I've got a list comprehensions which filter a list:
l = [obj for obj in objlist if not obj.mycond()]
but the object method mycond() can raise an Exception I must intercept. I need to collect all the errors at the end of the loop to show which object has created any problems and at the same time I want to be sure to loop all the list elements.
My solution was:
errors = []
copy = objlist[:]
for obj in copy:
try:
if (obj.mycond()):
# avoiding to touch the list in the loop directly
objlist.remove(obj)
except MyException as err:
errors = [err]
if (errors):
#do something
return objlist
In this post (How to delete list elements while cycling the list itself without duplicate it) I ask if there is a better method to cycle avoiding the list duplicate.
The community answer me to avoid in place list modification and use a list comprehensions that is applicable if I ignore the Exception problem.
Is there an alternative solution in your point of view ? Can I manage Exception in that manner using list comprehensions? In this kind of situation and using big lists (what I must consider big ?) I must find another alternative ?
I would use a little auxiliary function:
def f(obj, errs):
try: return not obj.mycond()
except MyException as err: errs.append((obj, err))
errs = []
l = [obj for obj in objlist if f(obj, errs)]
if errs:
emiterrorinfo(errs)
Note that this way you have in errs all the errant objects and the specific exception corresponding to each of them, so the diagnosis can be precise and complete; as well as the l you require, and your objlist still intact for possible further use. No list copy was needed, nor any changes to obj's class, and the code's overall structure is very simple.