On Fri, Sep 17, 2010 at 7:43 AM, Gregory, Matthew <matt.greg...@oregonstate.edu> wrote: > Sorry for what is probably a poor subject line ... > > Generally, I'm trying to understand the best way to set up an object's > __init__ if there are some required parameters and other parameters that can > be specified in a number of ways. As an example, I'm working on an envelope > class that describes a spatial envelope complete with cell size information. > Assuming that this envelope needs to have its upper left corner specified and > a cell size, I can *either* specify a lower right corner or the number of > rows and columns to finish the specification. Can folks provide guidance on > a good way to do this? Specifically, I'm wondering if some or all options > should be by position or by keyword. > > Here's my first cut of doing it all by keyword. > > class EnvelopeException(Exception): > pass > > class Envelope(object): > def __init__(self, **kwargs): > try: > self.x_min = kwargs['x_min'] > self.y_max = kwargs['y_max'] > self.cell_size = kwargs['cell_size'] > except KeyError: > err_str = 'Not all required parameters were specified' > raise EnvelopeException(err_str) > > try: > # First try n_cols and n_rows as keyword args > self.n_cols = kwargs['n_cols'] > self.n_rows = kwargs['n_rows'] > except KeyError: > try: > # Try x_max and y_min instead > self.x_max = kwargs['x_max'] > self.y_min = kwargs['y_min'] > except KeyError: > err_str = 'Either the number of rows and columns or ' > err_str += 'the lower-right coordinate was not specified.' > raise EnvelopeException(err_str) > > # calculate the remaining parts of the specification > ... > > (I realize that I could also specify the x_max and the n_rows or y_min and > n_cols and still derive the envelope, but that seemed nonintuitive to me, > although maybe I should consider it.)
Rather than present you with what I think (subjective) might be a "good solution", why don't you look up in the python documentation how you define methods and what you can do with them (parameters-wise). I'll summarize: def foo(self, a, b, c): ... def foor(self, a, b, *args): ... def foo(self, a, b, c=None): ... def foo(self, a, b, *args, **kwargs): ... There are probably other combinations, but these are probably the most common. The point I'm trying to make here is that this is really "up to you". Use a combination of variable arguments (*args) and maybe some arguments with default values (c=None) or just use **kwargs. Choice is yours :) cheers James -- -- James Mills -- -- "Problems are solved by method" _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
