Question

The full source code is @ PEP 333. These two lines:

status, response_headers = headers_sent[:] = headers_set

.. and ..

headers_set[:] = [status, response_headers]

What am I looking at here? How does [:] differ from giving nothing at all (just headers_set)? If someone could provide an explanation, I’d be really glad.

Answer 1

[:] means that you are overwriting the entire list’s contents.

>>> a = [1,2,3]
>>> a[:] = [3,4]
>>> a
[3, 4]
>>> a[]
  File "<stdin>", line 1
    a[]
      ^
SyntaxError: invalid syntax

And you can use the same syntax to overwrite some index range of the list:

>>> a[2:] = [3,4]
>>> a
[3, 4, 3, 4]