Print a space separated list in Python

Question

I am trying to solve the common "print a character after every item in a list, except the last one" problem.

I do not want to use list indexes. There are a couple of simple ways to do this in Python, but I am not sure how to apply them to my code.

Is there a more readable/couple liner way to do this?

def echo(*args):
    for data in args:
        if data is args[-1]:
            ender = ''
        else:
            ender = ' '

        if isinstance(data, six.string_types) \
                    or isinstance(data, six.integer_types):

            print_(data, end=ender)
        else:
            op = getattr(data, '__str__', None)
            if op:
                print_(data.__str__(), end=ender)
            else:
                LOG.error("echo called with an unsupported data type")
    print_('')

Answer 1

I might be misreading your code, but it looks like all you want is:

def echo(*args):
    six.print_(*args, sep=" ", end="")

For something a bit more manual:

def echo(*args):
    six.print_(" ".join(map(str, args)), end="")

---

In terms of reviewing your actual code, on top of the issues Morwenn raises, note that:

op = getattr(data, '__str__', None)
if op:

simplifies to:

if hasattr(data, '__str__'):

However, it is rare to call magic methods directly in Python; normally you'd just do str(data) and let Python call __str__. This is safer, as although an old-style class may not implement __str__, you can still get e.g. '<foo.bar object at 0x...>' via str(...).

---

I would include the filtering as a separate function:

def valid(arg):
    """Include a docstring to explain your filtering rules."""
    if (isinstance(data, (six.string_types, six.integer_types)) or
        hasattr(data, '__str__')):
        return True
    LOG.error("echo called with an unsupported data type")
    return False

Then echo can use filter:

def echo(*args):
    six.print_(*filter(valid, args), sep=" ", end="")

Answer 2

You could probably write a nested function that does the print and then you can handle the number of the iteration in the main function:

def echo(*args):

    # print a value with regard to its type
    def do_echo(data, ender):
        if isinstance(data, six.string_types) \
                    or isinstance(data, six.integer_types):

            print_(data, end=ender)
        else:
            op = getattr(data, '__str__', None)
            if op:
                print_(data.__str__(), end=ender)
            else:
                LOG.error("echo called with an unsupported data type")

    # print all the values but the last one
    for arg in args[:-1]:
        do_echo(arg, ender=' ')

    # print the last value
    do_echo(args[-1], ender='\n')

That was for the refactoring of the code logic. Now, I also have a couple of remarks:

• If args is empty, args[-1] will raise an IndexError. Therefore, it would be good to write this instead:

  # print the last value
  if args:
      do_echo(args[-1], ender='\n')
  

• The PEP8, the style guide for Python does not like explicit line continuations with the backslash and prefers implicit line continuations with parenthesis:

  if (isinstance(data, six.string_types) or
      isinstance(data, six.integer_types)):
  

  And as pointed by @jonrsharpe in the comments, isinstance can take a tuple of types instead of a simple type. Therefore, you can rewrite this condition as:

  if isinstance(data, (six.string_types, six.integer_types)):
  

• Why did you fully qualify six.string_types and six.integer_types but not six.print_? Being consistent with regard to the use of the six. prefix would be good.