Python 简明教程

Python - The try-except Block

Python Try-Except Block

在 Python 中，try-except 块用于优雅地处理异常和错误，确保您的程序即使在出现问题时也能继续运行。本教程将介绍 try-except 块的基本用法、其语法和最佳实践。

In Python, the try-except block is used to handle exceptions and errors gracefully, ensuring that your program can continue running even when something goes wrong. This tutorial will cover the basics of using the try-except block, its syntax, and best practices.

Python 中的 try-except 块用于捕获和处理异常。可能导致异常的代码放在 try 块中，而处理异常的代码放在 except 块中。

The try-except block in Python is used to catch and handle exceptions. The code that might cause an exception is placed inside the try block, and the code to handle the exception is placed inside the except block.

Syntax

以下是 Python 中 try-except 块的基本语法 −

Following is the basic syntax of the try-except block in Python −

try:
   # Code that might cause an exception
   risky_code()
except SomeException as e:
   # Code that runs if an exception occurs
   handle_exception(e)

Example

在此示例中，如果您输入非数字值，将引发“ ValueError ”。如果您输入零，将引发“ ZeroDivisionError ”。except 块处理这些异常并打印相应的错误消息 −

In this example, if you enter a non-numeric value, a ValueError will be raised. If you enter zero, a ZeroDivisionError will be raised. The except blocks handle these exceptions and prints appropriate error messages −

try:
   number = int(input("Enter a number: "))
   result = 10 / number
   print(f"Result: {result}")
except ZeroDivisionError as e:
   print("Error: Cannot divide by zero.")
except ValueError as e:
   print("Error: Invalid input. Please enter a valid number.")

Handling Multiple Exceptions

在 Python 中，您可以在单个 try-except 语句中使用多个 except 块来处理多种类型的异常。这样，您的代码就可以对执行期间可能发生的各种类型的错误做出不同的响应。

In Python, you can handle multiple types of exceptions using multiple except blocks within a single try-except statement. This allows your code to respond differently to different types of errors that may occur during execution.

Syntax

以下是 Python 中处理多种异常的基本语法 −

Following is the basic syntax for handling multiple exceptions in Python −

try:
   # Code that might raise exceptions
   risky_code()
except FirstExceptionType:
   # Handle the first type of exception
   handle_first_exception()
except SecondExceptionType:
   # Handle the second type of exception
   handle_second_exception()
# Add more except blocks as needed for other exception types

Example

在以下示例中 -

In the following example −

  1. If you enter zero as the divisor, a "ZeroDivisionError" will be raised, and the corresponding except ZeroDivisionError block will handle it by printing an error message.

  2. If you enter a non-numeric input for either the dividend or the divisor, a "ValueError" will be raised, and the except ValueError block will handle it by printing a different error message.

try:
   dividend = int(input("Enter the dividend: "))
   divisor = int(input("Enter the divisor: "))
   result = dividend / divisor
   print(f"Result of division: {result}")
except ZeroDivisionError:
   print("Error: Cannot divide by zero.")
except ValueError:
   print("Error: Invalid input. Please enter valid integers.")

Using Else Clause with Try-Except Block

在 Python 中，else 子句可以与 try-except 块结合使用，以指定仅在 try 块中未发生异常时才应运行的代码。这提供了一种方法来区分可能引发异常的主代码和仅在正常条件下才应执行的附加代码。

In Python, the else clause can be used in conjunction with the try-except block to specify code that should run only if no exceptions occur in the try block. This provides a way to differentiate between the main code that may raise exceptions and additional code that should only execute under normal conditions.

Syntax

以下是 Python 中 else 子句的基本语法 −

Following is the basic syntax of the else clause in Python −

try:
   # Code that might raise exceptions
   risky_code()
except SomeExceptionType:
   # Handle the exception
   handle_exception()
else:
   # Code that runs if no exceptions occurred
   no_exceptions_code()

Example

在以下示例中 -

In the following example −

  1. If you enter a non-integer input, a ValueError will be raised, and the corresponding except ValueError block will handle it.

  2. If you enter zero as the denominator, a ZeroDivisionError will be raised, and the corresponding except ZeroDivisionError block will handle it.

  3. If the division is successful (i.e., no exceptions are raised), the else block will execute and print the result of the division.

try:
   numerator = int(input("Enter the numerator: "))
   denominator = int(input("Enter the denominator: "))
   result = numerator / denominator
except ValueError:
   print("Error: Invalid input. Please enter valid integers.")
except ZeroDivisionError:
   print("Error: Cannot divide by zero.")
else:
   print(f"Result of division: {result}")

The Finally Clause

不管是否引发异常，finally 子句提供了一个机制来保证执行特定的代码。这对于执行清理操作（例如关闭文件或网络连接、释放锁或释放资源）很有用。

The finally clause provides a mechanism to guarantee that specific code will be executed, regardless of whether an exception is raised or not. This is useful for performing cleanup actions such as closing files or network connections, releasing locks, or freeing up resources.

Syntax

以下是在 Python 中 finally 子句的基本语法 −

Following is the basic syntax of the finally clause in Python −

try:
   # Code that might raise exceptions
   risky_code()
except SomeExceptionType:
   # Handle the exception
   handle_exception()
else:
   # Code that runs if no exceptions occurred
   no_exceptions_code()
finally:
   # Code that always runs, regardless of exceptions
   cleanup_code()

Example

在此示例中 -

In this example −

  1. If the file "example.txt" exists, its content is read and printed, and the else block confirms the successful operation.

  2. If the file is not found (FileNotFoundError), an appropriate error message is printed in the except block.

  3. The finally block ensures that the file is closed (file.close()) regardless of whether the file operation succeeds or an exception occurs.

try:
   file = open("example.txt", "r")
   content = file.read()
   print(content)
except FileNotFoundError:
   print("Error: The file was not found.")
else:
   print("File read operation successful.")
finally:
   if 'file' in locals():
      file.close()
   print("File operation is complete.")