Equivalents of Ruby's "!" methods?

Paul McGuire ptmcg at austin.rr.com
Mon Aug 25 15:21:09 CEST 2008


On Aug 25, 7:52 am, "Simon Mullis" <si... at mullis.co.uk> wrote:
> Consider the following hash:
>
> h = { "1" : "a\r", "2" : "b\n" }

In Python, this called a "dict".  "hash" sounds so... perlish.

>
> In order to strip the dict values in Python I (think) I can only do
> something like:
>
> for k,v in h.items:
>     h[k] = v.strip()
>
> While in Ruby - for the equivale dict/hash - I have the option of an
> in-place method:
>
> h.each_value { |v| val.strip! }
>
> Are there Python equivalents to the "!" methods in Ruby?
>
You have pretty much answered your own question.  "!" methods are not
specially named or annotated in Python, str.strip is the method that
corresponds to strip!.  It looks a little different since you left off
the ()'s in the Ruby example (as is your privilege and right in Ruby).

Pythonistically speaking, even though a dict is a mutable thing, I'm
learning that the accepted practice for methods like this is not so
much to update in place as it is to use generator expressions to
construct a new object.  For example, given a list of integers to 100,
instead of removing all of the even numbers (with the attendant
hassles of updating a list while iterating over it), just create a new
list of the numbers that are not even.

In your dict case, this would just be:

   h = dict( k,v.strip() for k,v in h.iteritems() )

Perhaps I am overgeneralizing from comments I have read here on c.l.py
about creating new lists instead updating old ones.  But in your loop,
you *will* end up doing a re-assignment of every value in the dict,
even if the original value had no whitespace to strip.

-- Paul



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