Example: Catching Built-in Exceptions

Python’s default exception handling is often exactly what you want—especially for code in a top-level script file, an error generally should terminate your program immediately. For many programs, there is no need to be more specific about errors in your code.

Sometimes, though, you’ll want to catch errors and recover from them instead. If you don’t want your program terminated when Python raises an exception, simply catch it by wrapping the program logic in a try. This is an important capability for programs such as network servers, which must keep running persistently. For example, the following code catches and recovers from the TypeError Python raises immediately when you try to concatenate a list and a string (the + operator expects the same sequence type on both sides):

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def kaboom(x, y):
    print(x + y)               # Trigger TypeError

try:
    kaboom([0,1,2], "spam")
except TypeError:              # Catch and recover here
    print('Hello world!')
print('resuming here')         # Continue here if exception or not

When the exception occurs in the function kaboom, control jumps to the try statement’s except clause, which prints a message. Since an exception is “dead” after it’s been caught like this, the program continues executing below the try rather than being terminated by Python. In effect, the code processes and clears the error, and your script recovers:

% python kaboom.py
Hello world!
resuming here

Notice that once you’ve caught an error, control resumes at the place where you caught it (i.e., after the try); there is no direct way to go back to the place where the exception occurred (here, in the function kaboom). In a sense, this makes exceptions more like simple jumps than function calls—there is no way to return to the code that triggered the error.

 

[74] As mentioned in the prior chapter, the text of error messages and stack traces tends to vary slightly over time and shells. Don’t be alarmed if your error messages don’t exactly match mine. When I ran this example in Python 3.0’s IDLE GUI, for instance, its error message text showed filenames with full absolute directory paths.