append special chars with "\"

[Alex Martelli]

tertius wrote:

> Is there a better way to append certain chars in a string with a
> backslash that the example below?
> 
> chr = "#$%^&_{}"          # special chars to look out for
> str = "123 45^ & 00 0_"   # string to convert
> n = ""                    # init new string
> for i in str:
>      if i in chr:          # if special character in str
>          n+='\\'           # append it with a backslash
>      n+=i
> print "old:",str
> print "new:",n

If you can afford some a priori preparation, make a dictionary
once and for all that maps each character (that isn't to be
represented by itself) to the string that represents it.  E.g.,
if you inevitably start with a chr string as above, you can
make the dictionary as follows:

charmap = {}
for c in chr: charmap[c] = c+'\\'

You can also write out the dict literal directly, and in
any case you only need to prepare this charmap once and
can then use it to prepare any number of translations.

Once you have this charmap, you can use its get method to
get the translations -- and prepare a *LIST OF STRINGS* to
be joined up at the end, that's MUCH, *MUCH* faster than
a loop using += on a string:

pieces = [charmap.get(c,c) for c in str]

and finally:

n = ''.join(pieces)


Alex