Mutable Strings - Any libraries that offer this?

Steven D'Aprano steve at
Mon Jul 20 16:21:15 CEST 2009

On Mon, 20 Jul 2009 21:08:22 +1000, Ben Finney wrote:

> casebash <walkraft at> writes:
>> I have searched this list and found out that Python doesn't have a
>> mutable string class (it had an inefficient one, but this was removed
>> in 3.0). Are there any libraries outside the core that offer this?
> A mutable string would not (AFAICT) be usefully implementable as a
> subclass of the built-in string types. So even if such a type existed,
> it would not be useable with all the functionality that works with
> strings.

If applications ignore duck-typing and do isinstance(value, str), it's 
arguably the application and not the value that is broken.

Besides, with the new __isinstance__ method, surely such a mutable string 
could claim to be an instance of string without needing to inherit from 

> What is it you're trying to do that makes you search for a mutable
> string type? It's likely that a better approach can be found.

When dealing with very large strings, it is wasteful to have to duplicate 
the entire string just to mutate a single character.

However, when dealing with very large strings, it's arguably better to 
use the rope data structure instead.


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