How to prevent subprocess to add spaces between some parameters?

Question

I'd like to execute some Mysql commands from Linux, using Python and its subprocess module.

Without Python, from a shell, my command line is:

mysql --database=mydb --host=localhost --port=3306 --password=  --execute="select * from mytable" --batch

With Python, I have:

cmd = ['mysql']
cmd.extend(['--database=', self._database])
cmd.extend(['--password=', self._password])
cmd.extend(['--execute=', query])

(...)

p = subprocess.Popen(cmd, shell = False, stdout = subprocess.PIPE, stderr = subprocess.PIPE)
out, err = p.communicate()
errcode = p.returncode

Unfortunately, it doesn't work (mysql just print usage) and I think, subprocess generate this kind of output (' '.join(cmd)):

mysql --database= mydb --host= localhost --port= 3306 --password=  --execute= "select * from mytable" --batch

ie. spaces are added between each parameters, separating = and value.

When I remove = in every parameters (cmd.extend(['--password', self._password])), it works fine, except when a parameter is void (so, I test if each parameter is void, then remove it if needed).

Finally, I found a workaround, by testing void parameters, but for future reference, is there any subprocess tip or usage I don't know to handle this kind of parameter= ? There must be some application when you have to use =, and I try to avoid Shell=True.

David Zwicker · Accepted Answer · 2013-03-21 17:38:22Z

6

Change that to

cmd = [
    'mysql',
    '--database=%s' % self._database,
    '--password=%s' % self._password,
    '--execute=%s' % query
]

(...)

p = subprocess.Popen(cmd, shell = False, stdout = subprocess.PIPE, stderr = subprocess.PIPE)
out, err = p.communicate()
errcode = p.returncode

such that the list of arguments contains switches with their respective arguments

answered Mar 21, 2013 at 17:38

David Zwicker

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2 Comments

Cyrille Over a year ago

So simple, yet, I didn't even think of that (I guess I need vacation :) ) Thank you

tdelaney Over a year ago

I like, but how about removing the extend and just do ['mysql', '--database=%s' % self._database, ...] while we're at it?

kindall · Accepted Answer · 2013-03-21 17:40:00Z

1

You're passing each argument containing an = sign as two separate arguments. Don't do that, because they're not two separate arguments.

cmd.append('--database=' + self._database)
cmd.append('--password='+ self._password)
cmd.append('--execute=' + query)

(David's suggestion to use [] to define the whole list at once rather than repeated append or extend calls is better style.)

answered Mar 21, 2013 at 17:40

kindall

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