Why are Python strings immutable? Best practices for using them

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  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?

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:]

Why are Python strings immutable?, , meaning you can change their content without changing their identity. An immutable object is an object that is not changeable and its state cannot be modified after it is created. In Python, a string is immutable. You cannot overwrite the values of immutable objects.

Mutability and Immutability in Python, Had not strings been immutable they could have not been keys in dicts. I'm making a choice driven game, it seems like a bad practice to use a "state" variable and change it to a string, Is Python the best language for string manipulation? Most languages have immutable strings. This includes Java, Python, and C#. Usually when concatenating strings, the language allocates an entirely new string and copies the content of the two strings into the new string. Immutability does tend to make programming easier.

  1. Immutable objects are automatically threadsafe.
  2. You will find that using Python strings is trivially easy in comparison to the extreme pain associated with strings in C.

Why string is called immutable in java?, Have you ever modified a variable without knowing it or wanting to? Let's now try this with strings. I know we covered a lot here, so take a deep breath, practice, repeat, and you'll soon be able to explain these Get unlimited access to the best stories on Medium — and support writers while you're at it. Strings are Immutable Strings are immutable in Python, which means you cannot change an existing string. The best you can do is create a new string that is a variation on the original.

Immutable strings can be keys in dictionaries and similar data structures, without the need to copy the strings. It is easier to make a mutable wrapper around an immutable string than the other way around.

Difference between Mutable and Immutable in Python, One is performance: knowing that a string is immutable makes it easy to lay it out at construction time — fixed and unchanging storage requirements. This is also  Textual data in Python is handled with str objects, more commonly known as strings. They are immutable sequences of Unicode code points. Unicode code points can represent a character. When it comes to storing textual data though, or sending it on the network, you may want to encode it, using an appropriate encoding for the medium you’re using.

Most languages have immutable strings. This includes Java, Python, and C#. Usually when concatenating strings, the language allocates an entirely new string and copies the content of the two strings into the new string.

Immutability does tend to make programming easier. Especially when dealing with a multi-threaded environment.

Why are Python strings immutable?, Its use case and why we need immutability with respect to Python? A string object will always represent the same value, it can't be modified. Mutable and immutable objects are handled differently in python. Immutable objects are quicker to access and are expensive to change because it involves the creation of a copy. Whereas mutable objects are easy to change. Use of mutable objects is recommended when there is a need to change the size or content of the object.

Why should I care about immutables in python? - DEV, Mutable vs Immutable Objects in Python - An object's mutability is determined by its like lists and dictionaries are mutable, meaning you can change their content Strings are immutable in Python, which means you cannot change an existing Having mutable variables means that calling the same method with the same  Python String Formatting Rule of Thumb: If your format strings are user-supplied, use Template Strings (#4) to avoid security issues. Otherwise, use Literal String Interpolation/f-Strings (#3) if you’re on Python 3.6+, and “New Style” str.format (#2) if you’re not.

Are strings actually immutable? - Python FAQ, Isaac, the definition of “same” or “different” has to do with the identity of an object. When a Python object is created, it resides in a certain  You have a lot of answers here, but none really explains why this happens. It's because python strings are immutable. That means you can't change them in-place, just make a changed copy of it and assign it to some other variable, or itself. Python has mutable variables, i.e. lists. – ljetibo Feb 27 '15 at 11:58

Mutable vs Immutable Objects in Python, We use cookies to ensure you have the best browsing experience on our The type of the object is defined at the runtime and it can't be changed Immutable Objects : These are of in-built types like int, float, bool, string, Use of mutable objects is recommended when there is a need to change the size Practice Tags :. Why are Python strings immutable? Best practices for using them Strings are immutable. Browse other questions tagged python-3.x or ask your own question.

Comments
  • There is a mutable string type, it is called bytearray
  • @JanneKarila: The semantics, however, are wildly different. It doesn't, for example, handle Unicode characters, just bytes. It's not a proper string -- with string methods -- it's an adjustable array of bytes.
  • Sooooooo I know this is a bit late, but could you explain what you mean by mutable strings not being reliable in dicts? List are mutable, but can be put in dicts, what would make mutable strings 'unreliable'?
  • @wnnmaw What larsmans means is that you wouldn't be able to reliably use strings as keys to a dict, since modifying a string would change its hash, which would change the bucket the string would go to. You can see this with lists yourself by trying to use a list as a key to a dictionary (a = {}; a[[1,2]] = 1 throws a TypeError).
  • But its easy to change strings: str = list(str); str[0] = "J"; str = ''.join(str)
  • I don't get it. What bad consequences could happen if you replace the first letter of name? Isn't it same if I assign a totally new value to name in this case?
  • Ruby (as one example) has mutable strings, and it has automatic memory management and can't have buffer overflows. As for replacing substrings being the only use case, what about (in-place-)appending? -1
  • There is no "in-place appending". That's just appending. x = x + y. Or x += y. You can't tell if it was in-place or done with immutable strings.
  • @delnan: Ruby? That's not part of this question.
  • You made (before the edit 31 minutes ago) a far more general (and, due to that generality, wrong) statement, to which I provided a counter-example. The part of the question you addressed in that part of your answer ("why make strings immutable?") didn't speak of C. And in-place appending is different from concatenating two immutable strings and putting the result back into a variable - the difference is plainly visible to all code that accesses the (old) string (instead of going through the updated variable).
  • I'd continue my argument from the (now confirmed) axioms, but it's getting ridiculous. @DavidHeffernan's comment indicates it has been stated frequently and clearly enough that one person got it. My urge to nitpick has been satisfied.
  • Can you give anexample?
  • An example of what?
  • "you would not be able to trust any externally-provided string" for this. Thanks
  • You write a function that accepts a string, but your caller mutates the string while your function is running.