Confused about .join()

I’m new to python and completely confused,

why ','.join('a','b','c')rised an error, a ",".join(['a','b','c'])didn’t.

','.join('a','b','c')

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

Why the code below have the same output?

In [3]: ",".join('a':1,'b':2,'c':3)
Out[3]: 'b,a,c'

In [4]: ",".join('a':2,'b':1,'c':3)
Out[4]: 'b,a,c'

In [5]: ",".join('a':3,'b':2,'c':1)
Out[5]: 'b,a,c'


','.join(('a', 'b', 'c'))

3 Answers
3

join takes only one argument of the iterable type. ','.join('a','b','c')gives join 3 arguments of type string instead of 1. Whereas ",".join(['a','b','c']) gives join only 1 argument of type list which is an iterable. Single or double quotes make no difference.

join

','.join('a','b','c')

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

For your second question, all 3 dictionaries give the same output since what value is stored for a key makes no difference. So whether a key has a value of 1 or 3 or perhaps some string, it wont affect the order. The order of a dictionary is inherently arbitrary. It cannot be predicted in advance and it is because of the fact how dictionaries are stored in memory.

The docs on str.join state that the method takes an iterable which is any type that can be iterated over (list, tuple, etc.):

str.join

iterable

str.join(iterable)¶

str.join(iterable)¶

Return a string which is the concatenation of the strings in iterable. A TypeError will be raised if there are any non-string values in iterable, including bytes objects. The separator between elements is the string providing this method.

TypeError

In your first example, you pass three arguments to a method which expects one so receive a TypeError, whereas you use the method as intended in the example where you pass a list (indicated so through the use of square brackets). In your typed examples, you are passing dictionaries which are also iterable, however, when iterated over, they yield their keys (note: the order is undetermined). So the keys of say: 'a':1,'b':2,'c':3 are: 'a', 'b' and 'c' and since these are all strings, the method works fine to join them together with a comma

TypeError

'a':1,'b':2,'c':3

'a'

'b'

'c'

Because with ','.join('a','b','c'), you're providing 3 arguments, each of which is a str, to str.join(), which is unexpected (expected 1).

','.join('a','b','c')

str

str.join()

With ','.join(['a','b','c']), you're providing only 1 argument which is a list, and it works well.

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

list

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