mirror of
https://github.com/python/cpython.git
synced 2025-11-04 07:31:38 +00:00
354 lines
13 KiB
ReStructuredText
354 lines
13 KiB
ReStructuredText
.. _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 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
|
|
|
|
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.
|
|
|
|
|