exceptions copyright © software carpentry 2010 this work is licensed under the creative commons...

Exceptions

Copyright © Software Carpentry 2010This work is licensed under the Creative Commons Attribution LicenseSee http://software-carpentry.org/license.html for more information.

Testing

Testing Exceptions

Things go wrong

Testing Exceptions

Things go wrongExternal

Testing Exceptions

Things go wrongExternal– Missing or badly-formatted file

Testing Exceptions

Things go wrongExternal– Missing or badly-formatted fileInternal

Testing Exceptions

Things go wrongExternal– Missing or badly-formatted fileInternal– Bug in code

Testing Exceptions

Things go wrongExternal– Missing or badly-formatted fileInternal– Bug in codeHandling errors sensibly is easy

Testing Exceptions

Option #1: return status code

Testing Exceptions

Option #1: return status codeparams, status = read_params(param_file)if status != OK: log.error('Failed to read', param_file) sys.exit(ERROR)

grid, status = read_grid(grid_file)if status != OK: log.error('Failed to read', grid_file) sys.exit(ERROR)

Testing Exceptions


This is what we really care about

params, status = read_params(param_file)if status != OK: log.error('Failed to read', param_file) sys.exit(ERROR)


Testing Exceptions


The rest is error handling



Testing Exceptions

Hard to see the forest for the trees

Testing Exceptions


Expectedflow of control

Testing Exceptions



Errorhandling

Testing Exceptions



Errorhandling

So people don't even try to handle errors

Testing Exceptions



Errorhandling

So people don't even try to handle errorsWhich makes them harder to find and fix whenthey do occur

Testing Exceptions

Option #2: use exceptions

Testing Exceptions

Option #2: use exceptionsSeparate "normal" operation from code thathandles "exceptional" cases

Testing Exceptions

Option #2: use exceptionsSeparate "normal" operation from code thathandles "exceptional" casesMake both easier to understand

Testing Exceptions

Rearrange this...



Testing Exceptions

...to put "normal" code in one place...



params = read_params(param_file) grid = read_grid(grid_file)

Testing Exceptions

...and error handling code in another




log.error('Failed to read', filename) sys.exit(ERROR)

Testing Exceptions

...and error handling code in another



Only need one copy of the error handling code


log.error('Failed to read', filename) sys.exit(ERROR)

Testing Exceptions

Join the two parts with try and except



try: params = read_params(param_file) grid = read_grid(grid_file)except: log.error('Failed to read', filename) sys.exit(ERROR)

Testing Exceptions

You have seen exceptions before

Testing Exceptions

You have seen exceptions before>>> open('nonexistent.txt', 'r')IOError: No such file or directory: 'nonexistent.txt'

Testing Exceptions

You have seen exceptions before>>> open('nonexistent.txt', 'r')IOError: No such file or directory: 'nonexistent.txt'

>>> values = [0, 1, 2]>>> values[99]IndexError: list index out of range

Testing Exceptions

Use try and except to deal with them yourself

Testing Exceptions

Use try and except to deal with them yourself>>> try:... reader = open('nonexistent.txt', 'r')... except IOError:... print 'Whoops!'

Whoops!

Testing Exceptions


Whoops! Blue indicates regular output

Testing Exceptions


Whoops! Try to do this...

Testing Exceptions


Whoops! ...and do this ifan IO error occurs

Testing Exceptions

Use try and except to deal with them yourself

...and do this ifan IO error occurs'IOError' is how Pythonreports 'file not found'

>>> try:... reader = open('nonexistent.txt', 'r')... except IOError:... print 'Whoops!'

Whoops!

Testing Exceptions

Can put many lines of code in a try block

Testing Exceptions

Can put many lines of code in a try blockAnd handle several errors afterward

Testing Exceptions


try: params = read_params(param_file) grid = read_grid(grid_file) entropy = lee_entropy(params, grid) write_entropy(entropy_file, entropy)except IOError: log_error_and_exit('IO error')except ArithmeticError: log_error_and_exit('Arithmetic error')

Testing Exceptions

Try to do this



Testing Exceptions


Handle I/Oerrors here


Testing Exceptions


and numericalerrors here


Testing Exceptions


These messagesaren't very helpful


Testing Exceptions

Python 2.6

try: x = 1/0except Exception, error: print error

Testing Exceptions

Python 2.6


Stores information about what went wrong

Testing Exceptions

Python 2.6


Stores information about what went wrongDifferent information for different kinds of errors

Testing Exceptions

Python 2.6


Python 2.7

try: x = 1/0except Exception as error: print error

More readable

Testing Exceptions

Better error messages

Testing Exceptions

try: params = read_params(param_file) grid = read_grid(grid_file) entropy = lee_entropy(params, grid) write_entropy(entropy_file, entropy)except IOError as err: log_error_and_exit('Cannot read/write' + err.filename)except ArithmeticError as err: log_error_and_exit(err.message)


Testing Exceptions


Better error messagesHelp user figure outwhich file

Testing Exceptions



No worse thanthe default

Testing Exceptions

A question of style

Testing Exceptions

A question of style

try: grid = read_grid(grid_file)except IOError: grid = default_grid()

Testing Exceptions

A question of style

if file_exists(grid_file): grid = read_grid(grid_file)else: grid = default_grid()


Testing Exceptions


A question of style

if file_exists(grid_file): grid = read_grid(grid_file)else: grid = default_grid()

Use exceptions for exceptional cases

Testing Exceptions

Another question of style

Testing Exceptions

Another question of styleBut first...

Testing Exceptions

Exceptions can be thrown a long way

Testing Exceptions



Testing Exceptions


These arefunction calls


Testing Exceptions

Any errorsthey don'tcatch...



Testing Exceptions



...are caughtand handledhere

Testing Exceptions

"Throw low, catch high"

Testing Exceptions

"Throw low, catch high"Many places where errors might occur

Testing Exceptions

"Throw low, catch high"Many places where errors might occurOnly a few where they can sensibly be handled

Testing Exceptions

"Throw low, catch high"Many places where errors might occurOnly a few where they can sensibly be handledA linear algebra library doesn't know how itscallers will want to report errors...

Testing Exceptions

"Throw low, catch high"Many places where errors might occurOnly a few where they can sensibly be handledA linear algebra library doesn't know how itscallers will want to report errors......so it shouldn't try to handle them itself

Testing Exceptions

You can raise exceptions yourself

Testing Exceptions

You can raise exceptions yourselfshould

Testing Exceptions

You can raise exceptions yourselfshould

def read_grid(grid_file): '''Read grid, checking consistency.'''

data = read_raw_data(grid_file) if not grid_consistent(data): raise Exception('Inconsistent grid: ' + grid_file) result = normalize_grid(data)

return result

Testing Exceptions

You can define new types of exceptions too

Testing Exceptions

You can define new types of exceptions tooshould

Testing Exceptions

You can define new types of exceptions tooshould

Need to understand classes and objects first

July 2010

created by

Greg Wilson

Copyright © Software Carpentry 2010This work is licensed under the Creative Commons Attribution LicenseSee http://software-carpentry.org/license.html for more information.

exceptions copyright © software carpentry 2010 this work is licensed under the creative commons...

Documents