doc: 阅读python文档

2025-08-17 05:28:33 +08:00
parent 68c21c80d1
commit 0e2445ccbf
1 changed files with 328 additions and 109 deletions
--- a/python/py.md
+++ b/python/py.md
@@ -57,14 +57,19 @@
    - [dir](#dir)
    - [package](#package)
      - [from ... import \*](#from--import-)
-  - [异常处理](#异常处理)
-    - [抛异常](#抛异常)
-      - [捕获异常后重新抛出异常](#捕获异常后重新抛出异常)
-      - [主动抛出异常](#主动抛出异常)
-  - [数据存储](#数据存储)
-    - [json.dump](#jsondump)
-    - [json.load](#jsonload)
-  - [http api](#http-api)
+  - [Errors and Exceptions](#errors-and-exceptions)
+    - [Syntax Errors](#syntax-errors)
+    - [Exceptions](#exceptions)
+    - [handling exceptions](#handling-exceptions)
+      - [python异常结构](#python异常结构)
+    - [rasing exception](#rasing-exception)
+    - [Exception Chaining](#exception-chaining)
+    - [用户自定义异常类型](#用户自定义异常类型)
+    - [define cleanup action](#define-cleanup-action)
+    - [predefined clean-up action](#predefined-clean-up-action)
+    - [rasing multi exceptions](#rasing-multi-exceptions)
+      - [except\*](#except)
+    - [enriching exceptions with notes](#enriching-exceptions-with-notes)
  - [多线程](#多线程)
  - [linux命令交互](#linux命令交互)
  - [正则](#正则)
@@ -786,124 +791,338 @@ echo.echofilter(input, output, delay=0.7, atten=4)
 __all__ = ["echo", "surround", "reverse"]
 ```

-## 异常处理
-在python中，可以通过如下语法来进行异常处理
+## Errors and Exceptions
+在python中存在两种不同的异常，`syntax erros`和`exceptions`
+
+### Syntax Errors
+syntax errors又被称为parsing errors，当parser检测到语法错误时，会抛出该error
+
+### Exceptions
+在python程序执行时，即使语法都正常，也有可能在执行时发生异常。在程序执行时被检测到的异常被称为`exceptions`，但是其时可以被处理的，并不会无条件的造成fatal。
+
+### handling exceptions
+可以在程序中对特定类型的异常进行处理，示例如下所示：
+```py
+while True:
+    try:
+        x = int(input("Please enter a number: "))
+        break
+    except ValueError:
+        print("Oops!  That was no valid number.  Try again...")
+```
+上述示例中针对`ValueError`类型的异常进行了捕获，并提示用户输入有效的数字。但是，在上述程序实例中，用户仍然能够通过`Ctrl + C`中断该程序。
+
+> `Ctrl + C`将会以抛出`KeyboardInterrupt`的形式表现。
+
+try statement中可以包含多个except，为不同类型的异常指定handler，最多只会有一个handler被执行。并且，一个except中也可以包含多个exceptions，示例如下所示：
+```py
+... except (RuntimeError, TypeError, NameError):
+    ...     pass
+```
+包含多个except的示例如下所示:
+```py
+class B(Exception):
+        pass
+
+    class C(B):
+        pass
+
+    class D(C):
+        pass
+
+    for cls in [B, C, D]:
+        try:
+                raise cls()
+            except D:
+                print("D")
+            except C:
+                print("C")
+            except B:
+                print("B")
+```
+上述示例中，会输出`B, C, D`。但是，如果修改`except B`的位置为第一位，那么输出将会变为`B, B, B`。
+
+在except语法中，除了指定异常类型外，还支持指定一个变量名来捕获异常，该变量中包含`args`属性，能够访问`构造该异常对象时传递的参数`，并且，异常中还定义了`__str__()`方法用于打印所有的异常参数。
+
+为捕获的异常绑定变量的示例如下所示：
 ```py
 try:
-    # code block
-    pass
-except ErrorType1:
-    # error type 1 handling code block
-    pass
-except ErrorType2:
-    # error type 2 handling code block
-    pass
-except Exception as result:
-    # handle other exception types
-    pass
-else:
-    # code block that no error occurs
-    pass
-finally:
-    # code block executed whether exception occurs
-    pass
+        raise Exception('spam', 'eggs')
+        except Exception as inst:
+                print(type(inst))    # the exception type
+            print(inst.args)     # arguments stored in .args
+            print(inst)          # __str__ allows args to be printed directly,
+                                 # but may be overridden in exception subclasses
+            x, y = inst.args     # unpack args
+            print('x =', x)
+            print('y =', y)
+
 ```

-### 抛异常
-#### 捕获异常后重新抛出异常
-如果python在捕获异常后需要重新对异常进行抛出，可以使用`raise`关键字
+输出内容为:
+```
+    <class 'Exception'>
+    ('spam', 'eggs')
+    ('spam', 'eggs')
+    x = spam
+    y = eggs
+```
+#### python异常结构
+`BaseException`是所有异常类的common base class；而`Exception`类则是`BaseException`的子类，同时也是所有非fatal异常类的base class。
+
+对于是`BaseException`子类但是不是`Exception`子类的类，其通常代表不应当被处理的异常，抛出该类异常通常代表程序应当被终止，其包含如下类：
+- `SystemExit`：由`system.exit()`抛出
+- `KeyboardInterrupt`：由想要中止程序的用户触发`Ctrl + C`
+
+`Exception`可以被用做匹配所有异常。
 ```py
+import sys
+
 try:
-    m = 10/0
-except Exception as e:
-    print(e)
+        f = open('myfile.txt')
+            s = f.readline()
+            i = int(s.strip())
+        except OSError as err:
+                print("OS error:", err)
+        except ValueError:
+            print("Could not convert data to an integer.")
+        except Exception as err:
+            print(f"Unexpected {err=}, {type(err)=}")
            raise
-finally:
-    print("handle correctly")
-```
-#### 主动抛出异常
-主动抛出异常时，也可使用`raise`关键字
-```py
-try:
-    raise Exception("fucking Nvidia raised their price!")
-except Exception as e:
-    print(e)
-finally:
-    print("my wallet is empty!")
 ```

-## 数据存储
-### json.dump
-可以通过`json.dump`方法将内容以json的格式存储到文件中：
+`try-except`语法还支持`else`，`else`必须跟在所有`except`之后，其代表`try block`中没有抛出任何异常的场景。
+
 ```py
-import json
-
-
-def save_as_json(data, fd):
-    json.dump(data, fd)
-
-
-waifu = {
-    "name": "Touma Kazusa",
-    "job": "Student",
-    "gender": "female",
-    "age": 17
-}
-
-is_save_success = True
-try:
-    with open("shiro.json", "w") as shiro_fd:
-        save_as_json(waifu, shiro_fd)
-except Exception as e:
-    print(e)
-    is_save_success = False
-    exit(1)
-finally:
-    if is_save_success:
-        print("save success!")
+for arg in sys.argv[1:]:
+    try:
+        f = open(arg, 'r')
+    except OSError:
+        print('cannot open', arg)
+    else:
+        print(arg, 'has', len(f.readlines()), 'lines')
+        f.close()
 ```
-### json.load
-可以通过`json.load`方法读取文件中的json格式内容：
+
+### rasing exception
+`raise`关键字支持抛出异常，示例如下
 ```py
-import json
-
-
-def read_as_json(fd):
-    return json.load(fd)
-
-
-is_read_success = True
-try:
-    with open("shiro.json", "r") as shiro_fd:
-        r_obj = read_as_json(shiro_fd)
-        print(r_obj)
-except Exception as e:
-    print(e)
-    is_read_success = False
-    exit(1)
-finally:
-    if is_read_success:
-        print("read success!")
+raise NameError('HiThere')
 ```
-## http api
-python安装requests包之后，可以访问http接口
+传递给raise的必须是`Exception`的实例或是`继承自Exception的类`，但传递的是类时，其会使用类的默认构造器来构造Exception实例。
+```py
+raise ValueError  # shorthand for 'raise ValueError()'
+```
+`raise`关键字也能用于对异常的重新抛出，示例如下：
 ```py
-import math
-import requests
-import time
-
 try:
-    body = requests.get("https://api.m.taobao.com/rest/api3.do", {
-        "api": "mtop.common.getTimestamp"
-    }).json()
-    ts = body['data']['t']
-    t = time.localtime(math.trunc(int(ts) / 1000))
-    time_str = time.strftime("%Y-%m-%d %H:%M:%S %z", t)
-    print(time_str)
-except Exception as e:
-    print(e)
+        raise NameError('HiThere')
+        except NameError:
+                print('An exception flew by!')
            raise
 ```
+### Exception Chaining
+如果在`except`处理异常时，处理逻辑又抛出了异常，那么其会将被处理的异常关联到新异常中，并且将被处理异常包含在error msg中：
+```py
+try:
+        open("database.sqlite")
+        except OSError:
+                raise RuntimeError("unable to handle error")
+```
+其输出如下：
+```
+Traceback (most recent call last):
+  File "<stdin>", line 2, in <module>
+    open("database.sqlite")
+    ~~~~^^^^^^^^^^^^^^^^^^^
+FileNotFoundError: [Errno 2] No such file or directory: 'database.sqlite'
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "<stdin>", line 4, in <module>
+    raise RuntimeError("unable to handle error")
+RuntimeError: unable to handle error
+```
+
+如果想要设置异常的cause，可以使用`from`语法，示例如下所示：
+```py
+def func():
+        raise ConnectionError
+
+    try:
+        func()
+        except ConnectionError as exc:
+                raise RuntimeError('Failed to open database') from exc
+```
+上述示例中，`raise ... from exc`直接表明RuntimeError由exc引发。
+
+输出示例如下所示：
+```py
+Traceback (most recent call last):
+  File "<stdin>", line 2, in <module>
+    func()
+    ~~~~^^
+  File "<stdin>", line 2, in func
+ConnectionError
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "<stdin>", line 4, in <module>
+    raise RuntimeError('Failed to open database') from exc
+RuntimeError: Failed to open database
+```
+通过`from None`也可以关闭异常chain，示例如下：
+```py
+try:
+        open('database.sqlite')
+        except OSError:
+                raise RuntimeError from None
+```
+示例如下所示：
+```py
+Traceback (most recent call last):
+  File "<stdin>", line 4, in <module>
+    raise RuntimeError from None
+RuntimeError
+```
+### 用户自定义异常类型
+用户如果要自定义异常，可以创建`Exception`类型的子类。
+
+大多数异常类型都以`Error`结尾，和standard exceptions的命名规范类似。
+
+### define cleanup action
+python支持通过`finally`来定义clean-up action，其在所有场景下都会被处理。
+```py
+try:
+    raise KeyboardInterrupt
+finally:
+    print('Goodbye, world!')
+```
+其中，finally执行时机如下：
+- 当抛出异常时，如果异常没有被except处理，那么异常在finally执行完后再次抛出
+- 当except或else中抛出异常时，那么异常在finally执行完后被抛出
+- 如果在finally中执行了`break, continue, return`等语句，那么异常不会被抛出
+- 如果try中执行了`break, continue, return`，那么finally会在`break, continue, return`执行之前执行
+- 如果finally中执行了return操作，那么其return的值将会覆盖try中return的值
+
+finally通常被用于释放资源等操作。
+
+### predefined clean-up action
+部分对象定义了对象不在被需要时的clean-up action,无论针对对象的操作是否成功，clean-up action都会被执行。
+
+故而，可以通过`with`对该类对象进行使用，并无需手动释放其资源，示例如下：
+```py
+with open("myfile.txt") as f:
+        for line in f:
+                print(line, end="")
+```
+在上述代码执行完后，文件会自动关闭，即使是在执行过程中遇到异常。
+
+### rasing multi exceptions
+在部分时候，可能需要抛出多个异常，此时可以通过`ExceptionGroup`来对exception list进行wrap，示例如下：
+```py
+def f():
+        excs = [OSError('error 1'), SystemError('error 2')]
+        raise ExceptionGroup('there were problems', excs)
+```
+
+#### except*
+当使用`except*`时，可以选择性处理group中特定类型的异常，并将其他异常传递到下游。
+
+```py
+def f():
+    raise ExceptionGroup(
+        "group1",
+        [
+            OSError(1),
+            SystemError(2),
+            ExceptionGroup(
+                "group2",
+                [
+                    OSError(3),
+                    RecursionError(4)
+                ]
+            )
+        ]
+    )
+
+try:
+    f()
+except* OSError as e:
+    print("There were OSErrors")
+except* SystemError as e:
+    print("There were SystemErrors")
+```
+### enriching exceptions with notes
+当创建异常时，可以通过`add_note(note)`方法来为异常补充信息，标准的traceback会按照其被添加的顺序渲染所有的note信息，note信息在异常信息之后
+```py
+try:
+    raise TypeError('bad type')
+except Exception as e:
+    e.add_note('Add some information')
+    e.add_note('Add some more information')
+    raise
+```
+其输出如下：
+```py
+Traceback (most recent call last):
+  File "<stdin>", line 2, in <module>
+    raise TypeError('bad type')
+TypeError: bad type
+Add some information
+Add some more information
+```
+```py
+def f():
+    raise OSError('operation failed')
+
+excs = []
+for i in range(3):
+    try:
+        f()
+    except Exception as e:
+        e.add_note(f'Happened in Iteration {i+1}')
+        excs.append(e)
+
+raise ExceptionGroup('We have some problems', excs)
+```
+其异常信息如下：
+```
+ + Exception Group Traceback (most recent call last):
+  |   File "<stdin>", line 1, in <module>
+  |     raise ExceptionGroup('We have some problems', excs)
+  | ExceptionGroup: We have some problems (3 sub-exceptions)
+  +-+---------------- 1 ----------------
+    | Traceback (most recent call last):
+    |   File "<stdin>", line 3, in <module>
+    |     f()
+    |     ~^^
+    |   File "<stdin>", line 2, in f
+    |     raise OSError('operation failed')
+    | OSError: operation failed
+    | Happened in Iteration 1
+    +---------------- 2 ----------------
+    | Traceback (most recent call last):
+    |   File "<stdin>", line 3, in <module>
+    |     f()
+    |     ~^^
+    |   File "<stdin>", line 2, in f
+    |     raise OSError('operation failed')
+    | OSError: operation failed
+    | Happened in Iteration 2
+    +---------------- 3 ----------------
+    | Traceback (most recent call last):
+    |   File "<stdin>", line 3, in <module>
+    |     f()
+    |     ~^^
+    |   File "<stdin>", line 2, in f
+    |     raise OSError('operation failed')
+    | OSError: operation failed
+    | Happened in Iteration 3
+    +------------------------------------
+```

 ## 多线程
 python可以通过引入threading和concurrent.futures两个包来引入多线程：