|An Introduction to Python|
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
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7.2.1 Methods of File Objects
The rest of the examples in this section will assume that a file
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
>>> f.read() 'This is the entire file.\n' >>> f.read() "
f.readline() reads a single line from the file; a newline
\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
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
>>> 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
truncate() which are less frequently
used; consult the Python Library Reference Manual for a complete guide to file
|ISBN 0954161769||An Introduction to Python||See the print edition|