Python doctest ValueError: "lacks blank after"

Question

I have the following function and docstring:

def vflip(self):
    """
    Vertically flips the characters on this canvas.

    >>> canvas = Canvas(4, 4)
    >>> canvas.fill('.')
    >>> canvas[0, 0] = 'A'
    >>> canvas[1, 1] = 'B'
    >>> canvas[2, 2] = 'C'
    >>> canvas[3, 3] = 'D'
    >>> print(canvas)
    A...
    .B..
    ..C.
    ...D
    >>> canvas.vflip()
    >>> print(canvas)
    ...D
    ..C.
    .B..
    A...
    """
    for y in range(0, self.height // 2):
        for x in range(0, self.width):
            self._chars[x][y], self._chars[x][self.height - 1 - y] = self._chars[x][self.height - 1 - y], self._chars[x][y]
    self._strDirty = True

Which produces the following error:

Traceback (most recent call last):
  File "__init__.py", line 1332, in <module>
    print(doctest.testmod())
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 1949, in testmod
    for test in finder.find(m, name, globs=globs, extraglobs=extraglobs):
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 932, in find
    self._find(tests, obj, name, module, source_lines, globs, {})
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 995, in _find
    globs, seen)
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 1029, in _find
    globs, seen)
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 982, in _find
    test = self._get_test(obj, name, module, globs, source_lines)
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 1066, in _get_test
    filename, lineno)
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 668, in get_doctest
    return DocTest(self.get_examples(string, name), globs,
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 682, in get_examples
    return [x for x in self.parse(string, name)
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 644, in parse
    self._parse_example(m, name, lineno)
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 702, in _parse_example
    self._check_prompt_blank(source_lines, indent, name, lineno)
  File "C:\Users\Al\AppData\Local\Programs\Python\Python37\lib\doctest.py", line 789, in _check_prompt_blank
    line[indent:indent+3], line))
ValueError: line 17 of the docstring for __main__.Canvas.vflip lacks blank after ...: '...D'

I'm not sure why I'm getting this error. Other StackOverflow posts usually indicate that there's no space between the >>> and the code on that line, but that's not the case here. Why am I getting this error?

Al Sweigart · Accepted Answer · 2018-08-27 19:11:29Z

3

This is a special case, where the output text '...D' mimics the other Python prompt. The interactive shell use >>> as the normal prompt, but any additional lines in a multi-line statement use ... as a secondary prompt. The '...D' is confusing doctest and making it think that this is a prompt.

For example, notice the two prompts in this basic code:

>>> for i in range(5):
...     print('Hello, world!')

answered Aug 27, 2018 at 19:11

Al Sweigart

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