On Wed, Nov 2, 2016 at 12:02 PM, Mikhail V <mikhail...@gmail.com> wrote:
> Actually even with ASCII (read for python 2.7) I would also be happy
> to have such function: say I just want to keep only digits so I write:
>
> digits = "0123456789"
> newstring = somestring.keep(digits)
>
well, with ascii, it's not too hard to make a translation table:
digits = "0123456789"
table = [(o if chr(o) in digits else None )for o in range(256)]
s = "some stuff and some 456 23 numbers 888"
s.translate(table)
'45623888'
but then there is the defaultdict way:
s.translate(defaultdict(*lambda*: *None*, {ord(c):c *for* c *in*
digits}.items()))
'45623888'
wasn't that easy? Granted, if you need to do this, you'd wrap it in a
function like Chris A. Suggested. But this really isn't easy or
discoverable -- it took me a fair bit of fidlding to get right, and I knew
I was looking for a defaultdict implementation.
Also:
In [43]: table
Out[43]:
defaultdict(<function __main__.<lambda>>,
{48: '0',
49: '1',
50: '2',
51: '3',
52: '4',
53: '5',
54: '6',
55: '7',
56: '8',
57: '9'})
In [44]: s.translate(table)
Out[44]: '45623888'
In [45]: table
Out[45]:
defaultdict(<function __main__.<lambda>>,
{32: None,
48: '0',
49: '1',
50: '2',
51: '3',
52: '4',
53: '5',
54: '6',
55: '7',
56: '8',
57: '9',
97: None,
98: None,
100: None,
101: None,
102: None,
109: None,
110: None,
111: None,
114: None,
115: None,
116: None,
117: None})
defaultdict puts an entry in for every ordinal checked -- this could get
big -- granted, probaly nt a big deal with modern computer memory, but
still...
it might even be worth making a NoneDict for this:
class NoneDict(dict):
"""
Dictionary implementation that always returns None when a key is not in
the dict,
rather than raising a KeyError
"""
def __getitem__(self, key):
try:
val = dict.__getitem__(self, key)
except KeyError:
val = None
return val
(see enclosed -- it works fine with translate)
(OK, that was fun, but no, not really that useful)
Despite I can do it other way, this would be much simpler and clearer
> way to do it. And I suppose it is quite common task not only for me.
>
That's the key question -- is this a common task? If so, then whie there
are ways to do it, they're not easy nor discoverable.
And while some of the guiding principles of this list are:
"not every two line function needs to be in the standard lib"
and
"put it up on PYPi, and see if a lot of people find it useful"
It's actually kind of silly to put a single function up as a PyPi package
-- and I doubt many people will find it if you did.
-CHB
--
Christopher Barker, Ph.D.
Oceanographer
Emergency Response Division
NOAA/NOS/OR&R (206) 526-6959 voice
7600 Sand Point Way NE (206) 526-6329 fax
Seattle, WA 98115 (206) 526-6317 main reception
chris.bar...@noaa.gov
class NoneDict(dict):
"""
Dictionary implimentaton that always returns None when a key is not in the dict,
rather than raising a KeyError
"""
def __getitem__(self, key):
try:
val = dict.__getitem__(self, key)
except KeyError:
val = None
return val
def test_basic():
'''
some simple tests
'''
test_dict = {'a': 23,
'b': 45,
}
arbitrary_keys = [23, 'that', (1, 2, 3)]
d = NoneDict(test_dict)
# do the ones in there work:
for key, val in test_dict.items():
print("trying:", key, val)
print(d[key])
assert d[key] == val
for key in arbitrary_keys:
print("trying:", key)
assert d[key] is None
# did the dict length change?
assert len(d) == len(test_dict)
print("all tests pass")
def test_with_translate():
"""
tests using a NoneDict with str.translate()
"""
digits = "0123456789"
# make a translate table:
table = NoneDict({ord(c): c for c in digits})
s = "some stuff and some 456 23 numbers 888"
final = s.translate(table)
for c in final:
assert c in digits
print("it works with translate")
if __name__ == "__main__":
test_basic()
test_with_translate()
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