2.1.7.9 File Objects

File objects are implemented using C's stdiopackage and can be created with the built-in function open() described in section 2.3, ``Built-in Functions.'' They are also returned by some other built-in functions and methods, e.g., posix.popen() and posix.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 (): Close the file. A closed file cannot be read or written anymore.

flush (): Flush the internal buffer, like stdio's fflush().

isatty (): Return 1 if the file is connected to a tty(-like) device, else 0.

fileno (): 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, e.g. module fcntl or os.read() and friends.

read ([size]): 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.

readline ([size]): Read one entire line from the file. A trailing newline character is kept in the string^2.7 (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 immediately. Note: Unlike stdio's fgets(), the returned string contains null characters ('\0') if they occurred in the input.

readlines ([sizehint]): 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.

seek (offset[, whence]): Set the file's current position, like stdio's fseek(). The whence argument is optional and defaults to 0 (absolute file positioning); other values are 1 (seek relative to the current position) and 2 (seek relative to the file's end). There is no return value.

tell (): Return the file's current position, like stdio's ftell().

truncate ([size]): 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 (str): Write a string to the file. There is no return value. Note: Due to buffering, the string may not actually show up in the file until the flush() or close() method is called.

writelines (list): Write a list of strings to the file. There is no return value. (The name is intended to match readlines(); writelines() does not add line separators.)

File objects also offer the following attributes:

closed: Boolean indicating the current state of the file object. This is a read-only attribute; the close() method changes the value.

mode: 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.

name: 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.

softspace: 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 classes implemented in Python; types implemented in C will have to provide a writable softspace attribute.

Footnotes

... string ^2.7: 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, e.g. 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!).

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