异常与警告

异常

写代码的时候，出现错误必不可免。

看下面这段代码：

import math

while True:
    text = input('> ')
    if text[0] == 'q':
        break
    x = float(text)
    y = math.log10(x)
    print(f"log10({x}) = {y}")

这段代码接收命令行的输入，输入为数字时，计算它的对数并输出，直到输入值为 q 为止。

乍看没什么问题，然而输入0或者负数时：

import math

while True:
    text = input('> ')
    if text[0] == 'q':
        break
    x = float(text)
    y = math.log10(x)
    print(f"log10({x}) = {y}")

> -1

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

Input In [1], in ()
      6     break
      7 x = float(text)
----> 8 y = math.log10(x)
      9 print(f"log10({x}) = {y}")

ValueError: math domain error

log10 函数会报错，因为不能接受非正值。

一旦报错，程序就会停止执行，如果不希望程序停止执行，那么可以添加一对 try & except：

import math

while True:
    try:
        text = input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = math.log10(x)
        print(f"log10({x}) = {y}")
    except ValueError:
        print("the value must be greater than 0")

> -1
the value must be greater than 0
> 0
the value must be greater than 0
> 1
log10(1.0) = 0.0
> q

捕捉不同的异常类型

假设将这里的 y 更改为 1 / math.log10(x)，此时输入 1：

import math

while True:
    try:
        text = input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print(f"1 / log10({x}) = {y}")
    except ValueError:
        print("the value must be greater than 0")

> 1

---------------------------------------------------------------------------

ZeroDivisionError                         Traceback (most recent call last)

Input In [3], in ()
      7         break
      8     x = float(text)
----> 9     y = 1 / math.log10(x)
     10     print(f"1 / log10({x}) = {y}")
     11 except ValueError:

ZeroDivisionError: float division by zero

程序仍然抛出了异常，原因是ZeroDivisionError不在可处理的异常中。

可以有两种方法处理这个问题，第一种是捕获异常的父类Exception：

import math

while True:
    try:
        text = input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print(f"1 / log10({x}) = {y}")
    except Exception:
        print("invalid value")

> 1
invalid value
> 0
invalid value
> -1
invalid value
> q

第二种是指定多个错误类型：

import math

while True:
    try:
        text = input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print(f"1 / log10({x}) = {y}")
    except (ZeroDivisionError, ValueError):
        print("invalid value")

> 1
invalid value
> 0
invalid value
> -1
invalid value
> q

还可以分开处理：

import math

while True:
    try:
        text = input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print(f"1 / log10({x}) = {y}")
    except ValueError:
        print("the value must be greater than 0")
    except ZeroDivisionError:
        print("the value must not be 1")

> -1
the value must be greater than 0
> 1
the value must not be 1
> 2
1 / log10(2.0) = 3.321928094887362
> q

还可以将异常的具体信息打出来：

import math

while True:
    try:
        text = input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print(f"1 / log10({x}) = {y}")
    except Exception as e:
        print(e)

> -1
math domain error
> 0
math domain error
> 1
float division by zero
> abcde
could not convert string to float: 'abcde'
> q

可以用raise主动抛出异常，例如判断月份是否在1-12之间：

month = 13

if not 1 <= month <= 12:
    raise ValueError(f"{month} must between 1 and 12!")

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

Input In [8], in ()
      1 month = 13
      3 if not 1 <= month <= 12:
----> 4     raise ValueError(f"{month} must between 1 and 12!")

ValueError: 13 must between 1 and 12!

finally

try/catch 块还有一个可选的关键词 finally。

不管 try 块有没有异常， finally 块的内容总是会被执行，而且会在抛出异常前执行，因此可以用来作为安全保证，比如确保打开的文件被关闭：

try:
    print(1)
finally:
    print('finally was called.')

1
finally was called.

如果有异常被抛出，finally的部分会在抛出异常前执行：

try:
    print(1 / 0)
finally:
    print('finally was called.')

finally was called.

---------------------------------------------------------------------------

ZeroDivisionError                         Traceback (most recent call last)

Input In [11], in ()
      1 try:
----> 2     print(1 / 0)
      3 finally:
      4     print('finally was called.')

ZeroDivisionError: division by zero

异常被处理了，则在最后执行：

try:
    print(1 / 0)
except Exception as e:
    print(e)
finally:
    print('finally was called.')

division by zero
finally was called.