

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:

syntax errors
divide by 0

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 [2]:

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



In [3]:

    
divide1(1.0, 2)









    



result = 0.500000



In [4]:

    
divide1(1.0, 0)









    



You can't divide by 0!!



In [5]:

    
divide1("x", 2)









    



You can't divide by 0!!

Question: What do you do when you get an exception?

You can get information about exceptions.



In [6]:

    
#1/0



In [7]:

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



In [8]:

    
divide2(1, "x")









    



Got an exception



In [9]:

    
# Why doesn't this catch the exception?
# How do we fix it?
divide2("x", 2)









    



Got an exception



In [10]:

    
# Exceptions in file handling
def read_safely(path):
  error = None
  try:
    with open(path, "r") as fd:
      lines = fd.readlines()
      print ('\n'.join(lines()))
  except FileNotFoundError as err:
    print("File %s does not exist. Try again." % path)



In [11]:

    
read_safely("unknown.txt")









    



File unknown.txt does not exist. Try again.



In [12]:

    
# Handle division by 0 by using a small number
SMALL_NUMBER = 1e-3
def divide2(numerator, denominator):
    try:
      result = numerator/denominator
    except ZeroDivisionError:
      result = numerator/SMALL_NUMBER
    print("result = %f" % result)



In [13]:

    
divide2(1,0)









    



result = 1000.000000

Generating Exceptions

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



In [14]:

    
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 [15]:

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



In [16]:

    
df = pd.DataFrame({'years': range(10) })
# Generates an exception
#func(df)

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.