Exceptions and Error Handling

• Exception Handling
• Exceptions
• Assertions

Python's Philosophy

Prepare for Failure

• LBYL stands for "look before you leap"
  • check all preconditions
• EAFP standers for "easier to ask forgiveness than permission"
  • prepare for consequences

Python prefers EAFP

• code's "happy path" is emphasized rather than being interspersed with error handling
• example: file processing (and some givens)
  • processing details are not important
  • process_file() opens a file and reads it

# ################################ Process file: LBYL version ################################


import os


p = '/path/to/datafile.dat'


if os.path.exists(p):
    process_file(p)
else:
    print(f'No such file as {p}')

### ISSUES
# what if file's contents aren't valid?
# what if path leads to directory?
# BONUS: there is race condition where file could be deleted AFTER check but BEFORE processing

# ################################ Process file: EAFP version ################################


p = '/path/to/datafile.dat'


try:
    process_file(f)
except OSError as e:
    print(f'Error: {e}')

Takeaways

• EAFP is enabled by exceptions
• without exceptions, error handling is interspersed in program flow
• exceptions can be handled non-locally
• EAFP plus Exceptions
  • exceptions are not easily ignored
  • error codes are silent by default
  • Exceptions plus EAFP makes it difficult for problems to be silently ignored

As a practice when using try/except, you should specify the type of exception being handled

try...except

• except block is passed argument of exception type(s)
  • may have multiple except blocks per try block
  • accepts tuple as argument, which allows for passing multiple types of exceptions