bug#43450: 28.0.50; Python shell doesn't echo if shebang

[Gunnar Horrigmo]

Open a .py file
M-x run-python RET
In .py file, write some statement, say "1+2" and evaluate it with
(f.ex.) python-shell-send-statement. Observe that python shell echoes
the result, "3", and prompts on a new line.

Add a shebang to the top of .py file (f.ex. "#!/usr/bin/pyton3" on my
system). Evaluate the same statement as over. Observe that python shell
doesn't echo the result, but does print a new prompt, but without a
newline. 

You can add and remove the shebang as much you want, and see the echo
behaviour change accordingly. You do _not_ need to save the file.

You can see a replay of the issue here:
https://asciinema.org/a/DOq15ldlyiKI5HKLrKpF7gIeg

I've verified this behaviour using emacs -Q on versions 28.0.50
(including master HEAD as of 2020-09-16) and 27.1.50. And it seems all
the python-shell-send-* functions exhibit the same, not just
python-shell-send-statement. 



In GNU Emacs 28.0.50 (build 4, x86_64-pc-linux-gnu, GTK+ Version 3.24.13, cairo 
version 1.16.0)
 of 2020-05-23 built on hot-rats.uio.no
Repository revision: f8581bcf6a1942ebd331cae20e32945a3a86a3d1
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Fedora 31 (Workstation Edition)

Recent messages:
Checking 71 files in /home/horrigmo/src/emacs/emacs/lisp/erc...
Checking 34 files in /home/horrigmo/src/emacs/emacs/lisp/emulation...
Checking 180 files in /home/horrigmo/src/emacs/emacs/lisp/emacs-lisp...
Checking 24 files in /home/horrigmo/src/emacs/emacs/lisp/cedet...
Checking 59 files in /home/horrigmo/src/emacs/emacs/lisp/calendar...
Checking 87 files in /home/horrigmo/src/emacs/emacs/lisp/calc...
Checking 105 files in /home/horrigmo/src/emacs/emacs/lisp/obsolete...
Checking for load-path shadows...done
C-x C-g is undefined [2 times]
Buffer *unsent mail to bug-gnu-emacs@gnu.org* modified; kill anyway? (y or n) y

Configured features:
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ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS PDUMPER LCMS2
GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: ERC

Minor modes in effect:
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  line-number-mode: t

Load-path shadows:
None found.

Features:
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erc-goodies erc erc-backend erc-compat thingatpt pp erc-loaddefs
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network-stream nsm gnus-agent gnus-srvr gnus-score score-mode nnvirtual
gnus-msg gnus-art mm-uu mml2015 nntp gnus-cache gnus-sum url url-proxy
url-privacy url-expand url-methods url-history shr url-cookie url-domsuf
url-util svg xml dom gnus-group gnus-undo powerline advice
powerline-separators color cyberpunk-theme powerline-themes cus-edit
cus-start cus-load tramp tramp-loaddefs trampver tramp-integration
files-x tramp-compat shell pcomplete comint ansi-color ring ls-lisp ido
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rmail-loaddefs rfc2047 rfc2045 ietf-drums text-property-search time-date
mail-utils mm-util mail-prsvr wid-edit server pinentry epa-file epa
derived epg epg-config yatemplate autoinsert cl-extra yasnippet
help-mode edmacro kmacro dired-sidebar face-remap dired-subtree
dired-hacks-utils dash dired dired-loaddefs bbdb-mua bbdb-com crm
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font-core term/tty-colors frame minibuffer cl-generic cham georgian
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japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 441208 78375)
 (symbols 48 30936 1)
 (strings 32 123601 7188)
 (string-bytes 1 3885469)
 (vectors 16 47667)
 (vector-slots 8 1304186 200108)
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 (intervals 56 4655 1922)
 (buffers 992 33))

-- 
Gunnar