Chapter 13 – Exceptions

Sections 13.1 - 13.3

\(\boxdot\) Exceptions

• An exception in Python is an event or state that cannot be controlled by the normal flow of the program.

• Divide by zero. This event can be controlled with proper selection or case handling.

  if den != 0.0:  
      quo = num / den   
  else:
      print("Denominator is equal to zero!")  

• In Python, all errors are handled as exceptions, but not all exceptions are errors.

• Syntax:

  try:  
      (code block)  
  except ExceptionName1:  
      (code block for this type of exception)  
  except ExceptionName2:
      (code block for this type of exception)  
  else:   (optional)  
  finally: (optional)  

\(\boxdot\) File Handling Exceptions

• Initial Program:

  def process_cc_data(f_n, an):
      f_i = open(f_n, 'r')
      f_o = open('A' + an + '.txt', 'w')
      for line in f_i:
          l_strip = line.strip()
          items = l_strip.split()
          if items[0] == an:
              mn = items[1][:2]
              dy = items[1][2:4]
              yr = items[1][4:]
              amt = items[4]
              rt = items[3]
              f_o.write("On {0:2s}/{1:2s}/{2:4s} your account had a charge of ${3:>6.2f} at {4:<30s}\n".format(mn, dy, yr, float(amt), rt))
      f_o.close()
  
  
  def main():
      f_name = 'cc_trans_u.txt'     # assuming an updated file  exists
      process_cc_data(f_name, '10056')
  
  
  main()

• The data set named cc_trans.txt:

  10056    01012017    c    Amazon                  33.85 
  10056    01022017    c    Oneota_Market           12.48 
  10056    01052017    c    Amazon                  47.56 
  10056    01062017    c    T_Bocks                  4.27 
  10056    01082017    c    Walmart                167.14 
  10056    01092017    c    Fairway                 82.92 
  10056    01142017    c    Walmart                 92.70 
  10056    01182017    c    Walmart                 88.15 
  10056    01212017    c    Amazon                  32.73 
  10056    01222017    c    Walmart                164.14 
  10056    01252017    c    Roscoes                 13.35 
  10056    01292017    c    Viking_Theatre          26.29 
  10289    01012017    c    Amazon                  51.49 
  10289    01022017    c    Pulpit_Rock             31.32 
  10289    01042017    c    Roscoes                  6.60 
  10289    01042017    c    Toppling_Goliath         7.21 
  10289    01062017    c    Fairway                 55.52 
  10289    01062017    c    Amazon                  26.47 
  10289    01072017    c    Walmart                 27.23 
  10289    01132017    c    Oneota_Market            7.93 
  10289    01182017    c    Toppling_Goliath        50.54 
  10289    01192017    c    Koreanna                23.65 
  10289    01242017    c    T_Bocks                 28.43 
  19542    01022017    c    Fairway                 44.87   

• File Exception Fix:

  def process_cc_data(f_n, an):
      f_i = open(f_n, 'r')
      f_o = open('A' + an + '.txt', 'w')
      for line in f_i:
          l_strip = line.strip()
          items = l_strip.split()
          if items[0] == an:
          mn = items[1][:2]
          dy = items[1][2:4]
          yr = items[1][4:]
          amt = items[4]
          rt = items[3]
          f_o.write("On {0:2s}/{1:2s}/{2:4s} your account had a charge of ${3: >6.2f} at {4:<30s}\n".format(mn, dy, yr, float(amt), rt))
     f_o.close()
  
  
  def main():
      try:
          f_name = 'cc_trans_u.txt'
          process_cc_data(f_name, '10056')
      except FileNotFoundError:
          print('Updated file not found')
          f_name = 'cc_trans.txt'
          process_cc_data(f_name, '10056')
      except PermissionError:
          print('File access not permited')
  
  
  main()