Brett_McS wrote:
> In C++ (gack!) I got used to creating a helper function with each class to
> check the class object initialisation parameters prior to creating the
> object.
>
> In Python, this would be
> -----------------------------------------------
> import example
>
> if example.ParametersOK(a, b, c, d):
> newObj = example.Example(a, b, c, d)
> else:
> print "Error in parameters"
> -----------------------------------------------
>
> I presume this would still be considered good practise in Python, or is
> there some other, preferred, method?
Use exceptions to signal wrong parameters and move the parametersOk() test
into the initializer
class Example:
def __init__(self, a, b, c, d):
if a < 0:
raise ValueError("Negative length not allowed")
#...
Write a factory if
- creating the Example instance carries too much overhead and wrong
parameters are likely, or
- the checks are costly and you often get parameters known to be OK.
def make_example(a, b, c, d):
if a < 0:
raise ValueError("Negative length not allowed")
#...
return Example(a, b, c, d)
Example object creation then becomes
try:
newObj = example.Example(1,2,3,4) # or make_example(...)
except ValueError, e:
print e # you will get a specific message here
If the checks still have to occur in multiple places in your code you are of
course free to factor them out in a separate function.
Peter
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