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Update file-related information in the FAQ.

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Antoine Pitrou 2010-09-15 10:08:31 +00:00
parent 0b65b0fc83
commit 6a11a98b7c
1 changed files with 27 additions and 19 deletions
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46
Doc/faq/library.rst
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@ -458,7 +458,7 @@ contents, use :func:`shutil.rmtree`.
To rename a file, use ``os.rename(old_path, new_path)``.
To truncate a file, open it using ``f = open(filename, "r+")``, and use
To truncate a file, open it using ``f = open(filename, "rb+")``, and use
``f.truncate(offset)``; offset defaults to the current seek position. There's
also ```os.ftruncate(fd, offset)`` for files opened with :func:`os.open`, where
``fd`` is the file descriptor (a small integer).
@ -487,9 +487,9 @@ in big-endian format from a file::
import struct
f = open(filename, "rb") # Open in binary mode for portability
s = f.read(8)
x, y, z = struct.unpack(">hhl", s)
with open(filename, "rb") as f:
s = f.read(8)
x, y, z = struct.unpack(">hhl", s)
The '>' in the format string forces big-endian data; the letter 'h' reads one
"short integer" (2 bytes), and 'l' reads one "long integer" (4 bytes) from the
@ -498,6 +498,13 @@ string.
For data that is more regular (e.g. a homogeneous list of ints or thefloats),
you can also use the :mod:`array` module.
.. note::
To read and write binary data, it is mandatory to open the file in
binary mode (here, passing ``"rb"`` to :func:`open`). If you use
``"r"`` instead (the default), the file will be open in text mode
and ``f.read()`` will return :class:`str` objects rather than
:class:`bytes` objects.
I can't seem to use os.read() on a pipe created with os.popen(); why?
---------------------------------------------------------------------
@ -603,28 +610,29 @@ For Unix, see a Usenet post by Mitch Chapman:
Why doesn't closing sys.stdout (stdin, stderr) really close it?
---------------------------------------------------------------
Python file objects are a high-level layer of abstraction on top of C streams,
which in turn are a medium-level layer of abstraction on top of (among other
things) low-level C file descriptors.
Python :term:`file objects <file object>` are a high-level layer of
abstraction on low-level C file descriptors.
For most file objects you create in Python via the built-in ``open``
constructor, ``f.close()`` marks the Python file object as being closed from
Python's point of view, and also arranges to close the underlying C stream.
This also happens automatically in ``f``'s destructor, when ``f`` becomes
garbage.
For most file objects you create in Python via the built-in :func:`open`
function, ``f.close()`` marks the Python file object as being closed from
Python's point of view, and also arranges to close the underlying C file
descriptor. This also happens automatically in ``f``'s destructor, when
``f`` becomes garbage.
But stdin, stdout and stderr are treated specially by Python, because of the
special status also given to them by C. Running ``sys.stdout.close()`` marks
the Python-level file object as being closed, but does *not* close the
associated C stream.
associated C file descriptor.
To close the underlying C stream for one of these three, you should first be
sure that's what you really want to do (e.g., you may confuse extension modules
trying to do I/O). If it is, use os.close::
To close the underlying C file descriptor for one of these three, you should
first be sure that's what you really want to do (e.g., you may confuse
extension modules trying to do I/O). If it is, use :func:`os.close`::
os.close(0) # close C's stdin stream
os.close(1) # close C's stdout stream
os.close(2) # close C's stderr stream
os.close(stdin.fileno())
os.close(stdout.fileno())
os.close(stderr.fileno())
Or you can use the numeric constants 0, 1 and 2, respectively.
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