Python学习手册之Python异常和文件

num1 = 5 
num2 = 0
print(num1/num2)

>>>
ZeroDivisionError: division by zero
>>>

Python 还有其他几个内置的异常，比如 ZeroDivisionError 和 OSError。第三方库也经常定义自己的异常。

try:
 num1 = 5
 num2 = 0
 print(num1/num2)
 print("Done!")
except ZeroDivisionError:
 print("Has a error")
 print("due to zero division")

>>>
Has a error
due to zero division
>>>

上面的例子里 except 语句定义了 ZeroDivisionError 异常捕捉。

try:
 var = 5
 print(var + "string!")
 print(var/2)
except ZeroDivisionError:
 print("Divided by zero!")
except (ValueError,TypeError):
 print("Has a error!")

>>>
Has a error!
>>>

try:
 var = 5
 print(var + "string!")
 print(var/2)
except:
 print("An error occurred")

>>>
An error occurred
>>>

这种异常处理方式通常用在处理用户输入。

try:
 str = "Hello"
 var = 5
 print(str)
 print(var/0)
except ZeroDivisionError:
 print("Divided by zero")
finally:
 print("This code will run no matter what")

>>>
Hello
Divided by zero
This code will run no matter what
>>>

try:
 print(1)
except:
 print("a")
finally:
 print("Hello")

>>>
1
Hello
>>>

try:
 print(1)
 print(5/0)
except ZeroDivisionError:
 print(Error_var)
finally:
 print("This is executed last!")

>>>
1
This is executed last!
ZeroDivisionError: division by zero
During handling of the above exception, another exception occurred:
NameError: name 'Error_var' is not defined
>>>

print(1)
raise ZeroDivisionError
print(5)

>>>
1
ZeroDivisionError
>>>

你需要声明你要抛出的例外名称。

age = 12
raise TypeError("Invalid type!")

>>>
TypeError("Invalid type!")
>>>

try:
 num= 5/0
except:
 print("Has a error")
 raise 

>>>
ZeroDivisionError: division by zero

Has a error
>>>

print(1)
assert 1 + 2 == 3
print("Yes")
assert 1 + 1 == 0
print("Yes")

>>>
1
Yes
AssertionError
>>>

程序员通常在函数的开头放置断言以检查有效的输入，在函数调用之后放置断言以检查有效的输出。
断言可以在运行是通过添加 -O 或 -OO 选项来关闭。

print(1)
assert 1 != 2
print(2)
assert True
print(3)
assert False
print(4)
assert 1 + 1 == 2
print(5)

>>>
1
2
3
AssertionError
>>>

num = 5
assert (num <= 0),"The num is bigger than zero"

>>>
AssertionError: The num is bigger than zero
>>>

AssertionError 异常可以像任何其他异常一样使用 try/except 语句被捕捉和处理，如果程序不处理，这种类型的异常将终止程序的运行。

f = open("test.txt")

注意：open 函数的参数是文件的路径。如果文件与程序位于同一个目录中，则可以不要指定路径直接使用文件名。

# read mode
open("test.txt", "r")
open("test.txt")

# write mode
open("test.txt", "w")

# binary write mode
open("test.txt", "wb")

特别注意：使用 "w" 模式时，如果文件已经存在，会把旧文件的内容全部都清除掉。

f = open("test.txt","w")
# close the file
f.close()

接下来我们会读取和写入文件。

f = open("test.txt","r")
content = f.read()
print(content)
f.close()

打印文件名为 "test.txt" 的所有内容。

f = open("test.txt","r")
print(f.read(32))
print(f.read(16))
print(f.read(8))
print(f.read())
f.close()

f = open("test.txt","r")
f.read()
print("reading")
print(f.read())
print("finished!")
f.close()

>>>
reading

finished!
>>>

f = open("test.txt","r")
str = f.read()
print(len(str))
f.close()

>>>
16
>>>

f = open("test.txt","r")
print(f.readlines())
f.close()

>>>
['I like Python!!!']
>>>

f = open("test.txt","r")
for line in f:
 print(line)
f.close()

>>>
Line 1 : Hello world!

Line 2 : I like Python！

Line 3 : I like Java！
>>>

在输出中，返回的行包含换行符，打印 print 函数在输出的末尾自动添加新行。

f = open("test.txt","w")
f.write("I am happy!")
f.close()

f = open("test.txt","r")
print(f.read())
f.close()

>>>
I am happy!
>>>

如果文件不存在，"w" 将会创建一个文件。

f = open("test.txt","r")
print("Reading...")
print(f.read())
print("Finished!")
f.close()

f = open("test.txt","w")
f.write("New text")
f.close()

f = open("test.txt","r")
print("Reading new contents")
print(f.read())
print("Finished!")
f.close()

>>>
Reading...
I am happy!
Finished!
Reading new contents
New text
Finished!
>>>

特别注意：原有的文件将被新内容重写。

msg = "I like Python!!!"
f = open("test.txt","w")
count = f.write(msg)
print(count)
f.close()

>>>
16
>>>

try:
 f = open("test.txt")
 print(f.read())
finally:
 f.close()

当异常发生时也能确保文件对象被正确关闭。

with open("test.txt") as f:
print(f.read())

使用 with 语句打开文件是个好习惯，就算有例外发生，with 语句也会自动帮我们关闭文件句柄。