Errors and Exceptions

ITEC 3160 Python Programming for Data Analysis,
Cengiz Günay

(License: CC BY-SA 4.0)

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What are exceptions?

• A way to break out of the linear execution of a program
• Usually for an exceptional situation (e.g., an error)
• Otherwise, it may cause your program to crash
• Why not instead use if statements to catch errors?

Examples with if statements vs exceptions

If statement:

value = count_inputs(...)
if value < -1:
  print("Error in counting inputs!")
  exit -1
# now do stuff with value
...

try:
  value = count_inputs(...)
  # now do stuff with value
  ...
except:
  print("Error in counting inputs!")
  raise

Exceptions in Python

• All exceptions should be subclasses of BaseException
• Other options and examples below and in official Python docs

import sys

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(s.strip())
except OSError as err:
    print("OS error: {0}".format(err))
except ValueError:
    print("Could not convert data to an integer.")
except:
    print("Unexpected error:", sys.exc_info()[0])
    raise
else:
    print("OMG no errors!")
finally:
    print("Error or not, I say this.")

Exercise time

Individually or in teams, implement ONE of these by using exceptions:

1. defaultdict functionality by using the regular dict
2. Implement input validation (only numbers) with loops
3. Create a file if not already exists

Attach runtime output screenshots showing valid and exception cases on submission site.