he-027

# HumanEval/27

Implement a file **`solution.py`** that completes the function below. Keep the given name and signature; your file must define it at module level.

```python


def flip_case(string: str) -> str:
    """ For a given string, flip lowercase characters to uppercase and uppercase to lowercase.
    >>> flip_case('Hello')
    'hELLO'
    """
```

<!-- imported from OpenAI HumanEval (HumanEval/27) -->

# Auto-generated from OpenAI HumanEval HumanEval/27. Do not edit by hand.
from solution import *  # noqa: F401,F403 (prompt helpers may be referenced by tests)
from solution import flip_case as candidate

METADATA = {
    'author': 'jt',
    'dataset': 'test'
}


def check(candidate):
    assert candidate('') == ''
    assert candidate('Hello!') == 'hELLO!'
    assert candidate('These violent delights have violent ends') == 'tHESE VIOLENT DELIGHTS HAVE VIOLENT ENDS'


def test_he_027():
    check(candidate)

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.

```python
def flip_case(string: str) -> str:
    """ For a given string, flip lowercase characters to uppercase and uppercase to lowercase.
    >>> flip_case('Hello')
    'hELLO'
    """
    return string.swapcase()
```

The task is to implement a function `flip_case` that flips the case of characters in a string - lowercase becomes uppercase and uppercase becomes lowercase.

Let me think about the implementation:
1. I need to iterate through each character in the string
2. For each character, check if it's lowercase or uppercase
3. If lowercase, convert to uppercase
4. If uppercase, convert to lowercase
5. Return the resulting string

I can use Python's built-in string methods:
- `str.islower()` to check if a character is lowercase
- `str.isupper()` to check if a character is uppercase
- `str.lower()` to convert to lowercase
- `str.upper()` to convert to uppercase

Or I can use `str.swapcase()` which does exactly this!

Let me verify with the example:
- Input: 'Hello'
- Expected output: 'hELLO'
- 'H' is uppercase → 'h'
- 'e' is lowercase → 'E'
- 'l' is lowercase → 'L'
- 'l' is lowercase → 'L'
- 'o' is lowercase → 'O'
- Result: 'hELLO' ✓

The simplest solution is to use `swapcase()`:

```python
def flip_case(string: str) -> str:
    return string.swapcase()
```

This is clean and efficient. Let me make sure this works correctly for edge cases:
- Empty string: '' → '' ✓
- All lowercase: 'hello' → 'HELLO' ✓
- All uppercase: 'HELLO' → 'hello' ✓
- Mixed case: 'Hello' → 'hELLO' ✓
- Non-alphabetic characters: '123' → '123' (they remain unchanged) ✓

This should work perfectly.