pycontw.md

The Benefit of Type Hints

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Masato Nakamura @ Nulab From Japan

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What is Nulab

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My Goal

You can try Type Hints from today!

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Not to talk about today

Details

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Today I will talk about the Introduction, Usage and Benefits of Type Hints

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Body -delete

• 1. Introduction
• 2. Usage
• 3. Benefits

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Introduction - delete

• What is Type Hinting?
• History
  • PEP 3123
  • PEP 484
  • PEP 526
• modules

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Usage -delete

• How to write
• What has changed?
• Background

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Benefits -delete

• Code Style
  • Simple is best
  • Explicit is better than Implicit.
• Editor
  • Code Completion
• static analysis
  • use with command line
  • use with Editor
  • use with CI

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Introduction

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What is Type Hints

• Traditionary Python code

def twice(num):
    return num * 2.0

• use Type Hint

def twice(num: double) -> double:
    return num * 2.0

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Type Hints

• using Type Hints

def twice(num: double) -> double:
    return num * 2.0

C-language Code

double twice(double str) {
   return str * 2.0;
}

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History

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History

Type Hints has History.

PEP	implement
3107	Python 3.0
484	Python 3.5
526	Ptyhon 3.6

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Python2

Python 2.x series did not have a way to qualify function arguments and return values, so many tools and libraries appeared to fill that gap.

def greeting(name):
    return 'Hello ' + name

greet = greeting("Masato")

• What is name?
• What does greeting return values?

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PEP 3107

add Function annotations, both for parameters and return values, are completely optional.You can write free text .

def compile(source: "something compilable",
            filename: "where the compilable thing comes from",
            mode: "is this a single statement or a suite?"):

the semantics were deliberately left undefined.

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PEP 484

There has now been enough 3rd party usage for static type analysis that the community would benefit from a standard vocabulary and baseline tools within the standard library.

def greeting(name: str) -> str:
    return 'Hello ' + name

• PEP-484 Type Hints

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Non Goal for PEP484

Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention.

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return to the code

def greeting(name: str) -> str:
    return 'Hello ' + name

greet = greeting("Masato") # type: str

Hmm , now I'd like to explain variables more easily😟

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PEP 526

from typing import List

# pep 484
a = [1,2,3] # type: List[int]

# We should add *comments*
path = None # type: Optional[str]  # Path to module source

# pep 526
a: List[int] = [1,2,3]
path: Optional[str] = None # Path to module sourde

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typing module

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What is typing module

-　Module entered when PEP 484 was implemented in 3.5 -　For providing python with frequently used types such as List, Dict

• Make the data structure easier as namedtuple

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from typing import List

a: List[int] = [1,2,3]
path: Optional[str] = None # Path to module sourde

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• NamedTuple

In Python 3.5

Point = namedtuple('Point', ['x', 'y'])
p = Point(x=1, y=2)
print(p.z)  # Error: Point has no attribute 'z'

In Python 3.6

from typing import NamedTuple

class Point(NamedTuple):
    x: int
    y: int

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Usage

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How to write

I talk about PEP526 style

def greeting(name: str) -> str:
    return 'Hello ' + name

greet: str = greeting("Masato")

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difference of PEP484 and PEP526

# pep 484
child # type: bool # cannot allow

# pep 526
child: bool # OK
if age < 18:
    child = True
else:
    child = False

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compatibility

# We can write pep484 style in 3.6（backward compatiblity！
hour = 24 # type: int

# PEP 526 style
hour: int; hour = 24


hour: int = 24

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implementation at Python

where is variable annotation？

at __annotaitons__

>>> answer:int = 42
>>> __annotations__
{'answer': <class 'int'>}

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Attention

We can find Class variables. ignored instance variable

>>> class Car:
...     stats: ClassVar[Dict[str, int]] = {}
...     def __init__(self) -> None:
...         self.seats = 4
>>> c: Car = Car()
>>> c.__annotations__
{'stats': typing.ClassVar[typing.Dict[str, int]]}
# only ClassVar!

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Attention

PEP3107 style

>>> alice: 'well done' = 'A+' # 
>>> bob: 'what a shame' = 'F-'
>>> __annotations__
{'alice': 'well done', 'bob': 'what a shame'}

But let's use for Type Hints as possible for the future.

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Type Hints based on typing module

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Benefits

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Benefits

• Code Style
• IDE
• statistic type analysis

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Code Style

def greeting(name: str) -> str:
    return 'Hello ' + name

We can check this by Type Hints

def greeting(name: str) -> str:
    return 42

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IDE

We can code completion in

• Editor
  • Visual Studio code
• PyCharm(IntelliJ)

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example

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statistic type analysis

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statistic type analysis

• for check variable type(no run code)
• python has some tools
  • mypy
  • pytypes (not talk)

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mypy

• https://github.com/python/mypy
• made by man named JukkaL & Guido
• can output report

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mypy

from typing import Dict, NamedTuple

class Car:
    stats: Dict[str, int] = {}
    def __init__(self) -> None:
        self.seats = 4
        
class Employee(NamedTuple):
    name: str
    id: int

c: Car = Car()
# print(c.__annotations__)
employee = Employee(name='Guido', id='x')

mypy --fast-parser --python-version 3.6 example.py
example.py:16: error: Argument 2 to "Employee" has incompatible type "str"; expected "int"

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mypy for Python3.6

• We can use!(PEP526 style)

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CI

• Jenkins
• Github + Travis CI
• Github + CircleCi

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Github + Travis CI

• we can run mypy
• we can get results
  • If you develop by team, You can get many benefits

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That's all

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Today I talked about the Introduction, Usage and Benefits of Type Hinting

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I hope you can try Type Hinting from today!

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Question?