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.

To solve this problem, we need to write a function that takes a string and flips the case of each character. Lowercase characters should become uppercase and vice versa, while non-alphabetic characters should remain unchanged.

### Approach
The approach involves iterating over each character in the input string and checking its case:
1. If the character is uppercase, convert it to lowercase.
2. If the character is lowercase, convert it to uppercase.
3. If the character is neither (such as a number or symbol), leave it unchanged.

This can be efficiently achieved using a list comprehension that processes each character according to the above rules and then joins the processed characters back into a string.

### Solution Code

```python
def flip_case(string: str) -> str:
    return ''.join([c.lower() if c.isupper() else c.upper() if c.islower() else c for c in string])
```

### Explanation
The function `flip_case` works as follows:
1. It iterates over each character `c` in the input string.
2. For each character, it checks if `c` is uppercase using `c.isupper()`. If true, it converts `c` to lowercase using `c.lower()`.
3. If the character is not uppercase, it checks if `c` is lowercase using `c.islower()`. If true, it converts `c` to uppercase using `c.upper()`.
4. If the character is neither uppercase nor lowercase, it remains unchanged.
5. The processed characters are joined together into a single string, which is then returned.

This approach ensures that each character is processed in constant time, leading to an overall time complexity of O(n), where n is the length of the input string. The space complexity is also O(n) due to the storage required for the resulting string.

Okay, I need to solve this problem where I have to write a Python function called flip_case. The function takes a string and flips the case of each character. So lowercase becomes uppercase and vice versa. Let's see how to approach this.

First, I should think about the input. The input is a string, which could be empty or have a mix of uppercase and lowercase letters. For each character in the string, I need to check if it's lowercase or uppercase and then change it.

Hmm, how do I check the case of each character? Oh right, in Python, each character has the islower() and isupper() methods. So for each character, I can check if it's lowercase. If it is, I convert it to uppercase using upper(). If it's uppercase, I convert it to lowercase using lower(). But wait, what about characters that are neither, like numbers or symbols? They should stay the same, right? So I don't need to do anything for those.

So the plan is: iterate over each character in the input string. For each character, check if it's lowercase. If yes, change to uppercase. Else, check if it's uppercase and change to lowercase. Else, leave as is.

Wait, but checking islower() and then else isupper() might not cover all cases. Because some characters might not be letters at all. So perhaps a better approach is to check if the character is lowercase, flip it. Else, check if it's uppercase, flip it. Else, leave it.

Alternatively, I can use a list comprehension to build the new string. For each char in the input string, I can write a condition: if char.islower(), then char.upper(), else if char.isupper(), then char.lower(), else char.

Yes, that makes sense. So the function can be written concisely using a generator expression within the str.join() method.

Let me think about the sample input. For example, 'Hello' should become 'hELLO'. Let's see: 'H' becomes 'h', 'e' becomes 'E', 'l' becomes 'L', another 'l' becomes 'L', 'o' becomes 'O'. So the output is 'hELLO', which matches the example.

Another test case: what about a string with numbers or symbols? Like 'Hello123!' should become 'hELLO123!'. So the function should leave the numbers and symbols unchanged.

So putting it all together, the function can be written as:

def flip_case(string: str) -> str:
    return ''.join([c.lower() if c.isupper() else c.upper() if c.islower() else c for c in string])

Wait, wait. Let me think about the order. For each character, first check if it's uppercase. If yes, convert to lowercase. Else, check if it's lowercase, convert to uppercase. Else, leave as is.

Yes, that's correct. So the list comprehension will handle each character appropriately.

Let me test this logic with the sample input. For 'Hello', the first character is 'H' which is uppercase, so it becomes 'h'. The next is 'e' which is lowercase, becomes 'E'. The 'l's are lowercase, so they become 'L's. The 'o' becomes 'O'. So the result is 'hELLO', which is correct.

What about a string with mixed cases, like 'HeLLo WoRLd'? It should become 'hElLl wOrLd'. Wait, let me see: 'H' becomes 'h', 'e' becomes 'E', 'L' becomes 'l', 'L' becomes 'l', 'o' becomes 'O', space remains, 'W' becomes 'w', 'o' becomes 'O', 'R' becomes 'r', 'L' becomes 'l', 'd' becomes 'D'. So the output would be 'hElLo wOrLd'.

Yes, that seems right.

So the code should work as intended. Now, I'll write the function as described.