Python Try Except
The try block lets you test a block of code for errors.
The except block lets you handle the error.
The else block lets you execute code when there is no error.
The finally block lets you execute code, regardless of the result of the try- and except blocks.
异常处理
When an error occurs, or exception as we call it, Python will normally stop and generate an error message.
These exceptions can be handled using the try statement
示例
The try block will generate an exception, because x is not defined
try
print(x)
except
print("An exception occurred")
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Since the try block raises an error, the except block will be executed.
Without the try block, the program will crash and raise an error
多种异常
You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error
示例
Print one message if the try block raises a NameError and another for other errors
try
print(x)
except NameError
print("Variable x is not defined")
except
print("Something else went wrong")
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Else
You can use the else keyword to define a block of code to be executed if no errors were raised
示例
In this example, the try block does not generate any error
try
print("Hello")
except
print("Something went wrong")
else
print("Nothing went wrong")
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Finally
The finally block, if specified, will be executed regardless if the try block raises an error or not.
示例
try
print(x)
except
print("Something went wrong")
finally
print("The 'try except' is finished")
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This can be useful to close objects and clean up resources
示例
Try to open and write to a file that is not writable
try
f = open("demofile.txt")
try
f.write("Lorum Ipsum")
except
print("Something went wrong when writing to the file")
finally
f.close()
except
print("Something went wrong when opening the file")
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The program can continue, without leaving the file object open.
引发异常
As a Python developer you can choose to throw an exception if a condition occurs.
To throw (or raise) an exception, use the raise keyword.
示例
Raise an error and stop the program if x is lower than 0
x = -1
if x < 0
raise Exception("Sorry, no numbers below zero")
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The raise keyword is used to raise an exception.
You can define what kind of error to raise, and the text to print to the user.
示例
Raise a TypeError if x is not an integer
x = "hello"
if not type(x) is int
raise TypeError("Only integers are allowed")
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