2.2.8 File Objects
File objects are implemented using C's
package and can be created with the built-in constructor
file() described in section
2.1, ``Built-in Functions.''2.10They are also returned
by some other built-in functions and methods, such as
os.popen() and os.fdopen() and the
makefile() method of socket objects.
When a file operation fails for an I/O-related reason, the exception
IOError is raised. This includes situations where the
operation is not defined for some reason, like seek() on a tty
device or writing a file opened for reading.
Files have the following methods:
Close the file. A closed file cannot be read or written anymore.
Any operation which requires that the file be open will raise a
ValueError after the file has been closed. Calling
close() more than once is allowed.
Flush the internal buffer, like
fflush(). This may be a no-op on some file-like
Return true if the file is connected to a tty(-like) device, else
If a file-like object is not associated
with a real file, this method should not be implemented.
Return the integer ``file descriptor'' that is used by the
underlying implementation to request I/O operations from the
operating system. This can be useful for other, lower level
interfaces that use file descriptors, such as the
fcntl module or
os.read() and friends. Note:
which do not have a real file descriptor should not provide
Read at most size bytes from the file (less if the read hits
EOF before obtaining size bytes). If the size
argument is negative or omitted, read all data until EOF is
reached. The bytes are returned as a string object. An empty
string is returned when EOF is encountered immediately. (For
certain files, like ttys, it makes sense to continue reading after
an EOF is hit.) Note that this method may call the underlying
C function fread() more than once in an effort to
acquire as close to size bytes as possible.
Read one entire line from the file. A trailing newline character is
kept in the string2.11 (but may be absent when a file ends with an
incomplete line). If the size argument is present and
non-negative, it is a maximum byte count (including the trailing
newline) and an incomplete line may be returned.
An empty string is returned when EOF is hit
stdio's fgets(), the
returned string contains null characters (
'\0') if they
occurred in the input.
Read until EOF using readline() and return a list containing
the lines thus read. If the optional sizehint argument is
present, instead of reading up to EOF, whole lines totalling
approximately sizehint bytes (possibly after rounding up to an
internal buffer size) are read. Objects implementing a file-like
interface may choose to ignore sizehint if it cannot be
implemented, or cannot be implemented efficiently.
xreadlines.xreadlines(file). (See the xreadlines module for more information.)
New in version 2.1.
- seek(offset[, whence])
Set the file's current position, like
The whence argument is optional and defaults to
(absolute file positioning); other values are
relative to the current position) and
2 (seek relative to the
file's end). There is no return value. Note that if the file is
opened for appending (mode
seek() operations will be undone at the next write. If the
file is only opened for writing in append mode (mode
this method is essentially a no-op, but it remains useful for files
opened in append mode with reading enabled (mode
Return the file's current position, like
Truncate the file's size. If the optional size argument
present, the file is truncated to (at most) that size. The size
defaults to the current position. Availability of this function
depends on the operating system version (for example, not all
Unix versions support this operation).
Write a string to the file. There is no return value. Due to
buffering, the string may not actually show up in the file until
the flush() or close() method is called.
Write a sequence of strings to the file. The sequence can be any
iterable object producing strings, typically a list of strings.
There is no return value.
(The name is intended to match readlines();
writelines() does not add line separators.)
Files support the iterator protocol. Each iteration returns the same
file.readline(), and iteration ends when the
readline() method returns an empty string.
File objects also offer a number of other interesting attributes.
These are not required for file-like objects, but should be
implemented if they make sense for the particular object.
Boolean indicating the current state of the file object. This is a
read-only attribute; the close() method changes the value.
It may not be available on all file-like objects.
The I/O mode for the file. If the file was created using the
open() built-in function, this will be the value of the
mode parameter. This is a read-only attribute and may not be
present on all file-like objects.
If the file object was created using open(), the name of
the file. Otherwise, some string that indicates the source of the
file object, of the form "<...>". This is a read-only
attribute and may not be present on all file-like objects.
Boolean that indicates whether a space character needs to be printed
before another value when using the print statement.
Classes that are trying to simulate a file object should also have a
writable softspace attribute, which should be initialized to
zero. This will be automatic for most classes implemented in Python
(care may be needed for objects that override attribute access); types
implemented in C will have to provide a writable
This attribute is not used to control the
print statement, but to allow the implementation of
print to keep track of its internal state.
See About this document... for information on suggesting changes.
- ... Functions.''2.10
is new in Python 2.2. The older built-in open() is an
alias for file().
- ... string2.11
The advantage of leaving the newline on is that an empty string
can be returned to mean EOF without being ambiguous. Another
advantage is that (in cases where it might matter, for example. if you
want to make an exact copy of a file while scanning its lines)
you can tell whether the last line of a file ended in a newline
or not (yes this happens!).