File Objects

File objects are implemented using C's stdio package and can be created with the built-in function open() described under Built-in Functions below. 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.

read([size])

Read at most size bytes from the file (less if the read hits EOF or no more data is immediately available on a pipe, tty or similar device). 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.)

readline([size])

Read one entire line from the file. A trailing newline character is kept in the string (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()

Read until EOF using readline() and return a list containing the lines thus 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 (e.g., 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.)