[Python-ideas] [Python-Dev] subprocess not escaping "^" on Windows

Ivan Pozdeev vano at mail.mipt.ru
Sun Jan 7 17:25:36 EST 2018

On 07.01.2018 22:32, Christian Tismer wrote:
> Hi Chris,
> On 07.01.18 18:18, Chris Angelico wrote:
>> Redirecting this part of the conversation to python-ideas.
>> On Mon, Jan 8, 2018 at 3:17 AM, Christian Tismer <tismer at stackless.com> wrote:
>>> As a side note: In most cases where shell=True is found, people
>>> seem to need evaluation of the PATH variable. To my understanding,
>>>>>> from subprocess import call
>>>>>> call(("ls",))
>>> works in Linux, but (with dir) not in Windows. But that is misleading
>>> because "dir" is a builtin command but "ls" is not. The same holds for
>>> "del" (Windows) and "rm" (Linux).
>> That's exactly what shell=True is for - if you want a shell feature,
>> you use the shell. What exactly would emulate_shell do? Would it
>> simply do a $PATH or %PATH% search, but otherwise function as
>> shell=False? Would it process redirection? Would it handle
>> interpolations? I think not, from your description:
>>> Perhaps it would be a good thing to emulate the builtin programs
>>> in python by some shell=True replacement (emulate_shell=True?)
>>> to match the normal user expectations without using the shell?
>> but it becomes difficult to draw the line. For instance, with
>> emulate_shell=True, what would you do with all the sh/bash built-ins:
>> https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html
>> https://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html
>> I'm thinking especially of the commands where bash has its own
>> handling of something that could otherwise be found in $PATH, like
>> pwd, time, and echo, but shells can do a lot of other things too.
>> When do you actually want to execute a shell built-in from Python but
>> without using the shell itself? You give the example of ls/dir, but if
>> that ever comes up in real-world code, I'd toss it out and recommend a
>> cross-platform os.listdir or equivalent. There are plenty of times
>> I've wanted a really quick way to redirect a standard stream from
>> Python, but that isn't part of what you're recommending. Can you give
>> a real-world example that would be improved by this?
>> I know this was just a side note in your original, but I'd like to
>> hear more about what would make it useful.
> No, I cannot. I just thought of a way to keep users from using
> "shell=True". I *think* they do it after they experience that
> "del" for instance is not found. They conclude "ah, I need the
> shell", which is not true.
Even putting aside the fact this is pure conjecture, the kind of people 
who make decisions like this will find a zillion more ways to shoot 
themselves in the foot. They don't need a cleaner syntax, they need to 
learn the basics of programming in a high-level language to understand 
how it's different from programming in the shell. In particular, why 
spawning a subprocess for something covered by a library function is a 
bad idea.
> So whatever you come up with, the effect should be that people
> no longer use the shell. THATs what I want, after bad experience with
> non-escaped "^" in a regex, that caused some really weird result.
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