python - Why is the join built-in having no influence on my code?

Question

I had a bug that I reduced down to this:

a = ['a','b','c']
print( "Before", a )
" ".join(a)
print( "After", a )

Which outputs this:

runfile('C:/program.py', wdir=r'C:/')

Before ['a', 'b', 'c']
After ['a', 'b', 'c']

What's going on here?

Answer 1

str.join does not operate in-place because string objects are immutable in Python. Instead, it returns an entirely new string object.

If you want a to reference this new object, you need to explicitly reassign it:

a = " ".join(a)

Demo:

>>> a = ['a','b','c']
>>> print "Before", a
Before ['a', 'b', 'c']
>>> a = " ".join(a)
>>> print "After", a
After a b c
>>>