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Diffstat (limited to 'Doc/tutorial/inputoutput.rst')
| -rw-r--r-- | Doc/tutorial/inputoutput.rst | 90 |
1 files changed, 68 insertions, 22 deletions
diff --git a/Doc/tutorial/inputoutput.rst b/Doc/tutorial/inputoutput.rst index f9eec97..fe33d12 100644 --- a/Doc/tutorial/inputoutput.rst +++ b/Doc/tutorial/inputoutput.rst @@ -27,16 +27,13 @@ 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. +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. 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. +:func:`repr`, but they are no longer used in modern Python code and are removed +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 @@ -94,7 +91,7 @@ Here are two ways to write a table of squares and cubes:: 10 100 1000 >>> for x in range(1,11): - ... print '%2d %3d %4d' % (x, x*x, x*x*x) + ... print '{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x) ... 1 1 1 2 4 8 @@ -129,42 +126,91 @@ with zeros. It understands about plus and minus signs:: >>> '3.14159265359'.zfill(5) '3.14159265359' -Using the ``%`` operator looks like this:: +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 %5.3f.' % math.pi + >>> print 'The value of PI is approximately {0:.3f}.'.format(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:: +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 '%-10s ==> %10d' % (name, phone) + ... print '{0:10} ==> {1:10d}'.format(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:: +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' % table + >>> 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: |