Errors and Exception Handling

In this lecture we will learn about Errors and Exception Handling in Python. You've definitely already encountered errors by this point in the course. For example:

In [1]:
print 'Hello
  File "<ipython-input-1-23e01f0d17c8>", line 1
    print 'Hello
               ^
SyntaxError: EOL while scanning string literal

Note how we get a SyntaxError, with the further description that it was an EOL (End of Line Error) while scanning the string literal. This is specific enough for us to see that we forgot a single quote at the end of the line. Understanding these various error types will help you debug your code much faster.

This type of error and description is known as an Exception. Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal.

You can check out the full list of built-in exceptions here. now lets learn how to handle errors and exceptions in our own code.

try and except

The basic terminology and syntax used to handle errors in Python is the try and except statements. The code which can cause an exception to occue is put in the try block and the handling of the exception is the implemented in the except block of code. The syntax form is:

try:
   You do your operations here...
   ...
except ExceptionI:
   If there is ExceptionI, then execute this block.
except ExceptionII:
   If there is ExceptionII, then execute this block.
   ...
else:
   If there is no exception then execute this block. 

We can also just check for any exception with just using except: To get a better understanding of all this lets check out an example: We will look at some code that opens and writes a file:

In [11]:
try:
    f = open('testfile','w')
    f.write('Test write this')
except IOError:
    # This will only check for an IOError exception and then execute this print statement
   print "Error: Could not find file or read data"
else:
   print "Content written successfully"
   f.close()
Content written successfully

Now lets see what would happen if we did not have write permission (opening only with 'r'):

In [14]:
try:
    f = open('testfile','r')
    f.write('Test write this')
except IOError:
    # This will only check for an IOError exception and then execute this print statement
   print "Error: Could not find file or read data"
else:
   print "Content written successfully"
   f.close()
Error: Could not find file or read data

Great! Notice how we only printed a statement! The code still ran and we were able to continue doing actions and running code blocks. This is extremely useful when you have to account for possible input errors in your code. You can be prepared for the error and keep running code, instead of your code just breaking as we saw above.

We could have also just said except: if we weren't sure what exception would occur. For example:

In [13]:
try:
    f = open('testfile','r')
    f.write('Test write this')
except:
    # This will check for any exception and then execute this print statement
   print "Error: Could not find file or read data"
else:
   print "Content written successfully"
   f.close()
Error: Could not find file or read data

Great! Now we don't actually need to memorize that list of exception types! Now what if we kept wanting to run code after the exception occurred? This is where finally comes in.

finally

The finally: block of code will always be run regardless if there was an exception in the try code block. The syntax is:

try:
   Code block here
   ...
   Due to any exception, this code may be skipped!
finally:
   This code block would always be executed.

For example:

In [16]:
try:
   f = open("testfile", "w")
   f.write("Test write statement")
finally:
   print "Always execute finally code blocks"
Always execute finally code blocks

We can use this in conjunction with except. Lets see a new example that will take into account a user putting in the wrong input:

In [33]:
def askint():
        try:
            val = int(raw_input("Please enter an integer: "))
        except:
            print "Looks like you did not enter an integer!"
            
        finally:
            print "Finally, I executed!"
        print val       
In [35]:
askint()
Please enter an integer: 5
Finally, I executed!
5
In [36]:
askint()
Please enter an integer: five
Looks like you did not enter an integer!
Finally, I executed!
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-36-6ee53d339e7e> in <module>()
----> 1 askint()

<ipython-input-33-728ec4c542c2> in askint()
      7         finally:
      8             print "Finally, I executed!"
----> 9         print val

UnboundLocalError: local variable 'val' referenced before assignment

Notice how we got an error when trying to print val (because it was never properly assigned) Lets remedy this by asking the user and checking to make sure the input type is an integer:

In [39]:
def askint():
        try:
            val = int(raw_input("Please enter an integer: "))
        except:
            print "Looks like you did not enter an integer!"
            val = int(raw_input("Try again-Please enter an integer: "))
        finally:
            print "Finally, I executed!"
        print val 
In [40]:
askint()
Please enter an integer: f
Looks like you did not enter an integer!
Try again-Please enter an integer: f
Finally, I executed!
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-40-6ee53d339e7e> in <module>()
----> 1 askint()

<ipython-input-39-e540976abf48> in askint()
      4         except:
      5             print "Looks like you did not enter an integer!"
----> 6             val = int(raw_input("Try again-Please enter an integer: "))
      7         finally:
      8             print "Finally, I executed!"

ValueError: invalid literal for int() with base 10: 'f'

Hmmm...that only did one check. How can we continually keep checking? We can use a while loop!

In [41]:
def askint():
    while True:
        try:
            val = int(raw_input("Please enter an integer: "))
        except:
            print "Looks like you did not enter an integer!"
            continue
        else:
            print 'Yep thats an integer!'
            break
        finally:
            print "Finally, I executed!"
        print val 
In [42]:
askint()
Please enter an integer: five
Looks like you did not enter an integer!
Finally, I executed!
Please enter an integer: five
Looks like you did not enter an integer!
Finally, I executed!
Please enter an integer: four
Looks like you did not enter an integer!
Finally, I executed!
Please enter an integer: 4
Yep thats an integer!
Finally, I executed!

Great! Now you know how to handle errors and exceptions in Python with the try, except, else, and finally notation!