Re: newbie : modifying a string in python

[Brian van den Broek]

Leeds, Mark said unto the world upon 2005-03-16 14:46:
I want to modify a string in the following way :
 

for s in stks:
      s = s.strip()
      if ( s[-2:] == 'GR' ):
              s[-2:]= 'GF'
 

so, if the last two characters are GR, I want to change
them to GF ( there will be other if statements also but I am
just putting this one here for simplicity ).
 

I think the code is fine but I vaguely remember
reading somewhere in the documentation that
python strings can't be changed ( only elements of lists can be ) ?.
Is that true or is it okay to do the above.
.
                                   thanks
Hi Mark,
well, what happens when you try?
>>> test_string = 'some text ending in GR'
>>> if test_string[-2:] == 'GR':
...     test_string[-2:] = 'GF'
...
Traceback (most recent call last):
  File "<interactive input>", line 2, in ?
TypeError: object doesn't support slice assignment
So, not so much :-)
You can do it thusly:
>>> if test_string[-2:] == 'GR':
...     test_string = test_string[:-2] + 'GF'
...
>>> test_string
'some text ending in GF'
But, a better way in that it uses .endswith and points towards 
generalization:

>>> old_chars = 'GR'
>>> new_chars = 'GF'
>>> test_string = 'some text ending in GR'
>>> if test_string.endswith(old_chars):
...     test_string = test_string[:-len(old_chars)] + new_chars
...
>>> test_string
'some text ending in GF'
>>>
Wrap that in a function:
def replace_at_end(string, old, new):
    # logic here with a return of the new string as the last line
and you've got something reusable :-)
(There may be better ways, still; I'm no expert :-)
HTH,
Brian vdB
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