only strip continuation prompts if regular prompts seen first

[minrk]

should be discussed before merge (ping @takluyver), as it is a significant and sensitive change.

I'm also not sure that it's the right fix, but worth a try.

closes #4059

[Carreau]

ipy prompt does seem to be unambiguous as they avoid not to end with : and that will help in case like when you can select the first line without leaving the first prompt not unselected. So would you like to call return _strip_prompts(continuation_re, continuation_re) ?

[minrk]

That might be preferable. I'd like feedback from @takluyver on whether this seems like the right approach.
Keeping the existing behavior just means making the same call, so I can leave ipy_prompt unchanged if that's the way we want to go.

[takluyver]

I think this is closer to what we were doing before, but we changed it because quite a few people, faced with an example like this:

>>> def f(a):
...     b = a * 2
...     return b + 1

will copy from immediately after the initial prompt, so we get something like this pasted:

def f(a):
...     b = a * 2
...     return b + 1

I think this case is rather more common than having the ellipsis singleton as the first element on the second line of the block (we already leave it if there isn't something that looks like a prompt in the first two lines).

We could make it slightly more specific by checking the first line only for the initial >>> prompt, not the ... continuation prompt. But there are also times when you want to copy and paste code from the middle of a function definition or a loop, in which case it may start with a ... prompt.

[minrk]

Logic updated after discussion with @takluyver:

1. two expressions: prompt_re and initial_re, with initial being optional, and defaulting to the same as prompt
2. prompt should match any prompt, not just continuation (unchanged from master)
3. initial is checked against the first line only, so that continuation prompts will not be stripped from the first line of the cell.
4. Any prompt will be stripped from the second line of the cell
5. If any prompt is found on the first two lines of the cell, prompts will be stripped from the rest of the cell.

Mostly the logic is unchanged - the only change being the addition of the smaller initial_re for checking only the first line, which allows us to avoid stripping ... in the first line.

There are still some cases that will fail, which I think is inevitable given the ambiguity. But I think the affected cases are rare enough and easy enough to workaround that this is at least an improvement over master:

1. paste a block only consisting of continuation prompts (works in master, fails with this PR)
2. line starting with intentional ellipsis only on the second line of the cell (no change from master)

workaround for 1. is to use initial prompt or no prompt on the first line.

[takluyver]

👍 I agree with the behaviour, and the implementation looks good. We'll give it a day or two in case anyone wants to discuss the behaviour more.

[asmeurer]

This seems like as good of a workaround as I can think of. You can probably be smarter in the qtconsole, but that's your decision if you want to implement the different logic there.