On 11/28/2014 11:01 AM, Ned Batchelder wrote:
On 11/28/14 10:22 AM, Dave Angel wrote:
On 11/28/2014 10:04 AM, fetchinson . wrote:
Hi all,
I have a feeling that I should solve this by a context manager but
since I've never used them I'm not sure what the optimal (in the
python sense) solution is. So basically what I do all the time is
this:
for line in open( 'myfile' ):
if not line:
# discard empty lines
continue
if line.startswith( '#' ):
# discard lines starting with #
continue
items = line.split( )
if not items:
# discard lines with only spaces, tabs, etc
continue
process( items )
You see I'd like to ignore lines which are empty, start with a #, or
are only white space. How would I write a context manager so that the
above simply becomes
with some_tricky_stuff( 'myfile' ) as items:
process( items )
I see what you're getting at, but a context manager is the wrong
paradigm. What you want is a generator. (untested)
def mygenerator(filename):
with open(filename) as f:
for line in f:
if not line: continue
if line.startswith('#'): continue
items = line.split()
if not items: continue
yield items
Now your caller simply does:
for items in mygenerator(filename):
process(items)
I think it's slightly better to leave the open outside the generator:
def interesting_lines(f):
for line in f:
line = line.strip()
if line.startswith('#'):
continue
if not line:
continue
yield line
with open("my_config.ini") as f:
for line in interesting_lines(f):
do_something(line)
This makes interesting_lines a pure filter, and doesn't care what sort
of sequence of strings it's operating on. This makes it easier to test,
and more flexible. The caller's code is also clearer in my opinion.
Thank you, I agree. I was trying to preserve the factoring that the OP
had implied. I notice you also factored out the split.
BTW: this example is taken verbatim from my PyCon presentation on
iteration, it you are interested: http://nedbatchelder.com/text/iter.html
Thanks for the link. I've started reading it, and I'll definitely read
the whole thing.
--
DaveA
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