prompter: Write a Python program that takes a list of numbers as input from the user, calculates the sum of all even numbers in the list, and returns the result. The program should check if the input is valid (i.e., if it's a list of numbers), and if it's not, it should return an error message. The program should also handle exceptions gracefully and return a user-friendly error message in case of any exceptions.

assistant: Here's a python program that uses the third-party docopt library to sum all the even numbers the user enters in a command line:

``` python
"""Sum Even.

Usage: sum_even <number>...
"""
from docopt import docopt


def sum_even(numbers):
    """Sum all the even numbers in a list."""
    assert isinstance(numbers, list), "numbers must be a list"

    total = 0
    for n in numbers:
        try:
            n_flt = float(n)
            if n_flt % 2 == 0:
                total += int(n_flt)  # it's an int if it's even, and python ints can be much larger than floats
        except ValueError:
            return f"{n} is not a number"
    return total


if __name__ == "__main__":
    arguments = docopt(__doc__, version='Sum All v1.0')

    numbers = arguments['<number>']
    try:
        print(sum_even(numbers))
    except Exception as e:
        print(f"Could not sum all even numbers:\n {e}")
```

Docopt can be installed with:
```
pip install docopt
```

The program can then be run with:
```
python sum_even.py 1 2 3 4.0 4.1 1e3
```

However, it doesn't have any specific code for handling inf or nan, which float would convert. It instead just skips those because they don't result in 0 after the %2.

It also requires a call to python instead of just calling sum_even. It's possible to make it its own command line argument by making the right setup file and pip installing it.

prompter: Thank you for your reply, that was very helpful. One thing, though, is that it required me to install a new pip package. I prefer using internal Python packages such as "sys". Could you replace the "docopt" package with an internal Python package?

assistant: Certainly, here's an updated Python program that uses only built-in modules to sum all even numbers in a list:

```python
import sys

def sum_even(numbers):
    """Sum all the even numbers in a list."""
    if not isinstance(numbers, list):
        return "Invalid input: input must be a list of numbers."

    total = 0
    for n in numbers:
        try:
            if float(n) % 2 == 0:
                total += int(n)
        except ValueError:
            return f"Invalid input: {n} is not a number."

    return total

if __name__ == "__main__":
    numbers = sys.argv[1:]
    try:
        print(sum_even(numbers))
    except Exception as e:
        print(f"Error: {e}")
```

To run the program, save it as a .py file (e.g. sum_even.py) and run it with the command python sum_even.py 2 4 6 8. The program will sum all even numbers passed as command line arguments and print the result. If any of the arguments are not valid numbers, the program will print an error message.

assistant: ```
"""Sum Even.

Usage: sum_even <number>...
"""
import sys


def sum_even(numbers):
    """Sum all the even numbers in a list."""
    assert isinstance(numbers, list), "numbers must be a list"

    total = 0
    for n in numbers:
        try:
            n_flt = float(n)
            if n_flt % 2 == 0:
                total += int(n_flt)  # it's an int if it's even, and python ints can be much larger than floats
        except ValueError:
            return f"{n} is not a number"
    return total


if __name__ == "__main__":
    numbers = sys.argv[1:]
    try:
        print(sum_even(numbers))
    except Exception as e:
        print(f"Could not sum all even numbers:\n {e}")
```

assistant: Sure, here's a Python program that takes a list of numbers as input from the user, calculates the sum of all even numbers in the list, and returns the result:

```python
def sum_even_numbers(input_list):
    try:
        even_numbers = [num for num in input_list if num % 2 == 0]
        return sum(even_numbers)
    except TypeError:
        return "Error: Invalid input. Please enter a list of numbers."

input_str = input("Enter a list of numbers, separated by spaces: ")
try:
    input_list = [int(num) for num in input_str.split()]
    result = sum_even_numbers(input_list)
    print("Sum of even numbers in the list:", result)
except ValueError:
    print("Error: Invalid input. Please enter only numbers separated by spaces.")
```

The `sum_even_numbers` function takes a list of numbers as input and returns the sum of all even numbers in the list. It first uses a list comprehension to create a list of even numbers from the input list, then uses the built-in `sum` function to add up the values in the list.

The `try` and `except` blocks handle exceptions that may occur during the execution of the program. Specifically, the program catches a `TypeError` if the input is not a list of numbers, and catches a `ValueError` if any of the input values cannot be converted to integers.

When the program is run, the user is prompted to enter a list of numbers separated by spaces. The program then attempts to convert this input string into a list of integers using a list comprehension. If the conversion is successful, the `sum_even_numbers` function is called with the resulting list as its argument, and the sum of even numbers in the list is printed. If the conversion fails, the program prints an error message instead.

prompter: thanks

assistant: Sure, I'm glad to help! Are you interested in any other Python code or a different topic?

assistant: My pleasure. Are there any other programming problems that you'd like help with?

assistant: No problem!

prompter: Please explain step by step how to link that code to a basic html frontend (include the html code as well).

assistant: ```
import sys
sum=0
valid=True
for i in sys.argv[1:]:
    if i.isnumeric():
        if int(i) % 2 == 0:
            sum+=int(i)
    else:
        print("Error: invalid input, expecting a list of numbers")
        valid=False

if valid==True:
    print(sum)   
```