| .. _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.) |
| |
| 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 |
| string types have some methods that perform 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. |
| |
| The :mod:`string` module contains a :class:`~string.Template` class which offers |
| yet another way to substitute values into strings. |
| |
| 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 no 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(1.0/7.0) |
| '0.142857142857' |
| >>> repr(1.0/7.0) |
| '0.14285714285714285' |
| >>> 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: by default it adds spaces between its arguments.) |
| |
| This example demonstrates the :meth:`str.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:`str.ljust` and |
| :meth:`str.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:`str.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 {} who say "{}!"'.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 :meth:`str.format` method. A number in the |
| brackets refers to the position of the object passed into the |
| :meth:`str.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 :meth:`str.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. |
| |
| ``'!s'`` (apply :func:`str`) and ``'!r'`` (apply :func:`repr`) can be used to |
| convert the value before it is formatted. :: |
| |
| >>> import math |
| >>> print 'The value of PI is approximately {}.'.format(math.pi) |
| The value of PI is approximately 3.14159265359. |
| >>> print 'The value of PI is approximately {!r}.'.format(math.pi) |
| The value of PI is approximately 3.141592653589793. |
| |
| An optional ``':'`` and format specifier can follow the field name. This allows |
| greater control over how the value is formatted. The following example |
| rounds 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 built-in function |
| :func:`vars`, which returns a dictionary containing all local variables. |
| |
| For a complete overview of string formatting 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 :c:func:`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. |
| |
| 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('workfile', 'w') |
| >>> print f |
| <open file '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'``. Python on 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() |
| '' |
| |
| For reading lines from a file, you can loop over the file object. This is memory |
| efficient, fast, and leads to simple code:: |
| |
| >>> for line in f: |
| print line, |
| |
| This is the first line of the file. |
| Second line of the file |
| |
| If you want to read all the lines of a file in a list you can also use |
| ``list(f)`` or ``f.readlines()``. |
| |
| ``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('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 <module> |
| 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('workfile', 'r') as f: |
| ... read_data = f.read() |
| >>> f.closed |
| True |
| |
| File objects have some additional methods, such as :meth:`~file.isatty` and |
| :meth:`~file.truncate` which are less frequently used; consult the Library |
| Reference for a complete guide to file objects. |
| |
| |
| .. _tut-json: |
| |
| Saving structured data with :mod:`json` |
| --------------------------------------- |
| |
| .. index:: module: json |
| |
| 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. When you want to save more complex data |
| types like nested lists and dictionaries, parsing and serializing by hand |
| becomes complicated. |
| |
| Rather than having users constantly writing and debugging code to save |
| complicated data types to files, Python allows you to use the popular data |
| interchange format called `JSON (JavaScript Object Notation) |
| <http://json.org>`_. The standard module called :mod:`json` can take Python |
| data hierarchies, and convert them to string representations; this process is |
| called :dfn:`serializing`. Reconstructing the data from the string representation |
| is called :dfn:`deserializing`. Between serializing and deserializing, the |
| string representing the object may have been stored in a file or data, or |
| sent over a network connection to some distant machine. |
| |
| .. note:: |
| The JSON format is commonly used by modern applications to allow for data |
| exchange. Many programmers are already familiar with it, which makes |
| it a good choice for interoperability. |
| |
| If you have an object ``x``, you can view its JSON string representation with a |
| simple line of code:: |
| |
| >>> import json |
| >>> json.dumps([1, 'simple', 'list']) |
| '[1, "simple", "list"]' |
| |
| Another variant of the :func:`~json.dumps` function, called :func:`~json.dump`, |
| simply serializes the object to a file. So if ``f`` is a :term:`file object` |
| opened for writing, we can do this:: |
| |
| json.dump(x, f) |
| |
| To decode the object again, if ``f`` is a :term:`file object` which has |
| been opened for reading:: |
| |
| x = json.load(f) |
| |
| This simple serialization technique can handle lists and dictionaries, but |
| serializing arbitrary class instances in JSON requires a bit of extra effort. |
| The reference for the :mod:`json` module contains an explanation of this. |
| |
| .. seealso:: |
| |
| :mod:`pickle` - the pickle module |
| |
| Contrary to :ref:`JSON <tut-json>`, *pickle* is a protocol which allows |
| the serialization of arbitrarily complex Python objects. As such, it is |
| specific to Python and cannot be used to communicate with applications |
| written in other languages. It is also insecure by default: |
| deserializing pickle data coming from an untrusted source can execute |
| arbitrary code, if the data was crafted by a skilled attacker. |
| |
