bug#55174: python-shell-send-statement in python.el sends malformed code

[Jin Choi <jsc@alum.mit.edu>]

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python-shell-send-statement mangles the sent code if the region is inactive and the current statement is indented. Example: with the indented statement

	a = 1

it will send the string “if True:” by itself. With a multi-line statement such as
	a = (1,
		2)

it will send
if True:
	2)

The first line is always truncated.

I’ve looked a little into what is happening, and it looks like python-shell-buffer-substring has seen a number of edits over time that don’t work together properly.

* python-shell-send-statement works properly if the region is active because it calls python-shell-send-region with no-cookie set to nil, but when it is not, it calls python-shell-send-region with no-cookie set to t.
* python-shell-send-region calls python-shell-buffer-substring, passing along no-cookie.
* python-shell-buffer-substring does the following:
	- the start position is adjusted to the beginning of the line if the statement was indented
	- if no-cookie is false, fillstr is set to be a coding cookie (e.g., “# -*- coding: utf-8 -*-“) and a number of newlines to get the statement to the correct line
	- Then, this code appears:

    (with-temp-buffer
      (python-mode)
      (when fillstr
        (insert fillstr)) ; inserts coding cookie and newlines if not no-cookie
      (insert substring) ; inserts code
      (goto-char (point-min))
      (when (not toplevel-p)
        (insert "if True:")
        (delete-region (point) (line-end-position))) ; inserts “if True:” and deletes the rest of the first line

This works when no-cookie is false, because it *deletes the cookie line*, incidentally negating any benefit the coding cookie was supposed to provide. It fails when an indented statement is sent and no-cookie is true, because the first line IS the line to be sent. Either it gets deleted entirely, or if it is a multiline statement, the first line is replaced with the “if True:”.

Note that setting no-cookie has the added effect of removing the newlines for the line number matching.

I don’t know what use case the no-cookie argument was supposed to address, but the current implementation seems incompatible with how it’s being used with indented statements.

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