

In [1]:

    
import numpy as np

Exceptions

An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.

You've already seen some exceptions in the Debugging lesson. *

Many programs want to know about exceptions when they occur. For example, if the input to a program is a file path. If the user inputs an invalid or non-existent path, the program generates an exception. It may be desired to provide a response to the user in this case.

It may also be that programs will generate exceptions. This is a way of indicating that there is an error in the inputs provided. In general, this is the preferred style for dealing with invalid inputs or states inside a python function rather than having an error return.

Catching Exceptions

Python provides a way to detect when an exception occurs. This is done by the use of a block of code surrounded by a "try" and "except" statement.



In [17]:

    
def divide(numerator, denominator):
    result = numerator/denominator
    print("result = %f" % result)



In [18]:

    
divide(1.0, 0)









    



---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-18-ebcb36a8aa31> in <module>
----> 1 divide(1.0, 0)

<ipython-input-17-2d5d8a17c73f> in divide(numerator, denominator)
      1 def divide(numerator, denominator):
----> 2     result = numerator/denominator
      3     print("result = %f" % result)

ZeroDivisionError: float division by zero



In [36]:

    
def divide1(numerator, denominator):
    try:
        result = numerator/denominator
        print("result = %f" % result)
    except:
        print("You can't divide by 0!")



In [37]:

    
divide1(1.0, 0)









    



You can't divide by 0!



In [38]:

    
divide1(1.0, 2)









    



result = 0.500000



In [39]:

    
divide1("x", 2)









    



You can't divide by 0!



In [40]:

    
def divide2(numerator, denominator):
    try:
        result = numerator / denominator
        print("result = %f" % result)
    except (ZeroDivisionError, TypeError):
        print("Got an exception: %s" % err)



In [41]:

    
divide2(1, "x")









    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-40-24ef63cb0d19> in divide2(numerator, denominator)
      2     try:
----> 3         result = numerator / denominator
      4         print("result = %f" % result)

TypeError: unsupported operand type(s) for /: 'int' and 'str'

During handling of the above exception, another exception occurred:

NameError                                 Traceback (most recent call last)
<ipython-input-41-601e0a2b30ff> in <module>
----> 1 divide2(1, "x")

<ipython-input-40-24ef63cb0d19> in divide2(numerator, denominator)
      4         print("result = %f" % result)
      5     except (ZeroDivisionError, TypeError):
----> 6         print("Got an exception: %s" % err)

NameError: name 'err' is not defined



In [42]:

    
divide2("x, 2)









    



  File "<ipython-input-42-c9151a5629ae>", line 1
    divide2("x, 2)
                  ^
SyntaxError: EOL while scanning string literal

Why didn't we catch this `SyntaxError`?



In [46]:

    
# Handle division by 0 by using a small number
SMALL_NUMBER = 1e-3
def divide3(numerator, denominator):
    try:
        result = numerator/denominator
    except ZeroDivisionError:
        result = numerator/SMALL_NUMBER
        print("result = %f" % result)
    except Exception as err:
        print("Different error than division by zero:", err)



In [48]:

    
divide3(1,0)









    



result = 1000.000000



In [49]:

    
divide3("1",0)









    



Different error than division by zero: unsupported operand type(s) for /: 'str' and 'int'

What do you do when you get an exception?

First, you can feel relieved that you caught a problematic element of your software! Yes, relieved. Silent fails are much worse. (Again, another plug for testing.)

Generating Exceptions

Why generate exceptions? (Don't I have enough unintentional errors?)



In [51]:

    
import pandas as pd
def func(df):
    """"
    :param pd.DataFrame df: should have a column named "hours"
    """
    if not "hours" in df.columns:
        raise ValueError("DataFrame should have a column named 'hours'.")



In [52]:

    
df = pd.DataFrame({'hours': range(10) })
func(df)



In [54]:

    
df = pd.DataFrame({'years': range(10) })
func(df)









    



---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-54-19120415e5b1> in <module>
      1 df = pd.DataFrame({'years': range(10) })
----> 2 func(df)

<ipython-input-51-09fe3ea9b633> in func(df)
      5     """
      6     if not "hours" in df.columns:
----> 7         raise ValueError("DataFrame should have a column named 'hours'.")

ValueError: DataFrame should have a column named 'hours'.

Class exercise

Choose one of the functions from the last exercise. Create two new functions:

The first function throws an exception if there is a negative argument.
The second function catches an exception if the modulo operator (%) throws an exception and attempts to correct it by coercing the argument to a positive integer.