open read and close a file in 1 line of code

You don't really have to close it - Python will do it automatically either during garbage collection or at program exit. But as @delnan noted, it's better practice to explicitly close it for various reasons.

So, what you can do to keep it short, simple and explicit:

with open('pagehead.section.htm', 'r') as f:
    output = f.read()

Now it's just two lines and pretty readable, I think.

Python Standard Library Pathlib module does what you looking for:

Path('pagehead.section.htm').read_text()

Don't forget to import Path:

jsk@dev1:~$ python3
Python 3.5.2 (default, Sep 10 2016, 08:21:44)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pathlib import Path
>>> (Path("/etc") / "hostname").read_text()
'dev1.example\n'

On Python 27 install backported pathlib or pathlib2

Using CPython, your file will be closed immediately after the line is executed, because the file object is immediately garbage collected. There are two drawbacks, though:

1. In Python implementations different from CPython, the file often isn't immediately closed, but rather at a later time, beyond your control.

2. In Python 3.2 or above, this will throw a ResourceWarning, if enabled.

Better to invest one additional line:

with open('pagehead.section.htm','r') as f:
    output = f.read()

This will ensure that the file is correctly closed under all circumstances.

What you can do is to use the with statement, and write the two steps on one line:

>>> with open('pagehead.section.htm', 'r') as fin: output = fin.read();
>>> print(output)
some content

The with statement will take care to call __exit__ function of the given object even if something bad happened in your code; it's close to the try... finally syntax. For object returned by open, __exit__ corresponds to file closure.

This statement has been introduced with Python 2.6.