Python 3.10 is coming!

看看Python 官网的文档 whatsnew,Python 3.10 已然距离我们越来越近了,然我们看看 Python 3.10 相较于 Python 3.9 有哪些改变吧

新特性

通过括号来组织多个上下文管理器

**的福音,以前在面对有多个上下文管理器时,我们只能采取多层嵌套的方式,像这样:

with open("read.txt", "r") as rfile:
  with open("write.txt", "w") as wfile:
    wfile.write(rfile.read())

会不会变成这样,[捂脸][捂脸][捂脸]
Python 3.10 is coming!

那在 Python3.10 中,我们可以这样写了:

with (
  open("read.txt", "r") as rfile,
  open("write.txt", "w") as wfile
):
    wfile.write(rfile.read())

更加友好的错误信息

语法错误

  1. 对于代码中未闭合括号或引号造成的语法错误,解释器可以提供具体出错的开头位置

比如下面这段错误代码(是因为大括号没有闭合):

# filename: syntax_error_1.py
expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
            38: 4, 39: 4, 45: 5, 46: 5, 47: 5, 48: 5, 49: 5, 54: 6,
some_other_code = foo()

Python 3.9 版本运行的报错信息,很难定位到错误原因:

> C:\Users\zioyi\.pyenv\pyenv-win\versions\3.9.6\python.exe syntax_error_1.py
  File "C:\Users\zioyi\python_code\syntax_error_1.py", line 3
    some_other_code = foo()
                    ^
SyntaxError: invalid syntax

再来看看 Python 3.10 的报错信息,就显得贴心很多了:

> C:\Users\zioyi\.pyenv\pyenv-win\versions\3.10.0b4\python.exe syntax_error_1.py
  File "C:\Users\zioyi\python_code\syntax_error_1.py", line 1
    expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
               ^
SyntaxError: '{' was never closed
  1. 对于语法错误,解释器将高亮显示构成语法错误代码的完整错误范围

先看一下 Python 3.9:

>>> foo(x, z for z in range(10), t, w)
  File "<stdin>", line 1
    foo(x, z for z in range(10), t, w)
           ^
SyntaxError: Generator expression must be parenthesized

再看一下 Python 3.10:

>>> foo(x, z for z in range(10), t, w)
  File "<stdin>", line 1
    foo(x, z for z in range(10), t, w)
           ^^^^^^^^^^^^^^^^^^^^
SyntaxError: Generator expression must be parenthesized

原来,^^^^^^^^^^^这就是高亮
3. 错误信息变得更加智能,不再只是无情的才裁判,只会返回SyntaxError: invalid syntax,更像是一位循循善诱的老师

来看看当我们写错时,老师(Pyhton 3.10) 是怎么引导我们的

>>> # 条件语句忘记加冒号
>>> if rocket.position > event_horizon
  File "<stdin>", line 1
    if rocket.position > event_horizon
                                      ^
SyntaxError: expected ':'

>>> # 解包元组时没写括号
>>> {x,y for x,y in zip('abcd', '1234')}
  File "<stdin>", line 1
    {x,y for x,y in zip('abcd', '1234')}
     ^
SyntaxError: did you forget parentheses around the comprehension target?

>>> # 构造字典时,两个元素之间没加逗号
>>> items = {
... x: 1,
... y: 2
... z: 3,
  File "<stdin>", line 3
    y: 2
       ^
SyntaxError: invalid syntax. Perhaps you forgot a comma?

>>> # 准备补货多个异常,却少写了括号
>>> try:
...     build_dyson_sphere()
... except NotEnoughScienceError, NotEnoughResourcesError:
  File "<stdin>", line 3
    except NotEnoughScienceError, NotEnoughResourcesError:
           ^
SyntaxError: multiple exception types must be parenthesized

缩进错误 IndentationErrors

现在缩进错误会有更多关于期望缩进的块类型的上下文信息,包括语句的位置:

>>> def foo():
...    if lel:
...    x = 2
  File "<stdin>", line 3
    x = 2
    ^
IndentationError: expected an indented block after 'if' statement in line 2

属性错误 AttributeErrors

当出现熟悉错误时,PyErr_Display函数会猜你喜欢,打印出你可能要的属性:

>>> collections.namedtoplo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'collections' has no attribute 'namedtoplo'. Did you mean: namedtuple?

命名错误 NameErrors

和属性错误一样,当出现命名错误时,PyErr_Display函数会猜你喜欢,打印出你可能要的变量名:

>>> schwarzschild_black_hole = None
>>> schwarschild_black_hole
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'schwarschild_black_hole' is not defined. Did you mean: schwarzschild_black_hole?

PyErr_Display真是一个懂事的函数

switch-casematch-case终于来了

别人(Java、Golang、C)有的,你(Pyhton)也会有的,而且只会更好!
响应全网呼声,Python 版switch-case,即match-case也终于来了,这个特性也是 Python 3.10 中最受瞩目的特性

语法

Python 版的switch-case,你要按照这个格式去写:

match subject:
    case <pattern_1>:
        <action_1>
    case <pattern_2>:
        <action_2>
    case <pattern_3>:
        <action_3>
    case _:
        <action_wildcard>

简单匹配

在没有match-case之前,我们是这样写的:

def http_error_if_else(status):
  if status == 400:
    return "Bad request"
  elif status == 404:
    return "Not found"
  elif status == 418:
    return "I'm a teapot"
  else:
    return "Something's wrong with the internet"
# 或者这样
def http_error_dict(status):
  http_status_to_error = {
    400: "Bad request",
    404: "Not found",
    418: "I'm a teapot"
  }
  return http_status_to_error.get(status, "Something's wrong with the internet")

虽然可以解决问题,但我们知道if-else在性能上和易读性上都没有switch-case好用
再看看 Python 3.10 里,我们就可以这样写了:

def http_error_match_case(status):
    match status:
        case 400:
            return "Bad request"
        case 404:
            return "Not found"
        case 418:
            return "I'm a teapot"
        case _:
            return "Something's wrong with the internet"

这就变得简单了许多

你还可以在case后面用使用|来串联多个,像这样:

case 401 | 403 | 404:
    return "Not allowed"

如果不在case语句中使用_兜底,则可能不存在匹配项。如果不存在匹配项,则行为为空操作。例如,如果状态500被传递,则会发生空操作。

同时匹配变量和常量

在进行匹配时,会按照case后的patternsubject尝试进行转换,来绑定pattern中的变量,.在这个例子中,一个数据点可以解包到它的 x 坐标和 y 坐标:

# point is an (x, y) tuple
match point:
    case (0, 0):
        print("Origin")
    case (0, y):
        print(f"Y={y}")
    case (x, 0):
        print(f"X={x}")
    case (x, y):
        print(f"X={x}, Y={y}")
    case _:
        raise ValueError("Not a point")
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