The inverse of .join
in [Python]
|
Prev: Efficiency/style issues of import <module> vs. from <module> import <name>, ...
Next: Running a program from another program.
From: Steven D'Aprano on 17 Jun 2010 23:01 On Thu, 17 Jun 2010 17:45:41 +0000, Neil Cerutti wrote: > What's the best way to do the inverse operation of the .join function? str.join is a many-to-one function, and so it doesn't have an inverse. You can't always get the input back unchanged: >>> L = ["a", "b", "c|d", "e"] >>> s = '|'.join(L) >>> s 'a|b|c|d|e' >>> s.split('|') ['a', 'b', 'c', 'd', 'e'] There's no general way of getting around this -- if split() takes input "a|b|c", there is no way even in principle for it to know which of these operations it should reverse: "|".join(["a", "b", "c"]) "|".join(["a|b", "c"]) "|".join(["a", "b|c"]) "|".join(["a|b|c"]) "b".join(["a|", "|c"]) The behaviour with the empty string is just a special case of this. -- Steven
From: Steven D'Aprano on 18 Jun 2010 01:00 On Thu, 17 Jun 2010 20:44:41 +0100, MRAB wrote: > Should .split grow an addition keyword argument to specify the desired > behaviour? Please no. > (Although it's simple enough to define your own function.) Exactly. -- Steven
From: Steven D'Aprano on 18 Jun 2010 01:02 On Thu, 17 Jun 2010 20:03:42 +0000, Neil Cerutti wrote: > I'm currently using the following without problems, while reading a data > file. One of the fields is a comma separated list, and may be empty. > > f = rec['codes'] > if f == "": > f = [] > else: > f = f.split(",") > > I just wondered if something smoother was available. Seems pretty smooth to me. What's wrong with it? I assume you've put it into a function for ease of use and reduction of code duplication. You could also use the ternary operator, in which case it's a mere two- liner and short enough to inline wherever you need it: f = rec['codes'] f = f.split(",") if f else [] -- Steven
From: Neil Cerutti on 18 Jun 2010 08:54 On 2010-06-18, Steven D'Aprano <steve(a)REMOVE-THIS-cybersource.com.au> wrote: > On Thu, 17 Jun 2010 20:03:42 +0000, Neil Cerutti wrote: >> I'm currently using the following without problems, while >> reading a data file. One of the fields is a comma separated >> list, and may be empty. >> >> f = rec['codes'] >> if f == "": >> f = [] >> else: >> f = f.split(",") >> >> I just wondered if something smoother was available. > > Seems pretty smooth to me. What's wrong with it? I assume > you've put it into a function for ease of use and reduction of > code duplication. The part that's wrong with it, and it's probably my fault, is that I can never think of it. I had to go dig it out of my code to remember what the special case was. > You could also use the ternary operator, in which case it's a > mere two- liner and short enough to inline wherever you need > it: > > f = rec['codes'] > f = f.split(",") if f else [] That's pretty cool. Thanks to everybody for their thoughts. -- Neil Cerutti
From: Jon Clements on 18 Jun 2010 11:35
On 17 June, 21:03, Neil Cerutti <ne...(a)norwich.edu> wrote: > On 2010-06-17, Robert Kern <robert.k...(a)gmail.com> wrote: > > > On 6/17/10 2:08 PM, Neil Cerutti wrote: > >> On 2010-06-17, Ian Kelly<ian.g.ke...(a)gmail.com> wrote: > >>> On Thu, Jun 17, 2010 at 11:45 AM, Neil Cerutti > >>> <ne...(a)norwich.edu> wrote: > >>>> What's the best way to do the inverse operation of the .join > >>>> function? > > >>> Use the str.split method? > > >> split is perfect except for what happens with an empty string. > > > Why don't you try it and find out? > > I'm currently using the following without problems, while reading > a data file. One of the fields is a comma separated list, and may > be empty. > > f = rec['codes'] > if f == "": > f = [] > else: > f = f.split(",") > > I just wondered if something smoother was available. > > -- > Neil Cerutti In terms of behaviour and 'safety', I'd go for: >>> rec = { 'code1': '1,2,3', 'code2': '' } >>> next(csv.reader([rec['code1']])) ['1', '2', '3'] >>> next(csv.reader([rec['code2']])) [] hth Jon. |