better way for ' '.join(args) + '\n'?

[Thomas Rachel]

Am 26.10.2012 09:49 schrieb Ulrich Eckhardt:
> Hi!
>
> General advise when assembling strings is to not concatenate them
> repeatedly but instead use string's join() function, because it avoids
> repeated reallocations and is at least as expressive as any alternative.
>
> What I have now is a case where I'm assembling lines of text for driving
> a program with a commandline interface.

Stop.

In this case, you think too complicated.

Just do

     subprocess.Popen(['prog', 'foo', 'bar', 'baz'])

-  is the most safest thing for this use case.

If it should not be possible for any reason, you should be aware of any 
traps you could catch - e.g., if you want to feed your string to a 
Bourne shell, you should escape the strings properly.

In such cases, I use


def shellquote(*strs):
	r"""Input: file names, output: ''-enclosed strings where every ' is 
replaced with '\''. Intended for usage with the shell."""
	# just take over everything except ';
	# replace ' with '\''
	# The shell sees ''' as ''\'''\'''\'''. Ugly, but works.
	return " ".join([
		"'"+st.replace("'","'\\''")+"'"
		for st in strs
	])


so I can use

shellquote('program name', 'argu"ment 1', '$arg 2',
     "even args containing a ' are ok")

For Windows, you'll have to modify this somehow.


HTH,

Thomas