<SYSTEM_TASK:>
Given the following text description, write Python code to implement the functionality described below step by step
<END_TASK>
<USER_TASK:>
Description:
 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
Step1:  Question
Step3:  Generating Exceptions
Why generate exceptions? (Don't I have enough unintentional errors?)
<ASSISTANT_TASK:>
Python Code:
def divide1(numerator, denominator):
    try:
      result = numerator/denominator
      print("result = %f" % result)
    except:
      print("You can't divide by 0!!")
divide1(1.0, 2)
divide1(1.0, 0)
divide1("x", 2)
Explanation: 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.
End of explanation
#1/0
def divide2(numerator, denominator):
    try:
      result = numerator/denominator
      print("result = %f" % result)
    except (ZeroDivisionError, TypeError):
      print("Got an exception")
divide2(1, "x")
# Why doesn't this catch the exception?
# How do we fix it?
divide2("x", 2)
# 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)
read_safely("unknown.txt")
# 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)
divide2(1,0)
Explanation: Question: What do you do when you get an exception?
You can get information about exceptions.
End of explanation
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'.")
df = pd.DataFrame({'hours': range(10) })
func(df)
df = pd.DataFrame({'years': range(10) })
# Generates an exception
#func(df)
Explanation: Generating Exceptions
Why generate exceptions? (Don't I have enough unintentional errors?)
End of explanation
<END_TASK>