str.join

The str.join method is used to join a list of strings together using the given delimiter:

'..'.join(['hello', '@pythonetc'])
# 'hello..@pythonetc'

Many newcomers are confused by this syntax. Why join is a method of str? Why not a function? Why not a method of list?

The first question is quite easy to answer: it's not a function to keep the built-in namespace clean. Also, the delimiter can be only str. The built-in functions are mostly the ones that can accept arguments of multiple types. For example, the argument len can be str, list, tuple, and any other collection. Exceptions, like chr, are caused by limitations of the parser: 1.ord() is a SyntaxError, and (1).ord() doesn't look nice.

On the second question: it's not a method of list because it supports any iterable, not only lists. For example:

def f():
    yield 'hello'
    yield '@pythonetc'

' '.join(f())
# 'hello @pythonetc'

If you know someone who starts learning Python, tell them: there are no stupid questions. It's great to question some ideas and decisions. Learning about the motivation behind them helps to better understand the language. The join is a method of str not "just because", and it's great to know why.