_1.add_item("milk") >>> cart_1.add_item("eggs") >>> list(cart_1.values()) [Item(milk), Item(eggs)] >>> print('milk' in cart_1) True 51
52. other ABCs that offer free mixin methods 52
53. Context Managers 53
54. file_handle = open("/path/to/file") try: content = file_handle.read() finally: file_handle.close() with open("/path/to/file") as file_handle: content = file_handle.read() 54
55. Defining your own Context Managers 4 __enter__ 4 __exit__ 55
56. class FileReader: def __init__(self, filename): self.filename = filename def __enter__(self): self.file_handle = open(self.filename) return self.file_handle def __exit__(self, exc_type, exc_value, traceback): if self.file_handle: self.file_handle.close() with FileReader("/path/to/file") as file_handle: content = file_handle.read() 56
57. @contextlib.contextmanager 57
58. Resources 4 Python Data Model Documentation 4 Python collections module documentation 4 A guide to Python's Magic Methods 58
59. Thank You! 59