Python: Combining string and lists

Question

I have a list of counters

counters = ['76195087', '963301809', '830123644', '60989448', '0', '0', '76195087', '4006066839', '390361581', '101817210', '0', '0']

and I would like to create a string using some of these counters....

cmd = 'my_command' + counters[0:1]

But I find that I am unable to concatenate strings and lists.

What I must have at the end is a string that looks like this:

my_command 76195087

How do I get these numbers out of their list and get them to behave like strings?

phihag · Accepted Answer · 2011-11-27 18:51:20Z

4

You can join strings in a list with, well, join:

cmd = 'my_command' + ''.join(counters[:1])

But you shouldn't construct a command like that in the first place and give it to os.popen or os.system. Instead, use the subprocess module, which handles the internals (and escapes problematic values):

import subprocess
# You may want to set some options in the following line ...
p = subprocess.Popen(['my_command'] + counters[:1])
p.communicate()

answered Nov 27, 2011 at 18:51

phihag

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sth · Accepted Answer · 2011-11-27 18:50:51Z

4

If you just want a single element of the list, just index that element:

cmd = 'my_command ' + counters[0]

If you want to join several elements, use the 'join()' method of strings:

cmd = 'my_command ' + " ".join(counters[0:2]) # add spaces between elements

answered Nov 27, 2011 at 18:50

sth

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Sven Marnach · Accepted Answer · 2011-11-27 20:49:51Z

3

If you just want to append a single counter, you can use

"my_command " + counters[0]

or

"%s %s" % (command, counters[0])

where command is a variable containing the command as a string. If you want to append more than one counter, ' '.join() is your friend:

>>> ' '.join([command] + counters[:3])
'my_command 76195087 963301809 830123644'

edited Nov 27, 2011 at 20:49

answered Nov 27, 2011 at 18:50

Sven Marnach

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3 Comments

TheWellington Over a year ago

are the double quotes significant? when I use single quotes I get the following message: TypeError: cannot concatenate 'str' and 'list' objects

TheWellington Over a year ago

also... I will need to use several of the counters as arguments in the command. How do I bring in several counters? + counters[0] + counters[3], etc?

Sven Marnach Over a year ago

@TheWellington The double quotes are the same as single quotes, and I doubt you are using the exact code above.

Óscar López · Accepted Answer · 2011-11-27 18:50:50Z

You have to access an element of the list, not sublists of the list, like this:

cmd = 'my_command' + counters[0]

Since I guess you're interested in using all the counters at some point, use a variable to store the index you're currently using, and increment it where you see fit (possibly inside a loop)

idx = 0
cmd1 = 'my_command' + counters[idx]
idx += 1
cmd2 = 'my_command' + counters[idx]

Of course, being careful of not incrementing the index variable beyond the size of the list.

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