How to get resulting subprocess command string

python, subprocess

Solution

As mentioned in a comment, `subprocess` comes with (not documented in the docs pages) `list2cmdline` that transforms a list of arguments into a single string. According to the source doc, `list2cmdline` is used mostly on Windows:

On Windows: the Popen class uses CreateProcess() to execute the child program, which operates on strings. If args is a sequence, it will be converted to a string using the list2cmdline method. Please note that not all MS Windows applications interpret the command line the same way: The list2cmdline is designed for applications using the same rules as the MS C runtime.

Nevertheless, it's quite usable on other OSes.

EDIT

Should you need the reverse operation (ie, splitting a command line into a list of properly tokenized arguments), you'll want to use the `shlex.split` function, as illustrated in the doc of `subprocess`.

>>> help(subprocess.list2cmdline)
Help on function list2cmdline in module subprocess:

list2cmdline(seq)
    Translate a sequence of arguments into a command line
    string, using the same rules as the MS C runtime:

    1) Arguments are delimited by white space, which is either a
       space or a tab.

    2) A string surrounded by double quotation marks is
       interpreted as a single argument, regardless of white space
       contained within.  A quoted string can be embedded in an
       argument.

    3) A double quotation mark preceded by a backslash is
       interpreted as a literal double quotation mark.

    4) Backslashes are interpreted literally, unless they
       immediately precede a double quotation mark.

    5) If backslashes immediately precede a double quotation mark,
       every pair of backslashes is interpreted as a literal
       backslash.  If the number of backslashes is odd, the last
       backslash escapes the next double quotation mark as
       described in rule 3.

Problem

I have Python subprocess calls which are formatted as a sequence of arguments (like `subprocess.Popen(['ls','-l'])` instead of a single string (i.e. `subprocess.Popen('ls -l')`). When using sequenced arguments like I did, is there a way to get the resulting string that is sent to the shell (for debugging purposes)? One simple approach would be to join all arguments together myself. But I doubt this would in all cases be identical to what subprocess does, since the main reason for using a sequence is to 'allow[s] the module to take care of any required escaping and quoting of arguments'.

Original source