Python 3.8正式发布重要新功能一览

> walrus = False 
> print(walrus) 
False
Python3.8中，可以使用 walrus 运算符将上面两个语句合并为一句
> 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)

> 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. 
 
[...]
float (/) 中 (/) 是什么意思？有关这部分内容的讨论可以参考下面的文档，今天的内容中不做为我们的重点内容
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("") 
'Hello, "", greeting="Awesome job") 
'Awesome job, "", 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) 
...text 是仅位置参数，因此不能使用关键字 text：
> headline("Positional-only Arguments") 
'"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'
border 既可以使用关键字，也可以不使用关键字指定：
> headline("Python 3.8", "=") 
'=================== Python 3.8 ===================' 
 
> headline("Real Python", border=":") 
':::::::::::::::::: Real Python :::::::::::::::::::'

> 
> headline("Python", "", width=38) 
'"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

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"

# 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

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'

> 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'"

> 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 
..

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

> 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

> # 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 "==""3.8" 
True

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

> 
> 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