# RSTR-PERF-201 ā Python `s += '...'` inside a loop
## Summary
Python strings are immutable. `s += chunk` inside a loop creates a
fresh string of length `|s| + |chunk|` each iteration, so the loop
is `O(nĀ²)` in the total output length.
CPython has an opportunistic optimisation that *sometimes* mutates
the string in place when the reference count is 1, but it's
implementation-specific and disappears the moment another reference
takes a peek (logging, debugger, intermediate assignment). The
explicit-list-and-`join` form is reliably linear.
## Severity
`Medium`.
## Languages
Python.
## What rastray flags
```python
out = ''
for line in lines:
out += line + '\n' # ā flagged
```
## What rastray deliberately does *not* flag
- `out = '\n'.join(lines)`.
- Numeric `+=`.
- Concat outside loops.
## How to fix it
Collect into a list and join once:
```python
parts = []
for line in lines:
parts.append(line + '\n')
out = ''.join(parts)
```
Or use an `io.StringIO` for explicit incremental writes:
```python
import io
buf = io.StringIO()
for line in lines:
buf.write(line)
buf.write('\n')
out = buf.getvalue()
```
Both forms are `O(n)`.
## References
- [Python performance tips ā concat](https://wiki.python.org/moin/PythonSpeed/PerformanceTips#String_Concatenation)