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What exactly does the .join() method do?

Posted by: admin January 29, 2018 Leave a comment

Questions:

I’m pretty new to Python and am completely confused by .join() which I have read is the preferred method for concatenating strings.

I tried:

strid = repr(595)
print array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
    .tostring().join(strid)

and got something like:

5wlfgALGbXOahekxSs9wlfgALGbXOahekxSs5

Why does it work like this? Shouldn’t the 595 just be automatically appended?

Answers:

Look carefully at your output:

5wlfgALGbXOahekxSs9wlfgALGbXOahekxSs5
^                 ^                 ^

I’ve highlighted the “5”, “9”, “5” of your original string. The Python join() method is a string method, and takes a list of things to join with the string. A simpler example might help explain:

>>> ",".join(["a", "b", "c"])
'a,b,c'

The “,” is inserted between each element of the given list. In your case, your “list” is the string representation “595”, which is treated as the list [“5”, “9”, “5”].

It appears that you’re looking for + instead:

print array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
.tostring() + strid

Questions:
Answers:

join takes an iterable thing as an argument. Usually it’s a list. The problem in your case is that a string is itself iterable, giving out each character in turn. Your code breaks down to this:

"wlfgALGbXOahekxSs".join("595")

which acts the same as this:

"wlfgALGbXOahekxSs".join(["5", "9", "5"])

and so produces your string:

"5wlfgALGbXOahekxSs9wlfgALGbXOahekxSs5"

Strings as iterables is one of the most confusing beginning issues with Python.

Questions:
Answers:

To append a string, just concatenate it with the + sign.

E.g.

>>> a = "Hello, "
>>> b = "world"
>>> str = a + b
>>> print str
Hello, world

join connects strings together with a separator. The separator is what you
place right before the join. E.g.

>>> "-".join([a,b])
'Hello, -world'

Join takes a list of strings as a parameter.

Questions:
Answers:

join() is for concatenating all list elements. For concatenating just two strings “+” would make more sense:

strid = repr(595)
print array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
    .tostring() + strid

Questions:
Answers:

To expand a bit more on what others are saying, if you wanted to use join to simply concatenate your two strings, you would do this:

strid = repr(595)
print ''.join([array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
    .tostring(), strid])

Questions:
Answers:

On providing this as input ,

li = ['server=mpilgrim', 'uid=sa', 'database=master', 'pwd=secret']
s = ";".join(li)
print(s)

Python returns this as output :

'server=mpilgrim;uid=sa;database=master;pwd=secret'