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Diffstat (limited to 'Doc/faq/library.rst')
| -rw-r--r-- | Doc/faq/library.rst | 46 |
1 files changed, 27 insertions, 19 deletions
diff --git a/Doc/faq/library.rst b/Doc/faq/library.rst index 662f423..067548f 100644 --- a/Doc/faq/library.rst +++ b/Doc/faq/library.rst @@ -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 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`:: -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:: + os.close(stdin.fileno()) + os.close(stdout.fileno()) + os.close(stderr.fileno()) - os.close(0) # close C's stdin stream - os.close(1) # close C's stdout stream - os.close(2) # close C's stderr stream +Or you can use the numeric constants 0, 1 and 2, respectively. Network/Internet Programming |