[Tutor] subprocess.call list vs. str argument

[Albert-Jan Roskam]

>________________________________
> From: Oscar Benjamin <oscar.j.benjamin at gmail.com>
>To: Albert-Jan Roskam <fomcl at yahoo.com> 
>Cc: Dave Angel <davea at davea.name>; eryksun <eryksun at gmail.com>; "Tutor at python.org" <tutor at python.org> 
>Sent: Wednesday, February 26, 2014 3:38 PM
>Subject: Re: [Tutor] subprocess.call list vs. str argument
> 
>
>On 26 February 2014 08:50, Albert-Jan Roskam <fomcl at yahoo.com> wrote:
>
>>
>> Yesterday evening (it was *late* so forgive me if I wrong) I realized that part of my confusion was also caused by the fact that I ran my code in Idle.
>> If I called subprocess.call with a list argument, it  returned code 0 (success) but the generated sphinx code was not the same as when I ran the program from the terminal. I concluded that this is some weird interaction between Idle (which may also run in a subprocess??) and my own program. I also had no problems when I ran (in the terminal): python -c "import subprocess; subprocess.call(['sphinx-apidoc'...(etc)])"
>>
>> I was surprised that the list elements of the argument for subprocess.call *have to be* separate tokens, e.g. (earlier in one of Danny's replies): a token (list element) '-f -F' won't work, it has to be two separate elements/tokens: '-f', '-F'. Apparently, subprocess.call is more than just a convenience function that " ".joins the list and escapes the resulting string.
>
>At the operating system level there is no "command line" as such on
>posix (Linux, Mac etc.). The command to run a program is a string
>giving the name of the program and you can also pass a list of strings
>to the program as "arguments". The command line is something that
>happens in the shell (e.g. bash). Whereas Python would use quotes and
>commas to separate strings in a list of strings the shell just assumes
>that whitespace separates strings.
>
>So in Python you would write
>    ['qwe', 'asd', 'zxc zxc']
>and it's clear that you have a list of three strings. In the shell
>(e.g. bash) you would type
>    qwe asd "zxc zxc"
>and the shell understands that as three separate strings. The quotes
>are needed to distinguish it from 4 strings because shell syntax
>assumes that spaces separate strings.
>
>So the idea of the "command line" as a single string that must be
>split on whitespace is just something that happens in the shell. It is
>supposed to be a convenient way of typing commands in the interactive
>shell i.e. it would slow me down if I has to put in square brackets
>quotes and commas every time I wanted to type a command in the shell
>like ['ls', '-l', '~/mystuff']. But this convenience comes at a price
>in that it becomes hard to handle strings that have spaces in them.
>I've never really figured out how to write a non-trivial shell script
>that can properly handle whitespace (it's easier to just avoid putting
>spaces in filenames - or write the script in Python).
>
>So really I think it's a good thing that Python lets you clearly
>delineate each string in the argument list:
>    subprocess.call(['ls', '-l', foldername])
>without needing to worry about whether or not foldername contains spaces.


Thank you everybody for all your replies! I will use a list from now on. Also good to know that this works a little differently under Windows. 


Albert-Jan