Exceptions

Many situations can result in an error.

• An attempt to read a file that isn’t present.
• An attempt to cast a string to a number when the string does not represent a number.
• An index out of bounds.
• A key requested is not in the dictionary.

All these conditions (and many more) are called exceptions. If you do not handle them, the interpreter will, and it will just stop with some error message. We can prepare for failure by catching exceptions ourselves.

Catching Exceptions

The interpreter throws an exception so we can catch it. We can do this with a try except block.

try:
    fin=open("datafile.csv","r")
except IOError:
    print("Unable to open file.")

This stops on the named exception IOError. Any other exception will not be caught.

It is not necessary to specify exceptions if you are unsure of the correct name; the following will be executed for any exception.

try: 
    something
except: 
    print("Error")

More processing than we have shown is usually required, of course. For example, if a file cannot be opened, we may choose to exit the script.

try: 
    fin=open("datafile.csv","r")
except IOError:
    print("Unable to open file.")
    exit()

The interpreter provides a large number of built-in exceptions. (See the documentation for a complete list.) Some of the more commonly encountered are

• EOFError
• IOError
• KeyboardInterrupt
• IndexError
• IKeyError
• NameError
• NotImplementedError
• TypeError
• ValueError
• ZeroDivisionError

You can provide more than one except for the same try

try:
   input=int(input("Please enter an integer:"))
except EOFError:
   print("You did not enter anything")
except ValueError:
   print("You did not enter an integer")

If you want your script to perform some action upon passing the try, use else

try:
    fout=open("outputfile.csv","w")
except IOError:
    print("Unable to open file")
    exit()
else:
    print("File successfully opened")

If you have code that should be executed regardless of whether the try passes or fails, use finally:

try:
   fin=datafile.read()
except IOError:
   print("Unable to read ",datafile)
finally:
   fin.close()

You cannot use an else in any try clause that contains a finally block.

With/As

In many case, especially those having to do with file operations, the with and as operators can replace the try/except/finally block. The following will close the file whether the operation was successful or not.

with open('myfile','w') as f:
    for line in f:
        print(line)

Exercise

Download the file numbers.csv. Write a program to read it and extract the columns as x,y. It has a one-line header. What problem did you encounter? Use an exception to exclude the line with the error.

Optionally, add exception handling to the file operations (opening, reading, etc.)

x=[]
y=[]
with open("numbers.csv") as fin:
    for line in fin:
        data=line.rstrip("\r\n").split(",")
        try:
            x.append(float(data[0]))
            y.append(float(data[1]))
        except:
            print("Can't convert data in line:",line)

print(x[3],y[3])