Python replace character without replace method

This works:

test_string = "Hallo"
# turn the string into a list
test_string  = list(test_string)
# change the character you want  
test_string[0] = "T"
# convert the list back to a string. 
test_string = "".join(test_string)

Strings are immutable, so if you don't want to convert to a mutable type, you'll have to create a new string. Here is an example function that does that:

def replace_(original_string, replace_string, index):
    return original_string[:index] + replace_string + original_string[index + len(replace_string):]

print(replace_("Hallo", "T", 0))
print(replace_("Hallo", "Te", 0))

This outputs:

Tallo
Tello

I'd like to note that I prefer the answers that convert to a list; this answer is only provided for a pure-string implementation.

So you want to change just the first letter of your string?

You can do

t = "T" + t[1:len(t)]

note: I've put "t" instead of "test_string"

Using a list comprehension and enumerate like

"".join(['T' if i == 0 else x for i, x in enumerate(test_string) ])

also works

Here's another solution using bytearray which is a little faster than MedAli's solution:

test_string = "Hello"
b = bytearray(test_string)  # Or Python3 bytearray(test_string, 'utf-8')
b[0] = ord('T')
str(b)

Strange that nobody did the obvious...

test_string = "T" + test_string[1:]