Handling a File Error

As already pointed out in the exceptions lesson, Python contains a vast set of built-in exceptions. One important class has to do with file handling. The example presented in the lesson will help to review file handling methods and put them in the context of handling exceptions.

Re-raising Exceptions

A very common scenario is that when an exception appears, you want to do something but then raise the same exception. This is a very common case when writing to a database or to different files. Imagine the case where you are storing information in two files, in the first one you store spectra and in the second one the temperature at which you acquire each one. You first save the spectra and then the temperature, and you know that each line on one file corresponds to one file on the second file.

Normally, you save first a spectrum and then you save the temperature. However, once in a while, when you try to read from the instrument, it crashes and the temperature is not recorded. If you don't save the temperature, you will have an inconsistency in your data, because a line is missing. At the same time, you don't want the experiment to keep going, because the instrument is frozen. Therefore, you can do the following:

[data already saved]

try:
    temp = instrument.readtemp()
except:
    remove_last_line(data_file)
    raise
save_temperature(temp)

What you can see here is that we try to read the temperature and if anything happens, we will catch it. We remove the last line from our data file, and then we just call raise. This command will simply re-raise anything that was caught by the except. With this strategy, we are sure that we have consistent data, that the program will not keep running and that the user will see all the proper information regarding what went wrong.