@pjz had some very good points, so I'll skip those in my review.

1. Both function names and variable names are written in snake_case in python.
2. Instead of printing an error message, raise an appropriate exception.
3. You append a period for input of length 1 and 2, but not for longer output.
4. Using python list slicing you can actually remove some of your special cases.

Here's two suggested improvements, one easier and one more pythonic. I also included a short testing snippet to validate that the exception is raised.

def comma_code(input_list):
    if len(input_list) == 0:
        # Raise an exception rather than just printing the error
        raise ValueError('List cannot be empty')
    # Convert everything to string (could also raise an exception if not string)
    string_list = [str(i) for i in input_list]
    # Handle the trivial case
    if len(string_list) == 1:
        return string_list[0]

    # This could be even more pythonic, but it would lose its readability
    more_than_two_items = len(string_list) > 2
    first_part = ', '.join(string_list[:-2])
    optional_separator = ', ' * more_than_two_items
    last_part = ', and '.join(string_list[-2:])
    
    formatted_string = first_part + optional_separator + last_part
    return formatted_string     

def comma_code_pythonic(input_list):
    if len(input_list) == 0:
        raise ValueError('List cannot be empty')

    string_list = [str(i) for i in input_list]

    last_part = ', and '.join(string_list[-2:])
    first_part = string_list[:-2]

    formatted_string = ', '.join(first_part + [last_part])
    return formatted_string     


# Try to place non-global variables below the function
spam = ['apples', 'bananas', 'tofu', 'cats']
for i in range(5):
    try:
        print(comma_code_pythonic(spam[:i]))
    except ValueError as e:
        print(e)

@pjz had some very good points, so I'll skip those in my review.

1. Both function names and variable names are written in snake_case in python.
2. Instead of printing an error message, raise an appropriate exception.
3. You append a period for input of length 1 and 2, but not for longer output.
4. Using python list slicing you can actually remove some of your special cases.

Here's two suggested improvements, one easier and one more pythonic. I also included a short testing snippet to validate that the exception is raised.

def comma_code(input_list):
    if len(input_list) == 0:
        # Raise an exception rather than just printing the error
        raise ValueError('List cannot be empty')
    # Convert everything to string (could also raise an exception if not string)
    string_list = [str(i) for i in input_list]
    # Handle the trivial case
    if len(string_list) == 1:
        return string_list[0]

    # This could be even more pythonic, but it would lose its readability
    more_than_two_items = len(string_list) > 2
    first_part = ', '.join(string_list[:-2])
    optional_separator = ', ' * more_than_two_items
    last_part = ', and '.join(string_list[-2:])
    
    formatted_string = first_part + optional_separator + last_part
    return formatted_string     

def comma_code_pythonic(input_list):
    if len(input_list) == 0:
        raise ValueError('List cannot be empty')

    string_list = [str(i) for i in input_list]

    last_part = ', and '.join(string_list[-2:])
    first_part = string_list[:-2]

    formatted_string = ', '.join(first_part + [last_part])
    return formatted_string     


# Try to place non-global variables below the function
spam = ['apples', 'bananas', 'tofu', 'cats'] 

for i in range(5):
    try:
        print("comma_code:", comma_code(spam[:i]))
    except ValueError as e:
        print(repr(e))

for i in range(5):
    try:
        print("comma_code_pythonic:", comma_code_pythonic(spam[:i]))
    except ValueError as e:
        print(repr(e))