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diff --git a/Doc/tutorial/inputoutput.rst b/Doc/tutorial/inputoutput.rst new file mode 100644 index 0000000..9c302af --- /dev/null +++ b/Doc/tutorial/inputoutput.rst @@ -0,0 +1,354 @@ +.. _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 ``%`` operator with a string as the left argument. The ``%`` +operator 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. + +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. Reverse quotes (``````) are equivalent to +:func:`repr`, but they are no longer used in modern Python code and will likely +not be in future versions of the language. + +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'))" + >>> # reverse quotes are convenient in interactive sessions: + ... `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 '%2d %3d %4d' % (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' + +Using the ``%`` operator looks like this:: + + >>> import math + >>> print 'The value of PI is approximately %5.3f.' % math.pi + The value of PI is approximately 3.142. + +If there is more than one format in the string, you need to pass a tuple as +right operand, as in this example:: + + >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678} + >>> for name, phone in table.items(): + ... print '%-10s ==> %10d' % (name, phone) + ... + Jack ==> 4098 + Dcab ==> 7678 + Sjoerd ==> 4127 + +Most formats work exactly as in C and require that you pass the proper type; +however, if you don't you get an exception, not a core dump. The ``%s`` format +is more relaxed: if the corresponding argument is not a string object, it is +converted to string using the :func:`str` built-in function. Using ``*`` to +pass the width or precision in as a separate (integer) argument is supported. +The C formats ``%n`` and ``%p`` are not supported. + +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 using form ``%(name)format``, as +shown here:: + + >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678} + >>> print 'Jack: %(Jack)d; Sjoerd: %(Sjoerd)d; Dcab: %(Dcab)d' % 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. + + +.. _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)``. + +.. % Opening files + +:: + + >>> 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 and the Macintosh, ``'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. + + +.. _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 alternate 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 + +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. + + |