新功能手册：

https://docs.python.org/3.8/whatsnew/3.8.html

• 使用赋值表达式简化一些代码结构
• 在你自己的函数中强制执行仅位置参数
• 指定更精确的类型提示
• 使用f字符串进行更简单的调试

>>> walrus = False	
>>> print(walrus)	
False

>>> print(walrus := True)	
True

inputs = list()	
current = input("Write something: ")	
while current != "quit":	inputs.append(current)	current = input("Write something: ")

inputs = list()	
while True:	current = input("Write something: ")	if current == "quit":	break	inputs.append(current)

inputs = list()	
while (current := input("Write something: ")) != "quit":	inputs.append(current)

PEP572中描述了复制表达式的所有细节，大家可以深入阅读。

https://www.python.org/dev/peps/pep-0572/ https://www.python.org/dev/peps/pep-0572/#examples

>>> float("3.8")	
3.8	>>> help(float)	
class float(object)	|  float(x=0, /)	|	|  Convert a string or number to a floating point number, if possible.	[...]

PEP 457 -- Notation For Positional-Only Parameters

https://www.python.org/dev/peps/pep-0457/

>>> float(x="3.8")	
Traceback (most recent call last):	File "<stdin>", line 1, in <module>	
TypeError: float() takes no keyword arguments

>>> def incr(x):	
...     return x + 1	
...	
>>> incr(3.8)	
4.8	>>> incr(x=3.8)	
4.8

>>> def incr(x, /):	
...     return x + 1	
...	
>>> incr(3.8)	
4.8	>>> incr(x=3.8)	
Traceback (most recent call last):	File "<stdin>", line 1, in <module>	
TypeError: incr() got some positional-only arguments passed as	keyword arguments: 'x'

>>> def greet(name, /, greeting="Hello"):	
...     return f"{greeting}, {name}"	
...	
>>> greet("Łukasz")	
'Hello, Łukasz'	>>> greet("Łukasz", greeting="Awesome job")	
'Awesome job, Łukasz'	>>> greet(name="Łukasz", greeting="Awesome job")	
Traceback (most recent call last):	File "<stdin>", line 1, in <module>	
TypeError: greet() got some positional-only arguments passed as	keyword arguments: 'name'

>>> def to_fahrenheit(*, celsius):	
...     return 32 + celsius * 9 / 5	
...	
>>> to_fahrenheit(40)	
Traceback (most recent call last):	File "<stdin>", line 1, in <module>	
TypeError: to_fahrenheit() takes 0 positional arguments but 1 was given	>>> to_fahrenheit(celsius=40)	
104.0

>>> def headline(text, /, border="♦", *, width=50):	
...     return f" {text} ".center(width, border)	
...

>>> headline("Positional-only Arguments")	
'♦♦♦♦♦♦♦♦♦♦♦ Positional-only Arguments ♦♦♦♦♦♦♦♦♦♦♦♦'	>>> headline(text="This doesn't work!")	
Traceback (most recent call last):	File "<stdin>", line 1, in <module>	
TypeError: headline() got some positional-only arguments passed as	keyword arguments: 'text'

>>> headline("Python 3.8", "=")	
'=================== Python 3.8 ==================='	>>> headline("Real Python", border=":")	
':::::::::::::::::: Real Python :::::::::::::::::::'

>>>	
>>> headline("Python", "?", width=38)	
'??????????????? Python ???????????????'	
>>> headline("Python", "?", 38)	
Traceback (most recent call last):	File "<stdin>", line 1, in <module>	
TypeError: headline() takes from 1 to 2 positional arguments	but 3 were given

• 文字类型
• 打字字典
• 最终对象
• 协定

def double(number: float) -> float:	return 2 * number

>>> double(3.14)	
6.28	>>> double("I'm not a float")	
"I'm not a floatI'm not a float"

PEP 484

https://www.python.org/dev/peps/pep-0484/

# draw_line.py	def draw_line(direction: str) -> None:	if direction == "horizontal":	...  # Draw horizontal line	elif direction == "vertical":	...  # Draw vertical line	else:	raise ValueError(f"invalid direction {direction!r}")	draw_line("up")

$ mypy draw_line.py	
draw_line.py:15: error:	Argument 1 to "draw_line" has incompatible type "Literal['up']";	expected "Union[Literal['horizontal'], Literal['vertical']]"	
Found 1 error in 1 file (checked 1 source file)

# calculator.py	
from typing import Union	ARABIC_TO_ROMAN = [(1000, "M"), (900, "CM"), (500, "D"), (400, "CD"),	(100, "C"), (90, "XC"), (50, "L"), (40, "XL"),	(10, "X"), (9, "IX"), (5, "V"), (4, "IV"), (1, "I")]	def _convert_to_roman_numeral(number: int) -> str:	"""Convert number to a roman numeral string"""	result = list()	for arabic, roman in ARABIC_TO_ROMAN:	count, number = divmod(number, arabic)	result.append(roman * count)	return "".join(result)	def add(num_1: int, num_2: int, to_roman: bool = True) -> Union[str, int]:	"""Add two numbers"""	result = num_1 + num_2	if to_roman:	return _convert_to_roman_numeral(result)	else:	return result

# calculator.py	from typing import Literal, overload, Union	ARABIC_TO_ROMAN = [(1000, "M"), (900, "CM"), (500, "D"), (400, "CD"),	(100, "C"), (90, "XC"), (50, "L"), (40, "XL"),	(10, "X"), (9, "IX"), (5, "V"), (4, "IV"), (1, "I")]	def _convert_to_roman_numeral(number: int) -> str:	"""Convert number to a roman numeral string"""	result = list()	for arabic, roman in ARABIC_TO_ROMAN:	count, number = divmod(number, arabic)	result.append(roman * count)	return "".join(result)	@overload	
def add(num_1: int, num_2: int, to_roman: Literal[True]) -> str: ...	
@overload	
def add(num_1: int, num_2: int, to_roman: Literal[False]) -> int: ...	def add(num_1: int, num_2: int, to_roman: bool = True) -> Union[str, int]:	"""Add two numbers"""	result = num_1 + num_2	if to_roman:	return _convert_to_roman_numeral(result)	else:	return result

from typing import Final	ID: Final = 1	...	ID += 1

from typing import final	@final	
class Base:	...	class Sub(Base):	..

from typing import TypedDict	class PythonVersion(TypedDict):	version: str	release_year: int	py38 = PythonVersion(version="3.8", release_year=2019

当我看到一只鸟走路像鸭子，游泳像鸭子，像鸭子一样嘎嘎叫时，我把它称为鸭子。

from typing import Protocol	class Named(Protocol):	name: str	def greet(obj: Named) -> None:	print(f"Hi {obj.name}"

>>>	
>>> style = "formatted"	
>>> f"This is a {style} string"	
'This is a formatted string'

官方文档

https://docs.python.org/3/library/string.html#format-specification-mini-language

>>> import math	
>>> r = 3.8	>>> f"Diameter {(diam := 2 * r)} gives circumference {math.pi * diam:.2f}"	
'Diameter 7.6 gives circumference 23.88'

>>> python = 3.8	
>>> f"{python=}"	
'python=3.8'

>>> python = 3.7	
>>> f"python={python}"	
'python=3.7'

>>> f"{name.upper()[::-1] = }"	
"name.upper()[::-1] = 'CIRE'"

• Barry Warsaw
• Brett Cannon
• Carol Willing
• Guido van Rossum
• Nick Coghlan

 PyCon 2019上的Python指导委员会。从左至右：Barry Warsaw，Brett Cannon，Carol Willing，Guido van Rossum和Nick Coghlan（图片来源：Geir Arne Hjelle）

• importlib.metadata

>>> from importlib import metadata	
>>> metadata.version("pip")	
'19.2.3'	>>> pip_metadata = metadata.metadata("pip")	
>>> list(pip_metadata)	
['Metadata-Version', 'Name', 'Version', 'Summary', 'Home-page', 'Author',	'Author-email', 'License', 'Keywords', 'Platform', 'Classifier',	'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier',	'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier',	'Classifier', 'Classifier', 'Requires-Python']	>>> pip_metadata["Home-page"]	
'https://pip.pypa.io/'	>>> pip_metadata["Requires-Python"]	
'>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*'	>>> len(metadata.files("pip"))	
668

>>> [p for p in metadata.files("realpython-reader") if p.suffix == ".py"]	
[PackagePath('reader/__init__.py'), PackagePath('reader/__main__.py'),	PackagePath('reader/feed.py'), PackagePath('reader/viewer.py')]	>>> init_path = _[0]  # Underscore access last returned value in the REPL	
>>> print(init_path.read_text())	
"""Real Python feed reader	Import the `feed` module to work with the Real Python feed:	>>> from reader import feed	>>> feed.get_titles()	['Logging in Python', 'The Best Python Books', ...]	See https://github.com/realpython/reader/ for more information	
"""	# Version of realpython-reader package	
__version__ = "1.0.0"	...

>>> metadata.requires("realpython-reader")	
['feedparser', 'html2text', 'importlib-resources', 'typing']

try:	from importlib import metadata	
except ImportError:	import importlib_metadata as metadata	...

• 新增和改进的数学和统计功能
  
  

>>> import math	
>>> math.prod((2, 8, 7, 7))	
784	>>> 2 * 8 * 7 * 7	
784

• statistics.fmean（）计算浮点数的平均值。
• statistics.geometric_mean（）计算浮点数的几何平均值。
• statistics.multimode（）查找序列中最频繁出现的值。
• statistics.quantiles（）计算用于将数据等概率分为n个连续区间的切点。

>>> import statistics	
>>> data = [9, 3, 2, 1, 1, 2, 7, 9]	
>>> statistics.fmean(data)	
4.25	>>> statistics.geometric_mean(data)	
3.013668912157617	>>> statistics.multimode(data)	
[9, 2, 1]	>>> statistics.quantiles(data, n=4)	
[1.25, 2.5, 8.5]

>>> import random	
>>> import statistics	
>>> from timeit import timeit	>>> # Create 10,000 random numbers	
>>> data = [random.random() for _ in range(10_000)]	>>> # Measure the time it takes to run mean() and fmean()	
>>> t_mean = [timeit("statistics.mean(data)", number=100, globals=globals())	
...           for _ in range(30)]	
>>> t_fmean = [timeit("statistics.fmean(data)", number=100, globals=globals())	
...            for _ in range(30)]	>>> # Create NormalDist objects based on the sampled timings	
>>> n_mean = statistics.NormalDist.from_samples(t_mean)	
>>> n_fmean = statistics.NormalDist.from_samples(t_fmean)	>>> # Look at sample mean and standard deviation	
>>> n_mean.mean, n_mean.stdev	
(0.825690647733245, 0.07788573997674526)	>>> n_fmean.mean, n_fmean.stdev	
(0.010488564966666065, 0.0008572332785645231)	>>> # Calculate the lower 1 percentile of mean	
>>> n_mean.quantiles(n=100)[0]	
0.64450132212

• 新增危险语法警告功能
  
  

>>> # Python 3.7	
>>> version = "3.7"	
>>> version is "3.7"	
False	>>> # Python 3.8	
>>> version = "3.8"	
>>> version is "3.8"	
<stdin>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?	
False	>>> version == "3.8"	
True

>>> [	
...   (1, 3)	
...   (2, 4)	
... ]	
<stdin>:2: SyntaxWarning: 'tuple' object is not callable; perhaps	you missed a comma?	
Traceback (most recent call last):	File "<stdin>", line 2, in <module>	
TypeError: 'tuple' object is not callable

• 优化

>>>	
>>> import collections	
>>> from timeit import timeit	
>>> Person = collections.namedtuple("Person", "name twitter")	
>>> raymond = Person("Raymond", "@raymondh")	>>> # Python 3.7	
>>> timeit("raymond.twitter", globals=globals())	
0.05876131607996285	>>> # Python 3.8	
>>> timeit("raymond.twitter", globals=globals())	
0.0377705999400132

>>> import sys	>>> # Python 3.7	
>>> sys.getsizeof(list(range(20191014)))	
181719232	>>> # Python 3.8	
>>> sys.getsizeof(list(range(20191014)))	
161528168

https://docs.python.org/3.8/whatsnew/3.8.html#porting-to-python-3-8

https://www.python.org/downloads/release/python-380/

https://realpython.com/python38-new-features/ https://docs.python.org/3.8/whatsnew/3.8.html

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