python 异常命名原则 python异常例子

文章目录

• 一.Python异常概述

• 1.1 处理ZeroDivisionError 异常
• 1.2 使用try-except 代码块
• 1.3 使用异常避免崩溃
• 1.4 处理FileNotFoundError 异常
• 1.5 分析文本
• 1.6 使用多个文件
• 1.7 失败时一声不吭
• 1.8 决定报告哪些错误

一.Python异常概述

Python使用被称为异常 的特殊对象来管理程序执行期间发生的错误。每当发生让Python不知所措的错误时，它都会创建一个异常对象。如果你编写了处理该异常的代码，程序将继续运行；如果你未对异常进行处理，程序将停止，并显示一个traceback，其中包含有关异常的报告。

异常是使用try-except 代码块处理的。try-except 代码块让Python执行指定的操作，同时告诉Python发生异常时怎么办。使用了try-except 代码块时，即便出现异常，程序也将继续运行：显示你编写的友好的错误消息，而不是令用户迷惑的traceback。

1.1 处理ZeroDivisionError 异常

下面来看一种导致Python引发异常的简单错误。你可能知道不能将一个数字除以0，但我们还是让Python这样做吧：

>>> print(5/0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

1.2 使用try-except 代码块

当你认为可能发生了错误时，可编写一个try-except 代码块来处理可能引发的异常。你让Python尝试运行一些代码，并告诉它如果这些代码引发了指定的异常，该怎么办。
处理ZeroDivisionError 异常的try-except 代码块类似于下面这样：

>>> try:
...     print(5/0)
... except ZeroDivisionError:
...     print("You can't divide by zero!")
...
You can't divide by zero!

1.3 使用异常避免崩溃

发生错误时，如果程序还有工作没有完成，妥善地处理错误就尤其重要。这种情况经常会出现在要求用户提供输入的程序中；如果程序能够妥善地处理无效输入，就能再提示用户提供有效输入，而不至于崩溃。

代码:

print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit.")

while True:
    first_number = input("\nFirst number:")
    if first_number == 'q':
        break
    second_number = input("Second number: ")
    if second_number == 'q':
        break

    try:
        answer = int(first_number) / int(second_number)
    except ZeroDivisionError:
        print("You can't divide by 0!")
    else:
        print(answer)

测试记录:

E:\python\learn_python1\venv\Scripts\python.exe E:/python/learn_python1/exception_test1.py
Give me two numbers, and I'll divide them.
Enter 'q' to quit.

First number:10
Second number: 2
5.0

First number:0
Second number: 0
You can't divide by 0!

First number:q

Process finished with exit code 0

1.4 处理FileNotFoundError 异常

使用文件时，一种常见的问题是找不到文件：你要查找的文件可能在其他地方、文件名可能不正确或者这个文件根本就不存在。对于所有这些情形，都可使用try-except 代码块以直观的方式进行处理。

代码:

filename = 'abcdef.txt'

try:
    with open(filename) as f_obj:
        contents = f_obj.read()
except FileNotFoundError:
    msg = "Sorry, the file " + filename + " does not exists."
    print(msg)

测试记录:

E:\python\learn_python1\venv\Scripts\python.exe E:/python/learn_python1/except_test2.py
Sorry, the file abcdef.txt does not exists.

Process finished with exit code 0

1.5 分析文本

计算文本中有多少的单词

文本:

I love programming.
I love creating new games.
I also love finding meaning in large databases.
I lowe creating apps that can run in a browser.

代码:

filename = 'E:/python/file_test/programming.txt'

try:
    with open(filename) as f_obj:
        contents = f_obj.read()
except FileNotFoundError:
    msg = "Sorry, the file " + filename + " does not exists."
    print(msg)
else:
    # 计算文件大致包含多少个单词
    words = contents.split()
    num_words = len(words)
    print("The file " + filename + " has about " + str(num_words) + " words.")

测试记录:

E:\python\learn_python1\venv\Scripts\python.exe E:/python/learn_python1/except_test2.py
The file E:/python/file_test/programming.txt has about 26 words.

Process finished with exit code 0

1.6 使用多个文件

在某些场景，我们可能会有需求对多个文件进行操作。

文本:
db.txt

Oracle		1
MySQL		2
SQL Server	3
PostgreSQL	4

programming.txt

I love programming.
I love creating new games.
I also love finding meaning in large databases.
I lowe creating apps that can run in a browser.

代码:

def count_words(filename):
    """计算一个文件中大致包含多少个单词"""
    try:
        with open(filename) as f_obj:
            contents = f_obj.read()
    except FileNotFoundError:
        msg = "Sorry, the file " + filename + "does not exist."
        print(msg)
    else:
        # 计算文件大致包含多少个单词
        words = contents.split()
        num_words = len(words)
        print("The file " + filename + " has about " + str(num_words) + " words.")

filenames = ['E:/python/file_test/programming.txt','E:/python/file_test/db.txt']
for filename in filenames:
    count_words(filename)

测试记录:

E:\python\learn_python1\venv\Scripts\python.exe E:/python/learn_python1/except_test3.py
The file E:/python/file_test/programming.txt has about 26 words.
The file E:/python/file_test/db.txt has about 9 words.

Process finished with exit code 0

1.7 失败时一声不吭

在前一个示例中，我们告诉用户有一个文件找不到。但并非每次捕获到异常时都需要告诉用户，有时候你希望程序在发生异常时一声不吭，就像什么都没有发生一样继续运行。

要让程序在失败时一声不吭，可像通常那样编写try 代码块，但在except 代码块中明确地告诉Python什么都不要做。Python有一个pass 语句，可在代码块中使用它来让Python什么都不要做。

文本:
db.txt

Oracle		1
MySQL		2
SQL Server	3
PostgreSQL	4

– 不存在的文本
not_exist.txt

programming.txt

I love programming.
I love creating new games.
I also love finding meaning in large databases.
I lowe creating apps that can run in a browser.

代码:

def count_words(filename):
    """计算一个文件中大致包含多少个单词"""
    try:
        with open(filename) as f_obj:
            contents = f_obj.read()
    except FileNotFoundError:
        pass
    else:
        # 计算文件大致包含多少个单词
        words = contents.split()
        num_words = len(words)
        print("The file " + filename + " has about " + str(num_words) + " words.")

filenames = ['E:/python/file_test/programming.txt','E:/python/file_test/not_exist.txt','E:/python/file_test/db.txt']
for filename in filenames:
    count_words(filename)

测试记录:

E:\python\learn_python1\venv\Scripts\python.exe E:/python/learn_python1/except_test3.py
The file E:/python/file_test/programming.txt has about 26 words.
The file E:/python/file_test/db.txt has about 9 words.

Process finished with exit code 0

1.8 决定报告哪些错误

在什么情况下该向用户报告错误？在什么情况下又应该在失败时一声不吭呢？如果用户知道要分析哪些文件，他们可能希望在有文件没有分析时出现一条消息，将其中的原因告诉他们。如果用户只想看到结果，而并不知道要分析哪些文件，可能就无需在有些文件不存在时告知他们。向用户显示他不想看到的信息可能会降低程序的可用性。Python的错误处理结构让你能够细致地控制与用户分享错误信息的程度，要分享多少信息由你决定。

编写得很好且经过详尽测试的代码不容易出现内部错误，如语法或逻辑错误，但只要程序依赖于外部因素，如用户输入、存在指定的文件、有网络链接，就有可能出现异常。凭借经验可判断该在程序的什么地方包含异常处理块，以及出现错误时该向用户提供多少相关的信息。

参考:

1.Python编程:从入门到实践