'str' object does not support item assignment

Question:

I would like to read some characters from a string s1 and put it into another string s2.

However, assigning to s2[j] gives an error:

s2[j] = s1[i]

# TypeError: 'str' object does not support item assignment

In C, this works:

int i = j = 0;
while (s1[i] != '')
    s2[j++] = s1[i++];

My attempt in Python:

s1 = "Hello World"
s2 = ""
j = 0

for i in range(len(s1)):
    s2[j] = s1[i]
    j = j + 1
Asked By: Rasmi Ranjan Nayak

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Answers:

In Python, strings are immutable, so you can’t change their characters in-place.

You can, however, do the following:

for c in s1:
    s2 += c

The reasons this works is that it’s a shortcut for:

for c in s1:
    s2 = s2 + c

The above creates a new string with each iteration, and stores the reference to that new string in s2.

Answered By: NPE

assigning to s2[j] gives an error

Strings are immutable so what you’ve done in C won’t be possible in Python. Instead, you’ll have to create a new string.

I would like to read some characters from a string and put it into
other string.

Use a slice:

>>> s1 = 'Hello world!!'
>>> s2 = s1[6:12]
>>> print(s2)
world!
Answered By: wim

Strings in Python are immutable (you cannot change them inplace).

What you are trying to do can be done in many ways:

Copy the string:

foo = 'Hello'
bar = foo

Create a new string by joining all characters of the old string:

new_string = ''.join(c for c in oldstring)

Slice and copy:

new_string = oldstring[:]
Answered By: Burhan Khalid

How about this solution:

str=”Hello World” (as stated in problem)
srr = str+ “”

Answered By: user2643900

The other answers are correct, but you can, of course, do something like:

>>> str1 = "mystring"
>>> list1 = list(str1)
>>> list1[5] = 'u'
>>> str1 = ''.join(list1)
>>> print(str1)
mystrung
>>> type(str1)
<type 'str'>

if you really want to.

Answered By: Crowman

Hi you should try the string split method:

i = "Hello world"
output = i.split()

j = 'is not enough'

print 'The', output[1], j
Answered By: Devzzone

Another approach if you wanted to swap out a specific character for another character:

def swap(input_string):
   if len(input_string) == 0:
     return input_string
   if input_string[0] == "x":
     return "y" + swap(input_string[1:])
   else:
     return input_string[0] + swap(input_string[1:])

Performant methods

If you are frequently performing index replacements, a more performant and memory-compact method is to convert to a different data structure. Then, convert back to string when you’re done.

list:

Easiest and simplest:

s = "TEXT"
s = list(s)
s[1] = "_"
s = "".join(s)

bytearray (ASCII):

This method uses less memory. The memory is also contiguous, though that doesn’t really matter much in Python if you’re doing single-element random access anyways:

ENC_TYPE = "ascii"
s = "TEXT"
s = bytearray(s, ENC_TYPE)
s[1] = ord("_")
s = s.decode(ENC_TYPE)

bytearray (UTF-32):

More generally, for characters outside the base ASCII set, I recommend using UTF-32 (or sometimes UTF-16), which will ensure alignment for random access:

ENC_TYPE = "utf32"
ENC_WIDTH = 4

def replace(s, i, replacement):
    start = ENC_WIDTH * (i + 1)
    end = ENC_WIDTH * (i + 2)
    s[start:end] = bytearray(replacement, ENC_TYPE)[ENC_WIDTH:]


s = "TEXT HI ひ RA ら GA が NA な DONE"
s = bytearray(s, ENC_TYPE)

# Performs s[1] = "_"
replace(s, 1, "_")

s = s.decode(ENC_TYPE)

Though this method may be more memory-compact than using list, it does require many more operations.

Answered By: Mateen Ulhaq

Other answers convert the string to a list or construct a new string character by character. These methods can be costly, especially for large strings. Instead, we can use slicing to get the parts of the string before and after the character that is changed, and combine those with the new character.

Here I modify the example code from Crowman’s answer to replace a single character in the string using string slicing instead of conversion to a list.

>>> str1 = "mystring"
>>> pos = 5
>>> new_char = 'u'
>>> str2 = str1[:pos] + new_char + str1[pos+1:]
>>> print(str2)
mystrung
>>> type(str2)
<class 'str'>
Answered By: Leland Hepworth

The ‘str’ is an immutable data type. Therefore str type object doesn’t support item assignment.

s1 = "Hello World"
s2 = ['']*len(s1)
j = 0
for i in range(len(s1)):
s2[j]=s1[i]
j = j + 1
print(''.join(s2)) # Hello World
Answered By: Ankit Patidar

In my genius rookie situation I tried to add keys / values to a dictionary incorrectly. like so.

mydict = {}
mydict = mydict['my_key'] = 'my_value'

Where it should be.

mydict['my_key'] = 'my_value' 
Answered By: John Carr
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