Python 异常
异常
- 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，当有错误发生时，都会创建一个异常对象，将程序继续运行，如果未对异常进行处理，程序将停止，并显示一个traceback，包含有关异常的报告
处理ZeroDivisionError异常
- 导致Python引发异常的简单错误，例如不能将一个数字除以0
- ZeroDivisionError异常对象
- 指出引发了哪种异常，并根据信息对程序进行修改
print(5/0)
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-3-fad870a50e27> in <module>()
----> 1 print(5/0)
ZeroDivisionError: division by zero
使用try-except代码块
- 编写try-except代码来处理可能引发的异常
try:
print(5/0)
except ZeroDivisionError:
print("You can't divide by zero!")
You can't divide by zero!
else代码块
- try代码块只能包含可能导致错误的代码。
- except代码块告诉Python，出现zerodivisionerror异常时该怎么办,或使用pass语句，使发生异常时一声不吭，直接跳过。
- else代码块存放try代码块成功执行的代码
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:")
try:
answer = int(first_number) / int(second_number)
except ZeroDivisionError:
print("You can't dicide by 0!")
# pass 直接跳过
else:
print(answer)
Give me two numbers,and I'll divide them.
Enter 'q' to quit.
First number:5
Second number:0
You can't dicide by 0!
First number:5
Second number:2
2.5
First number:q
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。
filename = 'alice.txt'
with open(filename) as f_obj:
contents = f_obj.read()
---------------------------------------------------------------------------
FileNotFoundError Traceback (most recent call last)
<ipython-input-11-665c307cd5e9> in <module>()
1 filename = 'alice.txt'
----> 2 with open(filename) as f_obj:
3 contents = f_obj.read()
FileNotFoundError: [Errno 2] No such file or directory: 'alice.txt'
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)
Sorry, the file alice.txt does not exist.
分析文本
- 使用方法split()，它根据一个字符串创建一个单词列表
- 方法 split() 以空格为分隔符将字符串分拆成多个部分，并将这些部分都存储到一个列表中。结果是一个包含字符串中所有单词的列表，虽然有些单词可能包含标点。
>>> title = 'Alice in Wonderland'
>>> title.split()
['Alice', 'in', 'Wonderland']
# coding=gbk
filename = 'alice.txt'
try:
with open(filename,encoding='UTF-8') as f:
contents = f.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.")
The file alice.txt has about 29465 words.
使用多个文件
- 将计算文件单词数量放置在一函数里，通过给函数实例化，确定文件名及位置，然后对该文件调用函数，得到计算结果
- 通过使用for循环来计算多个文件的单词，函数实例化，将文件放置列表中。
- 为达到显著效果，可以将某些文件放置其他路径下
def count_words(filename):
try:
with open(filename,encoding='UTF-8') as f:
contents = f.read()
except FileNotFoundError:
print("Sorry, the file " + filename + " does not exist.")
else:
words = contents.split()
numbers = len(words)
print("The file " + filename + " has about " + str(numbers) + " words.")
filenames = ['alice.txt','siddhartha.txt', 'moby_dick.txt', 'little_women.txt']
for filename in filenames:
count_words(filename)
The file alice.txt has about 29465 words.
Sorry, the file siddhartha.txt does not exist.
The file moby_dick.txt has about 215830 words.
The file little_women.txt has about 189079 words.
练习
# 1.加法运算：
# 提示用户停工数值输入时，常出现的一个问题是，用户提供的是文本而不是数字，在这种情况下，
# 当你尝试将输入转换为整数时，将引发TypeError异常，编写一个程序，提示用户输入两个数字，再将它们想加起来并打印结果
# 在用户输入的任何一个值不是数字时都不活TypeError异常，并打印一条友好的错误消息，对编写的程序进行测试：
# 先输入两个数字，在输入一些文本而不是数字。
try:
x = input("Give me a number: ")
x = int(x)
y = input("Give me another number: ")
y = int(y)
except ValueError:
print("Sorry, I really needed a number.")
else:
sum = x + y
print("The sum of " + str(x) + ' and ' + str(y) +' is ' + str(sum) + '.')
Give me a number: 23
Give me another number: 4
The sum of 23 and 4 is 27.
# 2.加法计算器：
# 编写的代码放在一个 while 循环中，让用户犯错（输入的是文本而不是数字）后能够继续输入数字。
print("Enter 'q' at any time to quit.\n")
while True:
try:
x = input("Give me a number: ")
if x == 'q':
break
x = int(x)
y = input("Give me antother number: ")
if x == 'q':
break
y = int(y)
except ValueError:
print("Sorry, I really needed a number.")
else:
sum = x + y
print("The sum of " + str(x) + ' and ' + str(y) + ' is ' + str(sum) + '.')
Enter 'q' at any time to quit.
Give me a number: 23
Give me antother number: 47
The sum of 23 and 47 is 70.
Give me a number: 3
Give me antother number: file
Sorry, I really needed a number.
Give me a number: -20
Give me antother number: 20
The sum of -20 and 20 is 0.
Give me a number: q
# 3.猫和狗：
# 创建两个文件 cats.txt 和 dogs.txt ，在第一个文件中至少存储三只猫的名字，在第二个文件中至少存储三条狗的名字。编写一个程序，尝试读取这些文件，
# 并将其内容打印到屏幕上。将这些代码放在一个 try-except 代码块中，以便在文件不存在时捕获 FileNotFound 错误，并打印一条友好的消息。将其中一个文件
# 移到另一个地方，并确认 except 代码块中的代码将正确地执行。
# cat.txt
xiao hua
hu zi
hua zai
# dog.txt
lao da
lao er
lao san
# cat_and_dog.py
def cat_and_dog(filename):
try :
with open(filename,'r') as f:
notes = f.read()
except FileNotFoundError:
print("Sorry, The file is not exict.")
else:
print(notes + '\n')
filenames = ['cat.txt','dog.txt']
for filename in filenames:
cat_and_dog(filename)
运行：
移出cat.txt
# 4.沉默的猫和狗
# 修改你在练习3中编写的 except 代码块，让程序在文件不存在时一言不发。使用pass
# cat_and_dog.py
def cat_and_dog(filename):
try :
with open(filename,'r') as f:
notes = f.read()
except FileNotFoundError:
pass
else:
print(notes + '\n')
filenames = ['cat.txt','dog.txt']
for filename in filenames:
cat_and_dog(filename)
运行：
# 5.常见单词：
# 针对文本可以使用方法 count() 来确定特定的单词或短语在字符串中出现了多少次。
# 例如，下面的代码计算 'row' 在一个字符串中出现了多少次：
line = "ROw ,row,row your boat"
line.count('row')
2
通过使用 lower() 将字符串转换为小写，可捕捉要查找的单词出现的所有次数，而不管其大小写格式如何。
line.lower().count('row')
3
