handling exceptions (class slides)

CSC – handling errors (or exceptions)

try: and except:

• Can be used to catch errors in a program without it having to crash!
• The programmer can provide more information and/or context as to what happened
• Can make debugging easier if you provide clear error messages

try: and except:

ages = [35, 35, 23, 18, 45, 18, 72]
try:
    print(ages[0] + ages[4])
except:
    print('Failed to print the sum of two ages')

80

ages = [35, 35, 23, 18, 45, 18, 72]
try:
    print(ages[0] + ages[7])
except:
    print('Failed to print the sum of two ages')

Failed to print the sum of two ages

try: and except:

ages = [35, 35, 23, 18, 45, 18, 72]
try:
    print(ages[0] + ages[7])
except:
    print('Failed to print the sum of two ages')
    print('Tried to get a value from an index that does not exist in the list')

Failed to print the sum of two ages
Tried to get a value from an index that does not exist in the list

ages = [35, 35, 23, 18, 45, 18, 72]
print(ages[0] + ages[7])

IndexError: list index out of range

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Attendance

Write a python function called valid_input that takes a string as argument and returns False if the string is not a year (check if the length is 4, and if the string is an integer).

Name your file year_validation.py

Write a function

1. Its name is read_numbers
2. It takes a string for a file name
3. It opens the file in read mode, it splits the lines by space
4. It returns a list with all float numbers in the file
5. Download test file

print( read_numbers("weather.txt") ) # [89.5, 50.2, 93.4, 69.0]

Write a function – solution

def read_numbers(file_name):
  floats = []
  not_floats = []
  f = open(file_name, "r")
  for line in f:
    words = line.strip().split(" ")
    for w in words:
      try:
        floats.append(float(w))
      except:
        not_floats.append(w)
  f.close()
  return floats

def main():
  print( read_numbers("weather.txt") ) # [89.5, 50.2, 93.4, 69.0]
  
main()

[89.5, 50.2, 93.4, 69.0]

Write a function – solution 2

def read_numbers(file_name):
  floats = []
  not_floats = []
  f = open(file_name, "r")
  for line in f:
    words = line.strip().split(" ")
    for w in words:
      try:
        floats.append(float(w))
      except Exception as err:
        not_floats.append(w)
        print(err)
      else:
        print("coverted", w, "successfully")
  f.close()
  return floats

def main():
  print( read_numbers("weather.txt") ) # [89.5, 50.2, 93.4, 69.0]
  
main()

could not convert string to float: 'Tucson'
could not convert string to float: '--'
could not convert string to float: 'high'
could not convert string to float: 'temperature:'
coverted 89.5 successfully
could not convert string to float: 'Tucson'
could not convert string to float: '--'
could not convert string to float: 'low'
could not convert string to float: 'temperature:'
coverted 50.2 successfully
could not convert string to float: 'Phoenix'
could not convert string to float: '--'
could not convert string to float: 'high'
could not convert string to float: 'temperature:'
coverted 93.4 successfully
could not convert string to float: 'Phoenix'
could not convert string to float: '--'
could not convert string to float: 'low'
could not convert string to float: 'temperature:'
coverted 69.0 successfully
[89.5, 50.2, 93.4, 69.0]