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.)
thanks for help,
matt
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