Why are Python strings immutable? Best practices for using them

Question

1. What are the design reasons of making Python strings immutable? How does it make programming easier?
2. I'm used to mutable strings, like the ones in C. How am I supposed to program without mutable strings? Are there any best practices?

Answer 1

When you receive a string, you'll be sure that it stays the same. Suppose that you'd construct a Foo as below with a string argument, and would then modify the string; then the Foo's name would suddenly change:

class Foo(object):     def __init__(self, name):         self.name = name  name = "Hello" foo = Foo(name) name[0] = "J" 

With mutable strings, you'd have to make copies all the time to prevent bad things from happening.

It also allows the convenience that a single character is no different from a string of length one, so all string operators apply to characters as well.

And lastly, if strings weren't immutable, you couldn't reliably use them as keys in a dict, since their hash value might suddenly change.

As for programming with immutable strings, just get used to treating them the same way you treat numbers: as values, not as objects. Changing the first letter of name would be

name = "J" + name[1:] 

Answer 2

Immutable strings greatly simplify memory allocation when compared with C strings: you don't guess at a length and over-allocate hoping you over-allocated enough.

They're more secure: you can never have a buffer overrun the way you can in C.

There is only one mutable string use case.

• replacing a substring or a single character

All other string use cases (concatenation, searching, etc., etc.) the mutability does not matter. In all other cases, mutability does not matter.

If you want to replace a character or a substring in Python, you simply create a new string

x = x[:place] + replacement + x[place+1:] 

That's the only code that novel or distinctive.

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For reasons I fail to understand, it appears important to add the following.

"There are other ways to avoid a string buffer overflow than immutable strings."

For the purposes of this question (about Python, specifically) immutable strings have a pleasant consequence of no buffer overflows. For other languages, other principles, rules and nuances apply.