[Tutor] Conflicted [Strings are immutable]

[Danny Yoo]

On Sat, 6 Apr 2002, Eve Kotyk wrote:

>
> > > I am however not quite clear as to why the whole line is being
> > > replace rather than just the string requested.    >
> >
> > >>> s = 'My name is mud'
> > >>> s.replace('mud', 'Sean')
> > 'My name is Sean'
> > >>> s
> > 'My name is mud'
>
> Ok, perhaps I'm being a little dim but if I do
>
> s = 'Spam is not ham'
> s.replace('Spam', '')
> and write it to a new file, should I not get 'is not ham'?


Strings are 'immutable' in Python --- they don't change.  Whenever we do
manipulations on strings, Python creates whole new strings and doesn't
modify the old ones.

It helps if we try your example in the interpreter:

###
>>> s = "Spam is not ham"
>>> s.replace('Spam', '')
' is not ham'
###

Up to this point, what we're seeing is that the expression:

    s.replace('Spam', '')

is giving back to us the string that we're expecting.  However, this does
not change what 's' itself contains:

###
>>> s
'Spam is not ham'
###


What you probably want to do is capture the string that the replace()
gives in another variable.  Here's an example:

###
>>> s_without_spam = s.replace('Spam', '')
>>> s_without_spam
' is not ham'
>>> s
'Spam is not ham'
###


The 's_without_spam' is something you can write to your file.  If we want
to make it look like s is being changed itself, we can just reuse 's' as
our variable:

###
>>> s
'Spam is not ham'
>>> s = s.replace(' ', '/')
>>> s
'Spam/is/not/ham'
###


Python strings are designed with this sense of immutability in them, just
like numbers.  If we multiply 3 and 4:

###
>>> 3 * 4
12
###

... this doesn't "hurt" 3 or 4 or make any changes to them.  Immutability
with strings is the same idea:

###
>>> "three" * 4
'threethreethreethree'
###


Please feel free to ask more questions about this; it's a somewhat subtle
issue but important to understand.  Good luck!