Greg Perry <gregp <at> liveammo.com> writes:
> Is it safe to say that classes are only useful for instances where reuse is a
key consideration? From my very
> limited perspective, it seems that classes are in most cases overkill for
simple tasks (such as reading
> the command line then calculating a hash/checksum to verify integrity).
Alan has already adressed your questions, I just have one addition on this
point. If this is *all* your application does, a class is indeed overkill.
However, if this functionality is part of a larger application, it could become
a small class.
Imagine an application that would offer some file operations, like:
- calculate hash
- split into chunks of 1.44 MB
- compress
- encrypt
You could have a generic ancestor class, say FileOperation and inherit from it
different classes implementing specific operations.
class FileOperation(object):
def __init__(self, filename):
self._filename = filename
def Perform(self):
pass # should be implemented in children
def ShowHelp(self):
pass # should be implemented in children
class HashFileOperation(FileOperation):
def Perform(self):
# calculate hash of self._filename
def ShowHelp(self):
print "Calculates MD5 hash of the file"
<etc>
Depending on a command line option, the application could instantiate one of the
operation classes and work with it. Assuming the command line would be 'app.py
<operation> <filename>' (e.g. 'app.py hash myfile.txt'), the application code
could look something like this:
# register all known operation classes and bind them
# to specific command line options
operations = {'hash': HashFileOperation,
'split': SplitFileOperation, <etc>}
opname = sys.argv[1]
# determine what operation class is necessary
opclass = operations[opname]
# instantiate that operation (make a file operation object)
fileop = opclass(sys.argv[2])
# check if it should display help or run the operation
if sys.argv[2] == '?':
fileop.ShowHelp()
else:
fileop.Perform()
Adding new operations would be a matter of implementing an appropriate class and
adding it to the operations dictionary. With a bit of Python magic you could
even get the operation classes to auto-register, so just writing an operation
class would automatically make it available in the application.
--
Yours,
Andrei
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