There are generally three types of errors that you might make when writing script:
1) syntax errors include spelling and punctuation mistakes. Making typos, leaving out a colon (:), or indenting incorrectly are very common.
There are also:
2) exceptions, for example, in which modules and functions (and other parts of Python) don't have what they need to run properly. There are many, many types of exceptions, and luckily they are identified in a very general sense in the error statements in the results, and the line numbers in which they occur can be found as well.
3) The other type of error can be very insidious: the logic error. In this one, your code might run fine, but the answer you get is wrong, because you've told Python to do something different than what you intended. Python will not know the difference; it's only following directions, and can't read our minds! This is an example of GIGO: garbage in: garbage out.
For our assignment, we looked at three scripts with syntax and exception errors. In Scripts 1 and 2, we repaired the errors, but in Script 3, we added a "try-except" loop, which gives Python an alternative path, in order to step around the error. We didn't actually fix the error, and the results tell us that it's there.
You don't necessarily have to repair an error right away. It is possible to use a "try-except" statement around a block with an error, which isolates and bypasses the error and goes on through the rest of the script. It "tries" the problematic line, then has an "except" route, which offers an alternative for Python, such as printing out a statement identifying the nature of the error. This way Python doesn't have to get hung up in the middle of the script.
I found that there are three excellent ways to find errors. The first two are rather rudimentary. 1) You can "comment out" most of the lines in the script. This means you type # in front of every line except the first one, then run it. If it runs okay, take the comment # off the next line or loop (for, while, etc) and run it again. You keep working your way downward through the script, de-commenting the lines one by one, until it runs properly. This way, you can eliminate the lines that are okay, and find and fix the ones that aren't.
2) Another way to check and see if a block of script is working right, if there are no results following it, is to insert a "print" statement in a new line after it. I used "print okay." This way, if "okay" appeared in the interactive window, I knew that everything above that print statement was okay.
3)PythonWin also has debugging tools that work in a similar way, but without having to comment out or add print statements, which can be cumbersome if you have a long script. You can turn on the debugger, then run the script in a "step-through" fashion one line or block at a time. At each step, you'll see that it was okay, or that it wasn't. This way, you can find the errors or confirm that a line is okay. You can also insert break points, which isolate the parts you want to run.
Below are screenshots of my results from the three assigned scripts. They are shown in the PythonWin Interactive Window, after I had repaired the errors or added a try-except statement:
Script 1. These are the results, after the script was corrected.
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| Script 1 Results |
Script 2: This script had 8 errors. These are the results, after they were identified and fixed.
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| Script 2 Results |
Script 3:
In this script, the error was not corrected. Rather, a try-except statement was inserted after the error was identified, and the results printed out the type of error occurring in Part A of the script, then went on to Part B and printed the results. Part B did not contain any errors, and was not directly affected by the error in Part A. However, until the error in Part A was dealt with, or "caught," the script would not continue to execute, and would not have been able to proceed to Part B and produce results.
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| Script 3 Results |
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