```python
import fileinput
with fileinput.input(files=('foo.txt',), inplace=True, backup='') as f:
for line in f:
print(f'prefix{line}', end='')
```
```
$ mypy test2.py
test2.py:3: error: "Iterable[str]" has no attribute "__enter__"; maybe "__iter__"?
test2.py:3: error: "Iterable[str]" has no attribute "__exit__"
```
```
$ mypy test2.py --custom-typeshed typeshed
$
```
