Tim Golden wrote:
> Dodging your question slightly (and at the risk of teaching
> my grandmother to suck eggs) I sometimes use this idiom for
> checking params. Obviously it only goes so far, but it's
> fairly compact:
> <Noddy example code>
> import os, sys
> if __name__ == '__main__':
> ARGS = None, "DEV"
> filename, db = \
> (j or i for i, j in map (None, ARGS, sys.argv[1:]))
> print sys.argv
> print filename, db
> </code>
Thank you for the example. It demonstrates perfectly how
much people miss this feature :)
Raymond Hettinger wrote:
> At first blush that example would make it seem like a good idea, but I
> don't see how the example could extend past the first index. If the
> port argument is optional, how would you know the index position of
> optional arguments to follow?
> With a dictionary, one could plausibly write:
> host = d.get('host', 'http://example.com')
> port = d.get('port', 8080)
> path = d.get('path', '/')
> But would this make sense with a list:
> host = s.get(0, 'http://example.com')
> port = d.get(1, 8080)
> path = d.get(2, '/')
> If positions 0 and 1 are optional, how do you expect to know whether
> "path" is going to be at position 2? This problem doesn't exist with
> dictionaries because the presence or absence of optional entries does
> not affect the key reference to other entries. Accordingly, I
> wouldn't expect that dict.get() would have a parallel list.get() with
> plausible use cases.
If you want to fill position 2, then positions 0 and 1 are mandatory.
It is the simplest possible option parsing, I didn't said it was the
most flexible :)
But perhaps it was a wrong example altogether.
Consider a couple more snippets, unrelated to command-line options.
(found by searching 'IndexError' in the python standard library)
this snippet from cmd.py:
try:
return self.completion_matches[state]
except IndexError:
return None
transforms into
return self.completion_matches.get(state)
another one from fileinput.py
try:
line = self._buffer[self._bufindex]
except IndexError:
pass
else:
self._bufindex += 1
self._lineno += 1
self._filelineno += 1
return line
line = self.readline()
becomes
line = self._buffer.get(self._bufindex)
if line:
self._bufindex += 1
self._lineno += 1
self._filelineno += 1
return line
line = self.readline()
both examples show reduction by 3 lines.
There's nothing dictionary-specific in 'get', it is
just a special use-case of '__getitem__' that is needed frequently.
Since list has '__getitem__' it deserves to have 'get' too.
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