Improve error message for OS command entered at >>> prompt

[stevendaprano]

BPO	45721
Nosy	@terryjreedy, @aroberge, @stevendaprano, @pablogsal, @AlexWaygood
Superseder	bpo-28140: Give better errors for OS commands, like 'pip', in REPL, script

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-11-28.16:53:49.063>
created_at = <Date 2021-11-05.01:53:40.279>
labels = ['type-feature', '3.11']
title = 'Improve error message for OS command entered at >>> prompt'
updated_at = <Date 2021-11-28.16:53:49.062>
user = 'https://github.com/stevendaprano'

bugs.python.org fields:

activity = <Date 2021-11-28.16:53:49.062>
actor = 'terry.reedy'
assignee = 'none'
closed = True
closed_date = <Date 2021-11-28.16:53:49.063>
closer = 'terry.reedy'
components = []
creation = <Date 2021-11-05.01:53:40.279>
creator = 'steven.daprano'
dependencies = []
files = []
hgrepos = []
issue_num = 45721
keywords = []
message_count = 4.0
messages = ['405764', '405774', '405787', '405841']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'aroberge', 'steven.daprano', 'pablogsal', 'AlexWaygood']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '28140'
type = 'enhancement'
url = 'https://bugs.python.org/issue45721'
versions = ['Python 3.11']

[stevendaprano]

A frequent newbie mistake is to call shell commands from inside the interactive interpreter. Most common is to call Python itself.

Here is an example where a Python instructor was allegedly unable to diagnose the issue for their students:

https://windowsquestions.com/2021/10/09/syntaxerror-invalid-syntax-perhaps-you-forgot-a-comma/

I think it would be a nice feature if the compiler recognised obvious cases of "user tried to call Python from the Python prompt", and suggested a fix. If the statement matches the regex r"python\s+" the error message might say "it looks like you are trying to run a shell command at the Python prompt" rather than suggest a missing comma.

[aroberge]

I think that "trying to run a shell command" might not be easy to understand by a beginner.

If the special case of r"python\s+" is added, perhaps r"pip\s+" should be considered as well since many sites on the Internet suggest to install packages using "pip install ..." instead of "python -m pip install ..."

[AlexWaygood]

+1 for adding r"pip\s" as a special case as well. A few months ago, I was trying to (remotely) help a beginner friend debug why his attempt to install a library wasn't working. It took me ages before I realised he was entering it into the interactive REPL rather than a terminal window.

[terryjreedy]

Agreed. 'pip ...' not working is a recurring question on stackoverflow, usually from an IDLE user. 'python' is much rarer. Note that IDLE's shell may be a beginner's only exposure to the word 'shell'. The standard Windows shell is the Command Prompt console, so one has to say 'Command Prompt command'. Would 'bash command' work elsewhere?