PyConZA 2015: "What's the point of Object Orientation?" by Iwan Vosloo

Object Orientation (OO) is often introduced in terms of how it is implemented by a specific language. However, understanding the theory underlying OO is not quite the same as understanding how OO concepts are supported by a particular language. It is insightful to understand the simple OO fundamentals and how these map to the particular implementation provided by Python.

In this talk I will first explain the very basics of OO from a language-neutral point of view with the aim of showing what OO can offer you. I will touch upon the simple mathematical theory underlying OO and how it can be used as a mental discipline to improve your natural capacity to reason about programs. I hope to give you enough information to help you distinguish between better and worse designs and to detect whether you're using OO as it was intended. I will show how these fundamentals map to Python, and compare the difference of Python's implementation to that of some other languages (even functional languages).

This talk is for anyone: whether you're new at Object Orientation, or a practitioner wondering whether OO is worth the effort you've spent trying to use it.

Transcript

1. PyConZA 2015 What’s the point of Object Orientation? Iwan Vosloo
2. PyConZA 2015 Introduction OOP: a part of OO Supporting mechanisms for OO Conceptual model of a problem domain Programming language map to Code
3. PyConZA 2015 Introduction Cause and effect
4. PyConZA 2015 Introduction Scaling up Courtesy: inkwellideas.com 100 lines of code?
5. PyConZA 2015 Introduction How about... 180 000 lines of code?
6. PyConZA 2015 Introduction Understanding opportunity useless information danger
7. PyConZA 2015 Concepts A conceptual model Chair
8. PyConZA 2015 Concepts Concepts as sets Chair Desk
9. PyConZA 2015 Concepts Refinement via subsets Chair Office Chair
10. PyConZA 2015 Relations Connecting objects Chair Desk
11. PyConZA 2015 Relations Relation as a concept Assignment
12. PyConZA 2015 Relations Multiplicity Chair Desk
13. PyConZA 2015 Notation It gets complicated... Chair Desk Assignment Office Chair
14. PyConZA 2015 Notation UML Desk Chair OfficeChair assignment 1 * Warning !!
15. PyConZA 2015 More on concepts Intangible concepts Portfolio Investment Instrument UnitTrust
16. PyConZA 2015 More on concepts Overlapping concepts Person Investor
17. PyConZA 2015 More on concepts Changing classification Person Employee Investor
18. PyConZA 2015 More on concepts Concept labels Concept Synonym Type Concept Label Chair
19. PyConZA 2015 Affecting objects Operations Price PriceFile UnitTrust Fund * load_prices( )
20. PyConZA 2015 Affecting objects Methods Operation Object 1..* operates on Method 1..* 1 is implemented by
21. PyConZA 2015 Summary OO concepts Object classifies Type 0..* subtype 0..* Method implemented by 1..* Relation links 2..* Operation 1..* operates on
22. PyConZA 2015 How can this help? Programs are programs Courtesy: inkwellideas.com Concept Concept
23. PyConZA 2015 How can this help? Understanding-structured Courtesy: inkwellideas.com parse_file: Operation parse_exel: Method parse_csv: Method
24. PyConZA 2015 How can this help? Focus on one thing FileFormat PriceFile
25. PyConZA 2015 How can this help? Zoom in FileFormat Excel CSV
26. PyConZA 2015 How can this help? Zoom in more def parse_file(price_file): for row in price_file: yield row.split(‘,’)
27. PyConZA 2015 Implementing OO OOP and Python Object Oriented Programming Classical OOP Prototype Based supporting mechanisms Conceptual model map to Code
28. PyConZA 2015 Classical OOP Types as classes an instance an instance an instance
29. PyConZA 2015 Python OOP basics Python cookies >>> class InvestmentInstrument: ... pass >>> fund = InvestmentInstrument() >>> fund <InvestmentInstrument object at 0x7f6815559fd0> >>> isinstance(fund, InvestmentInstrument) True >>> type(fund) <class ‘InvestmentInstrument’>
30. PyConZA 2015 Classical OOP Relationships as attributes Investment Instrument bool True False is_active is_active is_active
31. PyConZA 2015 Python OOP basics Attributes >>> fund = InvestmentInstrument() >>> fund.is_active = True >>> fund.is_active True >>> other_fund = InvestmentInstrument() >>> other_fund.is_active = False >>> fund.is_active is other_fund.is_active False
32. PyConZA 2015 Classical OOP Methods as (Python) methods Investment Instrument def activate(self): self.is_active = True indexed by class Operation
33. PyConZA 2015 Python OOP basics A Python method class InvestmentInstrument: def activate(self): self.is_active = True >>> fund = InvestmentInstrument() >>> InvestmentInstrument.activate(fund) >>> fund.is_active True >>> fund.activate()
34. PyConZA 2015 Python OOP basics Initialising instances class InvestmentInstrument: def __init__(self): self.is_active = False def activate(self): self.is_active = True >>> fund = InvestmentInstrument() >>> fund.is_active False >>> fund.activate()
35. PyConZA 2015 Classical OOP Subtyping as inheritance class InvestmentInstrument: def __init__(self): self.is_active = False def activate(self): self.is_active = True class UnitTrustFund(InvestmentInstrument): def value_of(self, units): return units * self.unit_price
36. PyConZA 2015 Classical OOP Subtyping as inheritance >>> fund = UnitTrustFund() >>> isinstance(fund, InvestmentInstrument) True >>> fund.activate() >>> fund.is_active True
37. PyConZA 2015 Classical OOP The ghost of operations FileFormat Excel CSV parse( )
38. PyConZA 2015 Basic OO concepts The ghost of operations class CSV(FileFormat): def parse(self, a_file): for row in a_file: yield row.split(‘,’) class Excel(FileFormat): def parse(self, a_file): # totally different stuff
39. PyConZA 2015 Basic OO concepts The ghost of operations for line in price_file.format.parse(price_file): # do stuff with line FileFormat PriceFile
40. PyConZA 2015 Basic OO concepts The ghost of operations if price_file.format.is_csv: lines = parse_csv(price_file) elif price_file.format.is_excel: lines = parse_excel(price_file) else: raise Exception(‘not supposed to get here’) for line in lines: # do stuff with line
41. PyConZA 2015 Design What’s design? Person Employee Investor Person Role Employee Investor Company
42. PyConZA 2015 Design Worse design CommissionCalc Bundles QuoteBasis QuoteFees Commissions AbstractCommission Bundle * QuoteBenefit Benefit Summary Calc * Benefit Grouping * * QuoteBenefit Bundles Benefit Calc * Commission Bundle Altered CommissionBundle * Lookup CommissionScale LookupCommission ScaleFactory * Calculation Constants
43. PyConZA 2015 Design Better design QuoteScenario Benefit Category * * * * CommisionBundle includedInAPI? negotiatedMax (R or %LOA) Commission Scale * Product VAT BenefitGrouping * Commission Band
44. PyConZA 2015 Design Inheritance vs subtyping Unordered List Menu Menu Unordered List
45. PyConZA 2015 Thanks www.reahl.org iwan@reahl.org Martin Fowler: Refactoring Martin Fowler: Analysis Patterns James Martin & James Odell: Object Oriented Methods: A Foundation
46. PyConZA 2015 Design Inheritance vs subtyping ObjectTable ObjectTable WithFilters ObjectTable Filter 0..* Report ObjectTable Filter * ObjectTable ByDate