How to make argparse accept "-4^2+5.3*abs(-2-1)/2" string argument?

[avi.e.gross at gmail.com]

Cameron,

You are technically correct but perhaps off the mark.

Yes, a python program only sees what is handed to it by some shell if invoked a certain way.

The issue here is what you tell people using your program about what they need to type to get it to work. That means if their shell is going to make changes in what they typed, they need to know how to avoid unintended changes. As one example not mentioned, whitespace disappears if not somehow protected as in quotes.

What the OP is being told is that their Python program only controls what is fed to it. A user needs to know enough to avoid doing silly things like provide an unquoted string containing reserved symbols like a pipe symbol or odd things may happen and their program may not even be called. 

So the documentation of how to use the program may need to spell some things out alongside suggesting use of "--" ...


-----Original Message-----
From: Python-list <python-list-bounces+avi.e.gross=gmail.com at python.org> On Behalf Of Cameron Simpson
Sent: Sunday, January 29, 2023 12:51 AM
To: python-list at python.org
Subject: Re: How to make argparse accept "-4^2+5.3*abs(-2-1)/2" string argument?

On 28Jan2023 18:55, Jach Feng <jfong at ms4.hinet.net> wrote:
>Mark Bourne 在 2023年1月28日 星期六晚上10:00:01 [UTC+8] 的信中寫道：
>> I notice you explain the need to enclose the equation in quotes if it 
>> contains spaces. That's not even a feature of your application, but 
>> of the shell used to call it. So why so much objection to explaining 
>> the need for "--"?
>>
>> Depending on the shell, there are other cases where quoting might be 
>> needed, e.g. if the equation includes a "*" or "?" and happens to 
>> look like a pattern matching files in the current directory (most 
>> shells I've used pass the pattern unchanged if it doesn't match any 
>> files). In bash, if a "$" is used I'd need to enclose that in 'single quotes'
>> (can't even use "double quotes" for that one). You can't really 
>> expect to document all that sort of thing, because it depends on 
>> which shell the user happens to run your application from - you just 
>> have to trust the user to know or learn how to use their shell.
>
>Thank you for detail explanation of the role the shell is involved in this problem. I'm very appreciated!

The shell has basicly _nothing_ to do with your problems. By the time you've got sys.argv in your Python programme you will have no idea whether quotes were used with an argument. (UNIX/POSIX, not Windows, where things are ... more complex.) This means you don't know if the use
typed:

     -4.5

or

     "-4.5"

You'll just get a string '4.5' in your Python programme both ways.

All the quotes in the shell do is delimit what things should be kept together as a single argument versus several, or where variables should be interpolated when computing arguments etc. It's just _shell_ punctuation and the invoked programme doesn't see it.

>It seems that a CLI app may become very complex when dealing with different kind of shell, and may not be possible to solve its problem.

It doesn't matter what shell is used. The just controls what punctuation the end user may need to use to invoke your programme. You programme doesn't need to care (and can't because it doesn't get the quotes etc, only their result).

>> So why so much objection to explaining the need for "--"?
>Because of using " to enclose a space separated string is a common 
>convention, and adding a "--" is not:-)

They're unrelated. As others have mentioned, "--" is _extremely_ common; almost _all_ UNIX command like programmes which handle -* style options honour the "--" convention. _argparse_ itself honours that convention, as does getopt etc.

The "--" convention has nothing to do with the shell.

Cheers,
Cameron Simpson <cs at cskk.id.au>
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