--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/symbian-qemu-0.9.1-12/python-2.6.1/Doc/tutorial/inputoutput.rst Fri Jul 31 15:01:17 2009 +0100
@@ -0,0 +1,407 @@
+.. _tut-io:
+
+****************
+Input and Output
+****************
+
+There are several ways to present the output of a program; data can be printed
+in a human-readable form, or written to a file for future use. This chapter will
+discuss some of the possibilities.
+
+
+.. _tut-formatting:
+
+Fancier Output Formatting
+=========================
+
+So far we've encountered two ways of writing values: *expression statements* and
+the :keyword:`print` statement. (A third way is using the :meth:`write` method
+of file objects; the standard output file can be referenced as ``sys.stdout``.
+See the Library Reference for more information on this.)
+
+.. index:: module: string
+
+Often you'll want more control over the formatting of your output than simply
+printing space-separated values. There are two ways to format your output; the
+first way is to do all the string handling yourself; using string slicing and
+concatenation operations you can create any layout you can imagine. The
+standard module :mod:`string` contains some useful operations for padding
+strings to a given column width; these will be discussed shortly. The second
+way is to use the :meth:`str.format` method.
+
+One question remains, of course: how do you convert values to strings? Luckily,
+Python has ways to convert any value to a string: pass it to the :func:`repr`
+or :func:`str` functions.
+
+The :func:`str` function is meant to return representations of values which are
+fairly human-readable, while :func:`repr` is meant to generate representations
+which can be read by the interpreter (or will force a :exc:`SyntaxError` if
+there is not equivalent syntax). For objects which don't have a particular
+representation for human consumption, :func:`str` will return the same value as
+:func:`repr`. Many values, such as numbers or structures like lists and
+dictionaries, have the same representation using either function. Strings and
+floating point numbers, in particular, have two distinct representations.
+
+Some examples::
+
+ >>> s = 'Hello, world.'
+ >>> str(s)
+ 'Hello, world.'
+ >>> repr(s)
+ "'Hello, world.'"
+ >>> str(0.1)
+ '0.1'
+ >>> repr(0.1)
+ '0.10000000000000001'
+ >>> x = 10 * 3.25
+ >>> y = 200 * 200
+ >>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
+ >>> print s
+ The value of x is 32.5, and y is 40000...
+ >>> # The repr() of a string adds string quotes and backslashes:
+ ... hello = 'hello, world\n'
+ >>> hellos = repr(hello)
+ >>> print hellos
+ 'hello, world\n'
+ >>> # The argument to repr() may be any Python object:
+ ... repr((x, y, ('spam', 'eggs')))
+ "(32.5, 40000, ('spam', 'eggs'))"
+
+Here are two ways to write a table of squares and cubes::
+
+ >>> for x in range(1, 11):
+ ... print repr(x).rjust(2), repr(x*x).rjust(3),
+ ... # Note trailing comma on previous line
+ ... print repr(x*x*x).rjust(4)
+ ...
+ 1 1 1
+ 2 4 8
+ 3 9 27
+ 4 16 64
+ 5 25 125
+ 6 36 216
+ 7 49 343
+ 8 64 512
+ 9 81 729
+ 10 100 1000
+
+ >>> for x in range(1,11):
+ ... print '{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x)
+ ...
+ 1 1 1
+ 2 4 8
+ 3 9 27
+ 4 16 64
+ 5 25 125
+ 6 36 216
+ 7 49 343
+ 8 64 512
+ 9 81 729
+ 10 100 1000
+
+(Note that in the first example, one space between each column was added by the
+way :keyword:`print` works: it always adds spaces between its arguments.)
+
+This example demonstrates the :meth:`rjust` method of string objects, which
+right-justifies a string in a field of a given width by padding it with spaces
+on the left. There are similar methods :meth:`ljust` and :meth:`center`. These
+methods do not write anything, they just return a new string. If the input
+string is too long, they don't truncate it, but return it unchanged; this will
+mess up your column lay-out but that's usually better than the alternative,
+which would be lying about a value. (If you really want truncation you can
+always add a slice operation, as in ``x.ljust(n)[:n]``.)
+
+There is another method, :meth:`zfill`, which pads a numeric string on the left
+with zeros. It understands about plus and minus signs::
+
+ >>> '12'.zfill(5)
+ '00012'
+ >>> '-3.14'.zfill(7)
+ '-003.14'
+ >>> '3.14159265359'.zfill(5)
+ '3.14159265359'
+
+Basic usage of the :meth:`str.format` method looks like this::
+
+ >>> print 'We are the {0} who say "{1}!"'.format('knights', 'Ni')
+ We are the knights who say "Ni!"
+
+The brackets and characters within them (called format fields) are replaced with
+the objects passed into the format method. The number in the brackets refers to
+the position of the object passed into the format method. ::
+
+ >>> print '{0} and {1}'.format('spam', 'eggs')
+ spam and eggs
+ >>> print '{1} and {0}'.format('spam', 'eggs')
+ eggs and spam
+
+If keyword arguments are used in the format method, their values are referred to
+by using the name of the argument. ::
+
+ >>> print 'This {food} is {adjective}.'.format(
+ ... food='spam', adjective='absolutely horrible')
+ This spam is absolutely horrible.
+
+Positional and keyword arguments can be arbitrarily combined::
+
+ >>> print 'The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',
+ ... other='Georg')
+ The story of Bill, Manfred, and Georg.
+
+An optional ``':``` and format specifier can follow the field name. This also
+greater control over how the value is formatted. The following example
+truncates the Pi to three places after the decimal.
+
+ >>> import math
+ >>> print 'The value of PI is approximately {0:.3f}.'.format(math.pi)
+ The value of PI is approximately 3.142.
+
+Passing an integer after the ``':'`` will cause that field to be a minimum
+number of characters wide. This is useful for making tables pretty.::
+
+ >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
+ >>> for name, phone in table.items():
+ ... print '{0:10} ==> {1:10d}'.format(name, phone)
+ ...
+ Jack ==> 4098
+ Dcab ==> 7678
+ Sjoerd ==> 4127
+
+If you have a really long format string that you don't want to split up, it
+would be nice if you could reference the variables to be formatted by name
+instead of by position. This can be done by simply passing the dict and using
+square brackets ``'[]'`` to access the keys ::
+
+ >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
+ >>> print ('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '
+ ... 'Dcab: {0[Dcab]:d}'.format(table))
+ Jack: 4098; Sjoerd: 4127; Dcab: 8637678
+
+This could also be done by passing the table as keyword arguments with the '**'
+notation.::
+
+ >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
+ >>> print 'Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)
+ Jack: 4098; Sjoerd: 4127; Dcab: 8637678
+
+This is particularly useful in combination with the new built-in :func:`vars`
+function, which returns a dictionary containing all local variables.
+
+For a complete overview of string formating with :meth:`str.format`, see
+:ref:`formatstrings`.
+
+
+Old string formatting
+---------------------
+
+The ``%`` operator can also be used for string formatting. It interprets the
+left argument much like a :cfunc:`sprintf`\ -style format string to be applied
+to the right argument, and returns the string resulting from this formatting
+operation. For example::
+
+ >>> import math
+ >>> print 'The value of PI is approximately %5.3f.' % math.pi
+ The value of PI is approximately 3.142.
+
+Since :meth:`str.format` is quite new, a lot of Python code still uses the ``%``
+operator. However, because this old style of formatting will eventually removed
+from the language :meth:`str.format` should generally be used.
+
+More information can be found in the :ref:`string-formatting` section.
+
+
+.. _tut-files:
+
+Reading and Writing Files
+=========================
+
+.. index::
+ builtin: open
+ object: file
+
+:func:`open` returns a file object, and is most commonly used with two
+arguments: ``open(filename, mode)``.
+
+::
+
+ >>> f = open('/tmp/workfile', 'w')
+ >>> print f
+ <open file '/tmp/workfile', mode 'w' at 80a0960>
+
+The first argument is a string containing the filename. The second argument is
+another string containing a few characters describing the way in which the file
+will be used. *mode* can be ``'r'`` when the file will only be read, ``'w'``
+for only writing (an existing file with the same name will be erased), and
+``'a'`` opens the file for appending; any data written to the file is
+automatically added to the end. ``'r+'`` opens the file for both reading and
+writing. The *mode* argument is optional; ``'r'`` will be assumed if it's
+omitted.
+
+On Windows, ``'b'`` appended to the mode opens the file in binary mode, so there
+are also modes like ``'rb'``, ``'wb'``, and ``'r+b'``. Windows makes a
+distinction between text and binary files; the end-of-line characters in text
+files are automatically altered slightly when data is read or written. This
+behind-the-scenes modification to file data is fine for ASCII text files, but
+it'll corrupt binary data like that in :file:`JPEG` or :file:`EXE` files. Be
+very careful to use binary mode when reading and writing such files. On Unix,
+it doesn't hurt to append a ``'b'`` to the mode, so you can use it
+platform-independently for all binary files.
+
+
+.. _tut-filemethods:
+
+Methods of File Objects
+-----------------------
+
+The rest of the examples in this section will assume that a file object called
+``f`` has already been created.
+
+To read a file's contents, call ``f.read(size)``, which reads some quantity of
+data and returns it as a string. *size* is an optional numeric argument. When
+*size* is omitted or negative, the entire contents of the file will be read and
+returned; it's your problem if the file is twice as large as your machine's
+memory. Otherwise, at most *size* bytes are read and returned. If the end of
+the file has been reached, ``f.read()`` will return an empty string (``""``).
+::
+
+ >>> f.read()
+ 'This is the entire file.\n'
+ >>> f.read()
+ ''
+
+``f.readline()`` reads a single line from the file; a newline character (``\n``)
+is left at the end of the string, and is only omitted on the last line of the
+file if the file doesn't end in a newline. This makes the return value
+unambiguous; if ``f.readline()`` returns an empty string, the end of the file
+has been reached, while a blank line is represented by ``'\n'``, a string
+containing only a single newline. ::
+
+ >>> f.readline()
+ 'This is the first line of the file.\n'
+ >>> f.readline()
+ 'Second line of the file\n'
+ >>> f.readline()
+ ''
+
+``f.readlines()`` returns a list containing all the lines of data in the file.
+If given an optional parameter *sizehint*, it reads that many bytes from the
+file and enough more to complete a line, and returns the lines from that. This
+is often used to allow efficient reading of a large file by lines, but without
+having to load the entire file in memory. Only complete lines will be returned.
+::
+
+ >>> f.readlines()
+ ['This is the first line of the file.\n', 'Second line of the file\n']
+
+An alternative approach to reading lines is to loop over the file object. This is
+memory efficient, fast, and leads to simpler code::
+
+ >>> for line in f:
+ print line,
+
+ This is the first line of the file.
+ Second line of the file
+
+The alternative approach is simpler but does not provide as fine-grained
+control. Since the two approaches manage line buffering differently, they
+should not be mixed.
+
+``f.write(string)`` writes the contents of *string* to the file, returning
+``None``. ::
+
+ >>> f.write('This is a test\n')
+
+To write something other than a string, it needs to be converted to a string
+first::
+
+ >>> value = ('the answer', 42)
+ >>> s = str(value)
+ >>> f.write(s)
+
+``f.tell()`` returns an integer giving the file object's current position in the
+file, measured in bytes from the beginning of the file. To change the file
+object's position, use ``f.seek(offset, from_what)``. The position is computed
+from adding *offset* to a reference point; the reference point is selected by
+the *from_what* argument. A *from_what* value of 0 measures from the beginning
+of the file, 1 uses the current file position, and 2 uses the end of the file as
+the reference point. *from_what* can be omitted and defaults to 0, using the
+beginning of the file as the reference point. ::
+
+ >>> f = open('/tmp/workfile', 'r+')
+ >>> f.write('0123456789abcdef')
+ >>> f.seek(5) # Go to the 6th byte in the file
+ >>> f.read(1)
+ '5'
+ >>> f.seek(-3, 2) # Go to the 3rd byte before the end
+ >>> f.read(1)
+ 'd'
+
+When you're done with a file, call ``f.close()`` to close it and free up any
+system resources taken up by the open file. After calling ``f.close()``,
+attempts to use the file object will automatically fail. ::
+
+ >>> f.close()
+ >>> f.read()
+ Traceback (most recent call last):
+ File "<stdin>", line 1, in ?
+ ValueError: I/O operation on closed file
+
+It is good practice to use the :keyword:`with` keyword when dealing with file
+objects. This has the advantage that the file is properly closed after its
+suite finishes, even if an exception is raised on the way. It is also much
+shorter than writing equivalent :keyword:`try`\ -\ :keyword:`finally` blocks::
+
+ >>> with open('/tmp/workfile', 'r') as f:
+ ... read_data = f.read()
+ >>> f.closed
+ True
+
+File objects have some additional methods, such as :meth:`isatty` and
+:meth:`truncate` which are less frequently used; consult the Library Reference
+for a complete guide to file objects.
+
+
+.. _tut-pickle:
+
+The :mod:`pickle` Module
+------------------------
+
+.. index:: module: pickle
+
+Strings can easily be written to and read from a file. Numbers take a bit more
+effort, since the :meth:`read` method only returns strings, which will have to
+be passed to a function like :func:`int`, which takes a string like ``'123'``
+and returns its numeric value 123. However, when you want to save more complex
+data types like lists, dictionaries, or class instances, things get a lot more
+complicated.
+
+Rather than have users be constantly writing and debugging code to save
+complicated data types, Python provides a standard module called :mod:`pickle`.
+This is an amazing module that can take almost any Python object (even some
+forms of Python code!), and convert it to a string representation; this process
+is called :dfn:`pickling`. Reconstructing the object from the string
+representation is called :dfn:`unpickling`. Between pickling and unpickling,
+the string representing the object may have been stored in a file or data, or
+sent over a network connection to some distant machine.
+
+If you have an object ``x``, and a file object ``f`` that's been opened for
+writing, the simplest way to pickle the object takes only one line of code::
+
+ pickle.dump(x, f)
+
+To unpickle the object again, if ``f`` is a file object which has been opened
+for reading::
+
+ x = pickle.load(f)
+
+(There are other variants of this, used when pickling many objects or when you
+don't want to write the pickled data to a file; consult the complete
+documentation for :mod:`pickle` in the Python Library Reference.)
+
+:mod:`pickle` is the standard way to make Python objects which can be stored and
+reused by other programs or by a future invocation of the same program; the
+technical term for this is a :dfn:`persistent` object. Because :mod:`pickle` is
+so widely used, many authors who write Python extensions take care to ensure
+that new data types such as matrices can be properly pickled and unpickled.
+
+