How to check if an item exist in a nested list

[Matt McCredie]

> Is there any way to check if an item in specific location in a multiple
> dimension nested exist?  For example something like:
>
> if M_list[line][row][d] exist:
>   do_something_0
> else:
>   do_something_1
>

Certainly:
<code>
try:
    M_list[line][row][d]
except IndexError:
    do_something_1
else:
    do_something_0
</code>

Assuming that you want to check because something in `do_someting_0' fails
if the item isn't defined, it is very likely that this can be shortened to:

<code>
try:
    do_something_0
except IndexError:
    do_something_1
</code>

The try/except should be wrapped as tightly as possible around the specific
code that actually throws the exception.

Matt
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