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### Function: Calculate Factorial in Python

```python
def factorial(n):
    if n < 0:
        return "Error: Factorial not defined for negative numbers."
    elif n == 0:
        return 1
    else:
        result = 1
        for i in range(1, n + 1):
            result *= i
        return result
```

### Explanation:
- **Function Definition**: The function `factorial` takes one argument, `n`.
- **Error Handling**: It checks if `n` is negative and returns an error message if so.
- **Base Case**: If `n` is 0, it returns 1, as 0! is defined to be 1.
- **Loop**: For positive `n`, it calculates the factorial by multiplying all integers from 1 to `n` and returns the result.