Python Error Handling with Exceptions

What is an exception?

An exception is a runtime condition that could cause your program to crash

Almost every modern programming language handles exceptions

This technique is generically referred to as validation

We will look at the Python try�except block that protects your program from crashes

What triggers an exception?

Exceptions can occur automatically or they can be raised manually

Exceptions that occur automatically are raised by a built-in library function or operation

For example, the expression:

a = 3 / 0

would raise a ZeroDivisionError exception by the division ( / ) operator

This python module handles simple ZeroDivisionError exceptions

Here is a Python module (TryExceptExample.py) that handles generic exceptions

Sometimes you will need to raise an exception manually

For example, you might want to exclude integers that fall outside the data range that is valid for the problem you�re trying to solve

There is no automatic trigger that will make this happen

If you want to limit the data range for a problem to all positive non-zero integers, you could do this:

>>> a = -1 # set a test value for our variable a

>>> if a < 0:

...�� raise ValueError("the value of a is less than 0")

...

Traceback (most recent call last):

� File "<stdin>", line 2, in <module>

ValueError: the value of a is less than 0

This Python module (RaiseValueErrorException.py) uses both automatically and manually generated exceptions and includes other improvements over TryExceptExample.py

Using try�except blocks

If an exception (runtime error) happens in a try clause, we can handle it in a corresponding except clause

try does not take any arguments

except can take a qualifier (one of many exception types) or not

If it does not take a qualifier, any exception will trigger it

By handling an exception in an except clause, we can take action to correct the error, or at least prevent an immediate crash

Exception classes

A try�except block can handle many different types of exceptions

Exceptions are defined as objects of Python classes

Many are �built-in� to a Python language distribution (like Anaconda and most others)

Typical built-in exception classes include IOError, TypeError, OverflowError, and many others

A single try clause can be followed by multiple except clauses

Think of the try clause as a place to put dangerous code

The except clauses handle specific things that go wrong in the try clause

You can create your own exception classes that inherit from Exception

Best practices with try�except processing

In general, put code in a try clause if it has the potential to cause a crash

Often this can be due to bad external data values

At best, we want the program to continue working and simply reject the bad data

At worst, we want it to crash gracefully, giving the user relevant post mortem information

Be specific�if you can, trap the exact exceptions

RaiseValueErrorException.py does this

TryExceptExample.py does not do this�it traps generic exceptions with except:

Typical uses in an ArcPy script

In a data entry box, you could check to see if a specific type of ArcGIS data object (like a shapefile) has been entered

If the user were to type in the path to a shapefile, you could verify that it exists and then tell the user if there was a problem

And a million other things�