9.Python之异常处理

• 异常处理
  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。如果编写了处理该异常的代码，程序会继续运行；否则程序会停止，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-exception代码块让python执行指定的操作，同时告诉python发生异常怎么办。使用了try-exception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。
• ZeroDivisionError
  一个数字除以0会触发ZeroDivisionError异常。

使用try-except代码块处理可能引发的异常，将可能导致错误的代码行放在try代码块内。

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

创建一个只执行除法运算的计算器：

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

while True:
	first_number=input("first 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 is "+ str(answer))

在除数为0的时候触发异常，导致程序崩溃，让用户看到traceback是很糟糕的事情。
使用try-except代码块进行异常处理：

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

while True:
	first_number=input("first 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 is "+ str(answer))

通过预测可能发生错误的代码，可以编写出健壮的程序，即使面临无效数据或缺少资源，也能够继续运行，从而抵御无意的用户错误和恶意的攻击。

• FileNotFoundError
  使用文件时，常见的问题是找不到文件：文件在别的地方、文件名不对或者文件根本不存在。

filename = "alice.txt"
with open(filename) as f_obj:
	contents = f_obj.read()

这个异常是函数open()导致的，因此需要将open()放在try-except代码块内。

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)

简单分析文本，使用split()方法可以以空格为分隔符将字符串拆分为多个部分，并将这些部分都存储到一个列表中。
将alice.txt文件放入文件夹中

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)
else:
	words = contents.split()
	num_words = len(words)
	print("The file "+filename +" has about "+ str(num_words)+" words.")

• pass
  使用pass可以让程序在异常的时候一声不吭，直接跳过。
  修改上一段程序为函数形式，以读取多个文件：

def count_words(filename):
	try:
		with open(filename) as f_obj:
			contents = f_obj.read()	
	except FileNotFoundError:
		msg="Sorry,the file "+ filename + " does not exist."
		print(msg)
	else:
		words = contents.split()
		num_words = len(words)
		print("The file "+filename +" has about "+ str(num_words)+" words.")	
	
filenames = ['alice.txt','little_women.txt','siddhartha.txt']
for filename in filenames:
	count_words(filename)

将异常处理代码改为pass

def count_words(filename):
	try:
		with open(filename) as f_obj:
			contents = f_obj.read()	
	except FileNotFoundError:
		#msg="Sorry,the file "+ filename + " does not exist."
		#print(msg)
		pass
	else:
		words = contents.split()
		num_words = len(words)
		print("The file "+filename +" has about "+ str(num_words)+" words.")	
	
filenames = ['alice.txt','little_women.txt','siddhartha.txt']
for filename in filenames:
	count_words(filename)

pass语句还充当了占位符，提醒你在程序的某个地方什么都没做，以后可能要做点什么。例如将找不到的文件的名称写入missing_fils.txt文件。

• 所有异常
  

python异常
头大。。。
就酱！