default value for list access?

[Bo Peng]

Dear list,

My program needs to do calculation based on a giving data structure 
(like a sparse matrix) with  lots of missing values (invalid 
indices/keys of lists/dictionaries).

The programing is difficult in that I have to use a try...except block 
for every item access. I have written a function

def getItem(item, idx, default):
   try:
     return item[idx]
   except:
     return default

Then I will have to write a[1]['a'][4] as

getItem( getItem( getItem(a,1,{}), 'a', []), 4, 0)

Is there any way to automatically return 0 when any exception is raised 
for such data acceses? (automatic wrapping a[...] as
  try:
    a[...]
  except:
    0
)

Many thanks in advance.

Bo