Facade Design Pattern
Videos
| Section | Video Links |
|---|---|
| Facade Overview |    |
| Facade Use Case | |
| Python Decimal | |
| Python Type Hints | |
Book
Overview
... Refer to Book, pause Video Lectures or subscribe to Medium Membership to read textual content.
Facade UML Diagram
Output
python ./facade/facade_concept.py
A
B
{'C': [1, 2, 3]}
A
B
{'C': [1, 2, 3]}Example Use Case
... Refer to Book, pause Video Lectures or subscribe to Medium Membership to read textual content.
Example UML Diagram
Output
python ./facade/client.py
---- Gamestate Snapshot ----
{'clock': 59, 'game_open': True, 'entries': [('sean', Decimal('5'))]}
---- Reports History ----
0 : 1614087127.327007 : new user `sean` created
1 : 1614087127.327007 : wallet for `sean` created and set to 0
2 : 1614087127.327007 : Give new user `sean` sign up bonus of 10
3 : 1614087127.327007 : Balance adjustment for `sean`. New balance = 10
4 : 1614087128.3278701 : Balance check for `sean` = 10
5 : 1614087128.3278701 : Balance adjustment for `sean`. New balance = 9
6 : 1614087128.3278701 : New entry `5` submitted by `sean`
---- Gamestate Snapshot ----
{'clock': 58, 'game_open': True, 'entries': [('sean', Decimal('5'))]}New Coding Concepts
Python decimal Module
The decimal module provides support for correctly rounded decimal floating-point arithmetic.
If representing money values in python, it is better to use the decimal type rather than float .
Floats will have rounding errors versus decimal.
from decimal import Decimal print(1.1 + 2.2) # adding floats print(Decimal('1.1') + Decimal('2.2')) # adding decimals
Outputs
Note how the float addition results in 3.3000000000000003 whereas the decimal addition result equals 3.3 .
Be aware though that when creating decimals, be sure to pass in a string representation, otherwise it will create a decimal from a float.
from decimal import * print(Decimal(1.1)) # decimal from float print(Decimal('1.1')) # decimal from string
Outputs
1.100000000000000088817841970012523233890533447265625
1.1
Python Decimal: https://docs.python.org/3/library/decimal.html
Type Hints
In the Facade use case example, I have added type hints to the method signatures and class attributes.
_clock: int = 0 _entries: list[tuple[str, Decimal]] = [] ... def get_balance(user_id: str) -> Decimal: "Get a players balance" ... ... def register_user(cls, new_user: dict[str, str]) -> str: "register a user" ...
See the extra : str after the user_id attribute, and the -> Decimal before the final colon in the get_balance() snippet.
This is indicating that if you use the get_balance() method, that the user_id should be a type of string, and that the method will return a Decimal .
Note that the Python runtime does not enforce the type hints and that they are optional. However, where they are beneficial is in the IDE of your choice or other third party tools such type checkers.
In VSCode, when typing code, it will show the types that the method needs.
For type checking, you can install an extra module called mypy
and then run it against your code,
mypy ./facade/client.py Success: no issues found in 1 source file
Mypy will also check any imported modules at the same time.
If working with money, then it is advisable to add extra checks to your code. Checking that type usage is consistent throughout your code, especially when using Decimals, is a good idea that will make your code more robust.
For example, if I wasn't consistent in using the Decimal throughout my code, then I would see a warning highlighted.
mypy ./facade/client.py facade/game_engine.py:45: error: Argument 1 to "append" of "list" has incompatible type "Tuple[str, int]"; expected "Tuple[str, Decimal]" facade/game_api.py:34: error: Argument 2 to "submit_entry" of "GameEngine" has incompatible type "Decimal"; expected "int" Found 2 errors in 2 files (checked 1 source file)
Summary
... Refer to Book, pause Video Lectures or subscribe to Medium Membership to read textual content.