So far, we have worked with the objects in Primary Memory. However Primary Memory is volatile. In order to save the current state of program, objects for future use, we have to save it in Secondary Memory. It is achieved via file handling.
open() returns a file object, and is most commonly used with two
mode is a string that
determines how the file should be opened. Normally, files are opened in
text mode, that means, you read and write strings from and to the file,
which are encoded in a specific encoding. If encoding is not specified,
the default is platform dependent (see open()). ‘b’ appended to the mode
opens the file in binary mode: now the data is read and written in the
form of bytes objects. This mode should be used for all files that don’t
r+- Read and Write, similarly
If no mode is specified, it is defaulted to
Normally, files are opened in text mode, that means, you read and
write strings from and to the file, which are encoded in a specific
encoding. If encoding is not specified, the default is platform
'b' appended to the mode opens the file in binary mode:
now the data is read and written in the form of bytes objects. This mode
should be used for all files that don’t contain text.
It is good practice to use the
with keyword when dealing with file
objects. The advantage is that the file is properly closed after its
suite finishes, even if an exception is raised at some point. Using with
is also much shorter than writing equivalent try-finally blocks:
>>> with open('workfile') as f: ... read_data = f.read() >>> f.closed True
If you’re not using the
with keyword, then you should call
f.close() to close the file and immediately free up any system
resources used by it. If you don’t explicitly close a file, Python’s
garbage collector will eventually destroy the object and close the open
file for you, but the file may stay open for a while. Another risk is
that different Python implementations will do this clean-up at different
After a file object is closed, either by a with statement or by 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 <module> ValueError: I/O operation on closed file
Methods of File Objects¶
The rest of the examples in this section will assume that a file object
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 (in text mode) or bytes
object (in binary mode).
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,
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() ''
For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:
>>> >>> for line in f: ... print(line, end='') ... This is the first line of the file. Second line of the file
If you want to read all the lines of a file in a list you can also use
f.write(string) writes the contents of string to the file, returning
the number of characters written.
>>> >>> f.write('This is a test\n') 15
Other types of objects need to be converted – either to a string (in text mode) or a bytes object (in binary mode) – before writing them:
>>> >>> value = ('the answer', 42) >>> s = str(value) # convert the tuple to string >>> f.write(s) 18
f.tell() returns an integer giving the file object’s current
position in the file represented as number of bytes from the beginning
of the file when in binary mode and an opaque number when in text mode.
To change the file object’s position, use
The position is computed from adding offset to a reference point; the
reference point is selected by the
from_what argument. A from_what
0 measures from the beginning of the file,
1 uses the
current file position, and
2 uses the end of the file as the
from_what can be omitted and defaults to 0, using
the beginning of the file as the reference point.
>>> >>> f = open('workfile', 'rb+') >>> f.write(b'0123456789abcdef') 16 >>> f.seek(5) # Go to the 6th byte in the file 5 >>> f.read(1) b'5' >>> f.seek(-3, 2) # Go to the 3rd byte before the end 13 >>> f.read(1) b'd'
In text files (those opened without a
b in the mode string), only
seeks relative to the beginning of the file are allowed (the exception
being seeking to the very file end with
seek(0, 2)) and the only
valid offset values are those returned from the
f.tell(), or zero.
Any other offset value produces undefined behaviour.
File objects have some additional methods, such as
truncate() which are less frequently used; consult the Library
Reference for a complete guide to file objects.
Note : This chapter is copied from Python Reference