Exceptions Syntax¶

There are many variations on the code that catches exceptions. Here is a brief summary, but other code variations are possible.

Catch All Exceptions¶

Catch all exceptions, regardless of their type. This will prevent your program from crashing, but this type of exception handling is rarely useful because you can’t do anything meaningful to recover from the abnormal condition.

try:
  # Your normal code goes here.
  # Your code should include function calls which might raise exceptions.
except:
  # If any exception was raised, then execute this code block.

Catch A Specific Exception¶

This is perhaps the most often used syntax. It catches one specific condition and tries to re-cover from the condition.

try:
  # Your normal code goes here.
  # Your code should include function calls which might raise exceptions.
except ExceptionName:
  # If ExceptionName was raised, then execute this block.

Catch Multiple Specific Exceptions¶

try:
  # Your normal code goes here.
  # Your code should include function calls which might raise exceptions.
except Exception_one:
  # If Exception_one was raised, then execute this block.
except Exception_two:
  # If Exception_two was raised, then execute this block.
else:
  # If there was no exception then execute this block.

Clean-up After Exceptions¶

If you have code that you want to be executed even if exceptions occur, you can include a finally code block:

try:
  # Your normal code goes here.
  # Your code might include function calls which might raise exceptions.
  # If an exception is raised, some of these statements might not be executed.
finally:
  # This block of code will always execute, even if there are exceptions raised

An Example of File I/O¶

One place where you will always want to include exception handling is when you read or write to a file. Here is a typical example of file processing. Note that the outer try: except: block takes care of a missing file or the fact that the existing file can’t be opened for writing. The inner try: except: block protects against output errors, such as trying to write to a device that is full. The finally code guarantees that the file is closed properly, even if there are errors during writing.

try:
   f = open("my_file.txt", "w")
   try:
      f.write("Writing some data to the file")
   finally:
      f.close()
except IOError:
   print "Error: my_file.txt does not exist or it can't be opened for output."

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