CODEWITHSUNDEEP
pythonliststring
Matt McCormick
Matt McCormick

Reputation: 13200

What exactly does the .join() method do?

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?

Upvotes: 222

Views: 600149

Answers (10)

Mohamed Hammou
Mohamed Hammou

Reputation: 11

string : str = "".join(["a", "b", "c"])

Output : "a,b,c"

string : str = "-".join(["a", "b", "c"])

Output : "a-b-c"

Simply, this function is used to convert from a list to a string. This function takes two arguments wich are : the list of elements and the character that will seperate the elements of the list ; as default, the character is a quote "," .

Upvotes: 0

Martin
Martin

Reputation: 3472

"".join may be used to copy the string in a list to a variable

>>> myList = list("Hello World")
>>> myString = "".join(myList)
>>> print(myList)
['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd']
>>> print(myString)
Hello World

Upvotes: 0

kourosh
kourosh

Reputation: 91

list = ["my", "name", "is", "kourosh"]   
" ".join(list)

If this is an input, using the JOIN method, we can add the distance between the words and also convert the list to the string.

This is Python output 

'my name is kourosh'

Upvotes: 0

ZakS
ZakS

Reputation: 1131

There is a good explanation of why it is costly to use + for concatenating a large number of strings here

Plus operator is perfectly fine solution to concatenate two Python strings. But if you keep adding more than two strings (n > 25) , you might want to think something else.

''.join([a, b, c]) trick is a performance optimization.

Upvotes: 2

Akash Kandpal
Akash Kandpal

Reputation: 3366

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'

Upvotes: 0

Greg Hewgill
Greg Hewgill

Reputation: 993015

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

Upvotes: 319

Jorenko
Jorenko

Reputation: 2664

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])

Upvotes: 4

Dan Loewenherz
Dan Loewenherz

Reputation: 11236

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.

Upvotes: 62

Ned Batchelder
Ned Batchelder

Reputation: 375574

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.

Upvotes: 98

Pēteris Caune
Pēteris Caune

Reputation: 45102

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

Upvotes: 8

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