Try/except vs. if/else

[Gerrit Holl]

Shu-Hsien Sheu wrote:
> In my understanding, using try/except rather than if/else is more 
> pythonic. However, sometimes it is difficult to use the later.
> For example, I want to search for a sub string in a list composed of 
> strings. It is considered "possitive" if there is a match, no matter how 
> many.
> 
> my_test = ['something', 'others', 'still others']
> 
> case 1: try/except
> 
> hit = 0
> for i in my_test:
>    try:
>       i.index('some')
>       hit = 1
>    except ValueError:
>       pass

> case 2: if/else
> 
> hit = 0
> for i in my_test:
>    if 'some' in i:
>       hit = 1

Much faster would be:
def check():
    for elem in my_test:
        if 'some' in elem:
            return True

...this way, it immediatly stops checking all following values once it finds
a single match.

> It seems to me that in a simple searching/matching, using if might be 
> better and the code is smaller. Try/except would have its strengh on 
> catching multiple errorrs.

Agreed.

> However, problems occur if the criteria is 
> composed of "or" rather than "and". For instance:
> 
> if (a in b) or (c in b):
>    *do something
> 
> try:
>    b.index(a)
>    b.index(c)
>    *do something
> except ValueError:
>   pass
> 
> The above two are very  different.

It would be more similar to use 'if (a in b) and (c in b)',
because that is what the try/except block does. If so, I
think it has the same effect.
I would absolutely prefer the former, because I don't like function
calls who neither change an object whose return value is
thrown away. 

regards,
Gerrit.

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