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Input and Output
A D V E R T I S E M E N T
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.
Fancier Output Formatting
So far we've encountered two ways of writing values: expression
statements and the print statement. (A third way
is using the 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 standard
module 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 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
repr() or str() functions.
Reverse quotes (``) are equivalent to repr(),
but they are no longer used in modern Python code and will likely not be in
future versions of the language.
The str() function is meant to return
representations of values which are fairly human-readable, while
repr() is meant to generate representations which can
be read by the interpreter (or will force a SyntaxError
if there is not equivalent syntax). For objects which don't have a particular
representation for human consumption, str() will
return the same value as 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 print works: it always adds spaces between its
arguments.)
This example demonstrates the 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
ljust() and 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, 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 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
vars() function, which returns a dictionary containing
all local variables.
Reading and Writing Files
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 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 JPEG or EXE files. Be
very careful to use binary mode when reading and writing such files.
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 isatty()
and truncate() which are less frequently used; consult
the Library Reference for a complete guide to file objects.
The pickle Module
Strings can easily be written to and read from a file. Numbers take a bit
more effort, since the read() method only returns
strings, which will have to be passed to a function like
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
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 pickling.
Reconstructing the object from the string representation is called
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:
To unpickle the object again, if f is a file object which has
been opened for reading:
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