From Python's source code of open, I think open is just a normal function.
Why can we use it like below?
with open('what_are_context_managers.txt', 'r') as infile:
for line in infile:
print('> {}'.format(line))
Since is neither implements __enter__ nor __exit__, nor uses contextlib.contextmanager decorator.
You are not using the open function as a context manager. It is the result of the open(...) call expression that is the context manager. open() returns a file object, and it is that object that has __enter__ and __exit__ methods; see the io.IOBase documentation:
IOBase is also a context manager and therefore supports the with statement.
You can read the with statement like this:
_context_manager = open('what_are_context_managers.txt', 'r')
with _context_manager as infile:
Note that it is the return value of _context_manager.__enter__() that ends up being assigned to infile here. For file objects, file.__enter__() returns self, so you can get access to the same object that way.
As a side-note; you got the wrong open() function. The actual definition of the open() built-in is an alias for io.open(), see the _iomodule.c source code. The alias is set in initstdio() in pylifecycle.c (where io.OpenWrapper is itself an alias for _io.open). And yes, the documentation states the alias points the other way for end-user ease.
