[Tutor] I need to ignore an error let the script continue to run

[Alan Gauld]

On 06/05/2020 01:36, Jim wrote:
> On 5/5/20 4:13 PM, Mark Lawrence wrote:

>>> def parse_header(msg):
>>>      with open('/home/jfb/' + email_msg, 'rb') as fp:
>>>          header = BytesHeaderParser(policy=default).parse(fp)
>>>          #  Now the header items can be accessed as a dictionary:
>>>          try:
>>>              username =  header['to'].addresses[0].username + '\n'
>>>          except:
>>
>> Never use a bare except as in the long term it's asking for trouble, 
>> catch what you know can occur and let anything else raise it's own 
>> exception.

> I just write small scripts for my own use, so I really haven't had the 
> occasion to use try/except much. I'm sure your advice it correct in most 
> cases. I'm using try/except more like an 'if'. I needed some way to test 
> if certain items were  not in the header data being returned. 

Where you are using try/except is not the issue, it is how.
Mark is saying you should never use a "bare except", by which he means
you should always specify the errors that you want to catch.
In your case its a KeyERrror so you should have:

         try:
            username =  header['to'].addresses[0].username + '\n'
         except KeyError:
            username = ' Blank \n'

The problem with your code is that it catches all errors (IOError,
OSError, anything at all that may go wrong in the library
function) and just blindly assigns 'blank'.

So you have no way of telling that something potentially serious
has gone wrong. Something that may cause more errors further down
your code, but you won't know where to look because the second
error will now point at the wrong place.

That's why Mark is telling you not to use a bare except. He is not
saying do not use try/except as a control mechanism, but just to
always specify the errors you expect to occur. Allow unexpected
ones to generate an error trace.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos