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Fernando Vásquez is an electronics engineer and software developer currently based in the world. He occasionally blogs about Python and Android programming.

Python Exceptions Cheatsheet

python exceptions
Published on 01 Sep 2019

Basic Example

for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except IOError:
        print(f'cannot open {arg}')
    else:
        """ if the try clause does not raise an exception """
        print arg, 'has', len(f.readlines()), 'lines'
        f.close()

Custom Error

keyword raise

class MyError(Exception):
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return repr(self.value)

try:
    raise MyError(2*2)
except MyError as e:
    print('My exception occurred, value:', e.value)

raise MyError('oops!') // prints traceback

Traceback

import logging
LOG_FILENAME = '/tmp/logging_example.out'
logging.basicConfig(filename=LOG_FILENAME, level=logging.DEBUG)

try:
    run_my_stuff()
except:
    logging.exception('Got exception on main handler')
    raise

Storing trace in string

import traceback
try:
    int('k')
except:
    var = traceback.format_exc()

import traceback
except Exception as e:
    e_type, e_val, _ = sys.exc_info()
    trace = traceback.format_exc()
    log.error(f"--\nError: [{e_type}: {e_val}]\n--\n{trace}")
    raise

Exception types

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StandardError
      |    +-- BufferError
      |    +-- ArithmeticError
      |    |    +-- FloatingPointError
      |    |    +-- OverflowError
      |    |    +-- ZeroDivisionError
      |    +-- AssertionError
      |    +-- AttributeError
      |    +-- EnvironmentError
      |    |    +-- IOError
      |    |    +-- OSError
      |    |         +-- WindowsError (Windows)
      |    |         +-- VMSError (VMS)
      |    +-- EOFError
      |    +-- ImportError
      |    +-- LookupError
      |    |    +-- IndexError
      |    |    +-- KeyError
      |    +-- MemoryError
      |    +-- NameError
      |    |    +-- UnboundLocalError
      |    +-- ReferenceError
      |    +-- RuntimeError
      |    |    +-- NotImplementedError
      |    +-- SyntaxError
      |    |    +-- IndentationError
      |    |         +-- TabError
      |    +-- SystemError
      |    +-- TypeError
      |    +-- ValueError
      |         +-- UnicodeError
      |              +-- UnicodeDecodeError
      |              +-- UnicodeEncodeError
      |              +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
	   +-- ImportWarning
	   +-- UnicodeWarning
	   +-- BytesWarning

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