How does Python's .join work?

Question

I was suggested the below solution but I didn't understand this line cmd += "("+ ",".join(args) +")" of the below code. I got frustated because of this.Can someone please help me

Question is Reading an input like this

12
insert 0 5
insert 1 10
insert 0 6
print 
remove 6
append 9
append 1
sort 
print
pop
reverse
print

My arttempt in Python language

n = input()
l = []
for _ in range(n):
    s = raw_input().split()
    cmd = s[0]
    args = s[1:]
    if cmd !="print":
        cmd += "("+ ",".join(args) +")"
        eval("l."+cmd)
    else:
        print l

Rahul I just want to understand this line cmd += "("+ ",".join(args) +")" — Dhrub kumar, Jun 17 '16 at 07:19
`eval()` is a terrible solution for this. See http://stackoverflow.com/questions/1832940/is-using-eval-in-python-a-bad-practice. You could for example use `getattr(l, cmd)(*(int(arg) for arg in args))` in this particular case. Even uncontrolled getattr can be evil. — Ilja Everilä, Jun 17 '16 at 07:26

score 2 · Answer 1 · answered Jun 17 '16 at 07:45

2

This line is there to ensure that your command is in the right syntax and has brackets and comma when you call it! I wrote it clearer in the code below:

cmd = 'insert'
arg1 = '1'
arg2 = '3'
arg3 = '10'

cmd = cmd + "(" + ",".join((arg1, arg2, arg3)) + ")"
print cmd

Output:

insert(1,3,10)

answered Jun 17 '16 at 07:45

Blind0ne

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"," is nothing but the separator for arguments We can use anything instead of , like / as well. – Viraj Wadate Dec 03 '18 at 13:26

score 1 · Answer 2 · answered Jun 17 '16 at 07:21

Consider the following input line:

insert 1 10

So s is the list ["insert", "1", "10"]. Note that the list elements are all strings.

So cmd is "insert" and args is ["1", "10"].

Now here's the part that you didn't understand. First, ",".join(args) just creates a single string composed of the elements of arg, separated by commas: "1,10".

This is then wrapped in parentheses: "(1,10)". Finally, it is appended to cmd resulting in the string "insert(1,10)".

It then prepends "l." to this, resulting in the string "l.insert(1,10)". This is then passed to eval.

So it's invoking the insert method on the list l, inserting the value 10 at position 1. I.e., it's executing l.insert(1,10).

Thanks a lot Tom. Understood completely :) – Dhrub kumar Jun 17 '16 at 07:27 — Dhrub kumar, Jun 17 '16 at 07:27

score 1 · Answer 3 · answered Jun 17 '16 at 07:26

Your question is what does the following line mean:

cmd += "("+ ",".join(args) +")"

cmd is a variable (with a string).

+= is an operator which adds the right side to the left side.

"(" is a string containing a left parens.

+ is an operator that adds the left and right of itself.

",".join(args) this makes the list, args, into a string separated by commas. For example:

args =['bob','sue','tom'], then: 
",".join(args)
> bob,sue,tom

again, + is an operator that adds the left and right of itself.

) is a string containing a right parens.

Thus, if args is ['bob',sue','tom], and cmd is names, then the output of this line would be:

cmd = "names"
args =["bob","sue","tom"]
cmd += "("+ ",".join(args) +")"
print cmd
> names(bob,sue,tom)

How does Python's .join work?

3 Answers3

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