String Basics

The Idea

A Python string is a fixed block of characters. Nothing edits it in place: slicing, .upper() and + each hand back a whole new string.

So s += ch in a loop can copy everything built so far, every time. An innocent-looking loop can quietly cost O(n²). CPython sometimes grows the string in place when nothing else refers to it, but the language does not promise that.

Real-World Example

A log shipper glues ten thousand lines into one payload with +=. Every line can re-copy the growing blob — megabytes of pointless memory traffic. Collecting the lines in a list and calling "".join() once at the end can turn seconds into milliseconds, with no other change.

The Code

s = "string"
# s[0] = "S"  ->  TypeError: strings are immutable

out = ""
for ch in s:                  # each += may copy the whole result again
    out += ch.upper()         # O(n²) across the loop, in general
print(out)                    # STRING

parts = []
for ch in s:
    parts.append(ch.upper())  # appends are O(1)
print("".join(parts))         # STRING — copied once, O(n)

s = "ab"
t = s
s += "c"
print(s, t)