Screwing Up looping in Generator

Deborah Swanson python at deborahswanson.net
Mon Jan 2 20:19:16 EST 2017


> > > Sayth Renshaw wrote, on January 03, 2017 6:54 AM
> > > >
> > > > Hi
> > > >
> > > > This is simple, but its getting me confused.
> > > >
> > > > I have a csv writer that opens a file and loops each line of the
> > > > file for each file and then closes, writing one file.
> > > >
> > > > I want to alter the behaviour to be a written file for
> each input
> > > > file. I saw a roundrobin example however it failed for
> me as you
> > > > cannot get len(generator) to use a while loop on. it exhausts
> > > >
> > > > should I use the same for again after the with open?
> > > >
> > > > rootobs in this code is my generator and I am already looping it
> > > > however def data_attr(roots):
> > > >     """Get the root object and iter items."""
> > > >     for file in rootobs:
> > > >         base = os.path.basename(file.name)
> > > >         write_to = os.path.join("output",
> > > > os.path.splitext(base)[0] + ".csv")
> > > >         with open(write_to, 'w', newline='') as csvf:
> > > >             race_writer = csv.writer(csvf, delimiter=',')
> > > >             race_writer.writerow(
> > > >                 ["meet_id", "meet_venue", "meet_date",
> > "meet_rail",
> > > >                  ...
> > > >                  # other categories here
> > > >                  ...
> > > >                     "jockeysurname", "jockeyfirstname"])
> > > >             for xml_data in roots:
> > > >                 ...
> > > >                 # parsing code
> > > >                         for noms in race_child:
> > > >                             if noms.tag == 'nomination':
> > > >                                 race_writer.writerow(
> > > >                                     [meet_id, meet_venue,
> > meet_date,
> > > >                                      ...
> > > >                                     #parsing info removed
> > > >
> > > noms.get("jockeyfirstname")])
> > > >
> > > > Cheers
> > > > Sayth
> > >
> > > What's the code for your generator? And I don't see where
> you call
> > > 'next'.
> >
> > I think you're expecting
> >
> >     for file in rootobs
> >
> > to get the next yield for you from rootobs, but unless
> > someone corrects me, I don't think you can expect a 'for'
> > statement to do that. You need to have a 'next' statement
> > inside your for loop to get the next yield from the generator.
> >
> > But I might not understand exactly what you're asking.
>
> You probably want something like :
>
> for f in rootobs:
>     file = next
>     base = os.path.basename(file.name)
>        .
>        .
>        .
>       (etc)
>
> Notice I changed your iterating variable name to 'f', so you
> can use 'file' throughout your code after you get the next
> one from rootobs.
>
> As written, you'll get a StopIteration exception when rootobs
> runs out of files, which you can catch with a try/except. Or
> you can just let the code end there if you're done.

Well rats, I'm embarrassed. It's been awhile since I've used a generator and I 
forgot that you have to create the generator object first and use it to call 
the next function. And I really don't think you can use a generator as your 
range in a for loop. So I'd use a 'while True', and break out of the loop when 
you hit the StopIteration exception:

files = rootobs()

while True:
  try:
    file = files.next()
  except StopIteration:
    break

    base = os.path.basename(file.name)
       .
       .
       .
      (etc)

(Now I'm just going to shut up, until somebody else says something.)



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