add TerminalWriter.chars_on_current_line read-only property

[nicoddemus]

As discussed in pytest-dev/pytest#2858

[RonnyPfannschmidt]

the unification with reline is needed to have correct data

[RonnyPfannschmidt]

this breaks in combination with reline and _lastlen

[nicoddemus]

True, thanks

[nicoddemus]

Hmmm not sure what to do here. Here's reline implementation:

    def reline(self, line, **kw):
        if not self.hasmarkup:
            raise ValueError("cannot use rewrite-line without terminal")
        self.write(line, **kw)
        self._checkfill(line)
        self.write('\r')
        self._lastlen = len(line)

It always ends up with a \r, so the cursor is at the start of the line; anything you write afterwards will begin from there, but it seems there are more characters on the line. What should chars_on_current_line return in this case?

And notice that reline always expects that it will be called multiple times for the same line only. IOW, if we want to write a "collecting" message using reline, we can only call reline() and never write() otherwise things will get out of sync.

As an exercise, what should be the value of chars_on_current_line at the end of this code:

w = TerminalWriter()
w.reline('Hello World')
w.reline('pytest')
assert w.chars_on_current_line == ??

0 or 6?

Note that the "cursor" is at the beginning of the line because of the \r added by reline() at this point, so if we now write:

w.write('foo')

We end up with fooest on the terminal (this is why I mention that the only write() which produces correct result after reline() is write('\n')).

Hmm perhaps the property should be named line_cursor_position or something to avoid ambiguity?

[RonnyPfannschmidt]

if its hard to exlain it might be a bad idea

im under the impression we are trying something at the wrong abstraction level and i'd like to avoid sinking too much time into terminalwriter

in addition we are currently spagettifying multiple concepts - we should take a step back and make what we need and what boundaries those need to have

[nicoddemus]

i'd like to avoid sinking too much time into terminalwriter

Perhaps we should just drop the idea of messing with TerminalWriter and leave the hack in pytest-dev/pytest#2858?

Alternatively, we might just leave reline as it is. reline() is not really used in pytest or xdist, so this patch as it stands is enough for my needs in pytest-dev/pytest#2858.

[RonnyPfannschmidt]

we can leave it as is heren but then
please document it as broken interaction and open a help wanted ticket to fix the interaction later on

[nicoddemus]

Sounds good, done: #164

[RonnyPfannschmidt]

thanks for the BTS preps, for pedantic-ness, please add a todo comment with a short description and a link to that issue

[nicoddemus]

@RonnyPfannschmidt added the note about #160 you asked for and rebased on master.