每当发生让python不知所措的错误的时候，都会创建一个异常对象。如果编写了处理处理改异常的代码，程序将继续运行，如果未对异常处理，程序将停止，并显示一个traceback，其中包含有关异常的报告。
异常使用 try - except 代码块处理，执行指定操作的同时告诉python发生异常了怎么办？
1. 处理ZeroDivisionError异常
print(3/0)
被除数时不能为0的，但在python中为0了会怎么呢？
Traceback (most recent call last):
File "division.py", line 1, in <module>
print(3/0)
ZeroDivisionError: division by zeropython停止运行，并指出引发了哪种异常。发生异常也希望程序能继续运行怎么办呢？
2. 使用 try - except 代码块
当有可能发生错误时，可编写一个 try - except 代码块来处理可能引发的异常：
try:
print(3/0)
except ZeroDivisionError:
print("You can't divide by zero!")运行结果：
You can't divide by zero!3. 使用异常避免崩溃
发生错误时，如果程序还有工作没完成，妥善处理错误就尤为重要。
# 一个只执行除法运算的简单计算器
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
answer = int(first_number)/int(second_number)
print(answer)如果用户给第二个数字输入0，就会出现：
Traceback (most recent call last):
File "division.py", line 12, in <module>
answer = int(first_number)/int(second_number)
ZeroDivisionError: division by zero这样的信息会把不懂技术的用户搞糊；会对怀有恶意的用户暴露重要信息。
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4. else代码块
# 一个只执行除法运算的简单计算器
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 zero!")
else:
print(answer)运行结果：
Give me two numbers, and I'll divide them.
Enter 'q' to quit.
First number: 3
Second number: 0
You can't divide by zero!
First number: 4
Second number: 2
2.0
First number: q尝试执行 try 代码块中的代码：若引发了异常，执行except代码块的代码；若没有发生异常，执行else代码块。
5. 处理FileNotFoundError异常
使用文件时，最常见的问题就是找不到文件：文件可能在其他地方、文件名可能不对或者文件就不存在。尝试读取一个不存在的文件：
filename = 'alice.txt'
with open(filename) as f_obj:
contents = f_obj.read()运行结果：
Traceback (most recent call last):
File "alice.py", line 3, in <module>
with open(filename) as f_obj:
FileNotFoundError: [Errno 2] No such file or directory: 'alice.txt'上述代码运行错误是函数open()导致的，因此要处理这个错误，必须把try语句放在包含open()的代码前：
filename = 'alice.txt'
try:
with open(filename) as f_obj:
contents = f_obj.read()
except FileNotFoundError:
msg = "Sorry, the file " + filename + " does not exist."
print(msg)运行结果：
Sorry, the file alice.txt does not exist.告知用户文件不存在。
6. 分析文本
分析包含整本书的文本文件: 在网站http://www.gutenberg.org/下载文学文本The Lay of the Nibelung Men.txt，提取文本，并尝试计算包含了多少个单词：
filename = 'The-Lay-of-the-Nibelung-Men.txt'
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() # 以空格为分隔符，将字符串分拆成多个部分，并将这些部分存储在一个列表中
num_words = len(words)
print("The file " + filename + " has about " + str(num_words) + " words.")运行结果：
The file The-Lay-of-the-Nibelung-Men.txt has about 124918 words.7. 使用多个文件
def count_words(filename):
"""计算一个文件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.")定义一个可以计算文件有多少单词的函数，之后就可调用该函数来计算多个文件的：
filenames = ['A-Case-of-Sunburn.txt', 'moby_dick.txt', 'The-Lay-of-the-Nibelung-Men.txt']
for filename in filenames:
count_words(filename)运行结果：
The file A-Case-of-Sunburn.txt has about 9416 words.
Sorry, the file moby_dick.txt does not exist.
The file The-Lay-of-the-Nibelung-Men.txt has about 124918 words.8. 失败时一声不吭
若希望在不捕获到异常时一声不吭，继续执行：
try:
...
except FileNotFoundError:
pass
else:
...在捕获到异常时，执行pass语句。