今天 Python3.8 發布啦，它是 Python2 終結前最后一個大版本，我們一起看看這個版本都添加了那些新功能和特性。

PEP 572: Assignment Expressions

PEP 572 的標題是賦值表達式，也叫做「命名表達式」，不過它現在被廣泛的別名是「海象運算符」(The Walrus Operator)。因為:= 很像海象「眼睛小，長著兩枚長長的牙」這個特點 ^_^。

具體內容可以看我之前寫的文章: PEP572: 海象運算符 ，在這里給大家展示個通過用 PEP 572 改寫的一行實現斐波那契數列的例子:

In : (lambda f: f(f, int(input('Input: ')), 1, 0, 1))(lambda f, t, i, a, b: print(f'fib({i}) = {b}') or t == i or f ...: (f, t, i + 1, b, a + b))Input: 10fib(1) = 1fib(2) = 1fib(3) = 2fib(4) = 3fib(5) = 5fib(6) = 8fib(7) = 13fib(8) = 21fib(9) = 34fib(10) = 55Out: True

基于 Raymond Hettinger 版本改寫:

In : [(t:=(t[1], sum(t)) if i else (0,1))[1] for i in range(10)]Out: [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]PEP 570: Python Positional-Only parameters

PEP 570 說白了就是 強制使用者用位置參數

具體的可以看我之前寫的文章: PEP570 新語法：只接受位置參數

PEP 578: Python Runtime Audit Hooks

現在可以給 Python 運行時添加審計鉤子:

In : import sys...: import urllib.request...:...:...: def audit_hook(event, args):...: if event in ['urllib.Request']:...: print(f'Network {event=} {args=}')...:...: sys.addaudithook(audit_hook)In : urllib.request.urlopen('https://httpbin.org/get?a=1')Network event='urllib.Request' args=('https://httpbin.org/get?a=1', None, {}, 'GET')Out: <http.client.HTTPResponse at 0x10e394310>

目前支持審計的事件名字和 API 可以看 PEP 文檔 (延伸閱讀鏈接 2)， urllib.Request 是其中之一。另外還可以自定義事件:

In : def audit_hook(event, args):...: if event in ['make_request']:...: print(f'Network {event=} {args=}')...:In : sys.addaudithook(audit_hook)In : sys.audit('make_request', 'https://baidu.com')Network event='make_request' args=('https://baidu.com',)In : sys.audit('make_request', 'https://douban.com')Network event='make_request' args=('https://douban.com',)Multiprocessing shared memory

可以跨進程直接訪問同一內存 (共享):

# IPython進程AIn : from multiprocessing import shared_memoryIn : a = shared_memory.ShareableList([1, 'a', 0.1])In : aOut: ShareableList([1, 'a', 0.1], name='psm_d5d6ba1b') # 注意name# IPython進程B(另外一個終端進入IPython)In : from multiprocessing import shared_memoryIn : b = shared_memory.ShareableList(name='psm_d5d6ba1b') # 使用name就可以共享內存In : bOut: ShareableList([1, 'a', 0.1], name='psm_d5d6ba1b')New importlib.metadata module

使用新的 importlib.metadata 模塊可以直接讀取第三方包的元數據:

In : from importlib.metadata import version, files, requires, distributionIn : version('flask')Out: '1.1.1'In : requires('requests')Out:['chardet (<3.1.0,>=3.0.2)', 'idna (<2.9,>=2.5)', 'urllib3 (!=1.25.0,!=1.25.1,<1.26,>=1.21.1)', 'certifi (>=2017.4.17)', "pyOpenSSL (>=0.14) ; extra == 'security'", "cryptography (>=1.3.4) ; extra == 'security'", "idna (>=2.0.0) ; extra == 'security'", "PySocks (!=1.5.7,>=1.5.6) ; extra == 'socks'", 'win-inet-pton ; (sys_platform == "win32" and python_version == "2.7") and extra == /'socks/'']In : dist = distribution('celery')In : dist.versionOut: '4.3.0'In : dist.metadata['Requires-Python']Out: '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'In : dist.metadata['License']In : dist.entry_pointsOut:[EntryPoint(name='celery', value='celery.__main__:main', group='console_scripts'), EntryPoint(name='celery', value='celery.contrib.pytest', group='pytest11')]In : files('celery')[8]Out: PackagePath('celery/__init__.py')In : dist.locate_file(files('celery')[8])Out: PosixPath('/Users/dongweiming/test/venv/lib/python3.8/site-packages/celery/__init__.py')functools.cached_property