CODEWITHSUNDEEP
pythonstringimmutability
ignoramus
ignoramus

Reputation:

Overcoming the "disadvantages" of string immutability

I want to change the value of a particular string index, but unfortunately

string[4] = "a"

raises a TypeError, because strings are immutable ("item assignment is not supported").

So instead I use the rather clumsy

string = string[:4] + "a" + string[4:]

Is there a better way of doing this?

Upvotes: 6

Views: 960

Answers (3)

Eli Bendersky
Eli Bendersky

Reputation: 273416

The strings in Python are immutable, just like numbers and tuples. This means that you can create them, move them around, but not change them. Why is this so ? For a few reasons (you can find a better discussion online):

  • By design, strings in Python are considered elemental and unchangeable. This spurs better, safer programming styles.
  • The immutability of strings has efficiency benefits, chiefly in the area of lower storage requirements.
  • It also makes strings safer to use as dictionary keys

If you look around the Python web a little, you’ll notice that the most frequent advice to "how to change my string" is "design your code so that you won’t have to change it". Fair enough, but what other options are there ? Here are a few:

  • name = name[:2] + ‘G’ + name[3:] - this is an inefficient way to do the job. Python’s slice semantics ensure that this works correctly in all cases (as long as your index is in range), but involving several string copies and concatenations, it’s hardly your best shot at efficient code. Although if you don’t care for that (and most chances are you don’t), it’s a solid solution.
  • Use the MutableString class from module UserString. While no more efficient than the previous method (it performs the same trick under the hood), it is more consistent syntactically with normal string usage.
  • Use a list instead of a string to store mutable data. Convert back and forth using list and join. Depending on what you really need, ord and chr may also be useful.
  • Use an array object. This is perhaps your best option if you use the string to hold constrained data, such as ‘binary’ bytes, and want fast code.

Plagiarized from my own page on Python insights :-)

Upvotes: 9

ezod
ezod

Reputation: 7411

>>> from UserString import MutableString
>>> s = MutableString('abcd')
>>> s[2] = 'e'
>>> print s
abed

Upvotes: 0

ghostdog74
ghostdog74

Reputation: 342333

mystring=list("abcdef")
mystring[4]="Z"
print ''.join(mystring)

Upvotes: 3

Related Questions