Our code won’t always be able to run without issue. So many things can go wrong when running code — network connectivity can fail, file system permissions can be wrong, bad inputs can be passed to our scripts. Thankfully, we can normally tell when these types of issues *might* happen in our code and we can handle them. In this hands-on lab, we’ll add exception handling to an existing script that can run into many of these issues. To feel comfortable completing this lab, you’ll want to know how to perform exception handling in Python (watch the “Handling Exceptions with `try`, `except`, `else`, and `finally`” video from the Certified Associate in Python Programming Certification course).
Successfully complete this lab by achieving the following learning objectives:
- Add Exception Handling Around Setting the `name` Variable
The first expression in the
exception_handling.pyscript reads from
sys.argv, which is a list. If the script is run without any additional positional arguments, there won’t be an item at the index of
1and we’ll receive an
IndexError. Let’s add exception handling around this line to catch the error, print an error message about the number of required arguments, and then use
sys.exit(1)to stop the script and exit with a non-zero status code.
import sys # 1) Fetch name from the first argument passed to the script. # If there is no argument, then exist with a readable error message. # The potential error here is an IndexError. try: name = sys.argv except IndexError: print("Error: please provide a name and a repeat count to the script.") sys.exit(1) # Remainder of file omitted
Now if we run the script without any arguments, we should see this error message:
$ python3.7 exception_handling.py Error: please provide a name and a repeat count to the script. $ python3.7 exception_handling.py Kevin Traceback (most recent call last): File "exception_handling.py", line 16, in <module> repeat_count = int(sys.argv) IndexError: list index out of range
This next error is related to the next bit of logic in the script.
- Handle Potential `ValueError` and `IndexError` When Setting `repeat_count` Variable
When we set the
repeat_countvariable, we index
sys.argvat the index of
2so we can once again run into an
IndexError. In that case, we’ll want to provide the same error message we did for the
namevariable. Additionally, we can also run into a
ValueErrorif the value provided cannot be cast to an integer, so we’ll need to have a second
exceptstatement that handles that and provides a more useful error message.
# previous code omitted # 2) Convert the second argument to be an integer so that we can repeatedly print out # the name argument. We should either a ValueError or an IndexError if there weren't enough # arguments provided. try: repeat_count = int(sys.argv) except IndexError: print("Error: please provide a name and a repeat count to the script.") sys.exit(1) except ValueError: print("Error: the repeat count needs to be a number.") sys.exit(1) # remaining code omitted
- Handle Potential File IO Errors When Writing to `name_repeated.txt`
The last section of this script deals with interacting with a file, so there are numerous potential errors we could run into. Depending on the version of Python we use to run the script, we could get either an
PermissionErrorif we don’t have permissions to interact with a file. To make this script backward-compatible, we’ll catch
OSErrorand would also be caught. Additionally, we’re going to catch
IOErrorsince this is another common error that happens when interacting with files. These error types will both be caught by the same
exceptstatement. Our requirements state that we need to print to the screen if we can’t write to a file, and we’re going to keep our
tryblock small by writing to the file in the
elsebranch if we can read from it.
# previous code omitted # 3) Open a file called `name_repeated.txt` in the `root_files` directory and write a line for each time the name was # repeated. If the user running the script doesn't have write permissions for the directory that # we're writing to then we might see a PermissionError. PermissionError is new as of 3.3, so let's instead catch # OSError and IOError to make the script more backward compatible. If an error is caught print to the screen instead. try: f = open("root_files/name_repeated.txt", "w") except (OSError, IOError) as err: print("Error: unable to write file (", err, ")") for i in range(1, repeat_count + 1): print(i, "-", name) else: names = [name + "n" for i in range(1, repeat_count + 1)] f.writelines(names) f.close()
Now we’ve successfully handled all the common errors we might run into when running this script.