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  1. The first I'll write a simple program:
s = "Taxi" 
output will be: 
Taxi
  1. Then I'll replace the First index (T) To (S): but before I do anything How string is immutable and I can replace it ,, Is it immutable because the id of the first index will be take the old id ..

FOR illustration :

id(T) = 1100
and id(S) = 1100
help-ukraine-now
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joox
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  • Can't see what `T` is. But yes, `str` is immutable, as in you can't do `s[0] = 'M'` – han solo Aug 24 '20 at 08:31
  • Checkout [Mutability and Immutability in Python — Let’s Break It Down](https://medium.com/datadriveninvestor/mutable-and-immutable-python-2093deeac8d9#:~:text=An%20immutable%20object%20is%20an,can%20assign%20the%20variable%20again.&text=Since%20a%20string%20is%20immutable%2C%20it%20created%20a%20new%20string%20object.) – DarrylG Aug 24 '20 at 08:34
  • You should read the following as well: https://nedbatchelder.com/text/names.html – juanpa.arrivillaga Aug 24 '20 at 08:54
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    "Then I'll replace the First index (T) To (S): but before I do anything How string is immutable and I can replace it ,, Is it immutable because the id of the first index will be take the old id .." What does this even mean? Please be specific. – juanpa.arrivillaga Aug 24 '20 at 08:55

1 Answers1

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The fact that the ids were the same implies that they are pointing on the same object.
String in Python is immutable, you can't assign to it's indexes values.



Immutability means that we can change the original object: 

# List is mutable
l = [1, 2, 3]
# That's OK
l[0] = 5

# String is NOT mutable
s = '123'
# That's NOT OK - and WON'T work!
s[0] = '5'




The following assignment means that we create another pointer, to the same string:

S = "Taxi"
T = "Zaxi"
# The following line of code will cause 'S' point on 'T' string
# The string referenced by 'S' before has not changed! (and not available)
S = T
# Assertion will pass, as we changed S to point on T, now they point on same string
assert id(S) == id(T)


The fact that strings are immutable makes it memory-inefficient to generate new strings based on existing strings.
In C#, where strings are immutable too, there is a class called StringBuilder which mitigates this in-efficiency.
On other alternatives to StringBuilder in Python read more in this great thread.

Aviv Yaniv
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    This doesn't demonstrate immutabliity at all. Try this with mutable types, `S = [1, 2, 3]` and `T = ['a','b','c']` and `S = T` will give you `id(S) == id(T)`, indeed, for python **for any objects** `a` and `b`, `a = b` will imply `a is b`, *that's merely the semantics of assignment* – juanpa.arrivillaga Aug 24 '20 at 08:50
  • @juanpa.arrivillaga That's *exactly* what I wrote. Added more clarification regarding the meaning of mutability versus non-mutability. Thank you for your comment. – Aviv Yaniv Aug 24 '20 at 08:55
  • I think I follow you now, yes. – juanpa.arrivillaga Aug 24 '20 at 08:56
  • thank you , acutely i am beginner in the language and i try to know everything – joox Aug 24 '20 at 09:04