Python Standard Library

The base64 Module

The base64 encoding scheme is used to convert arbitrary binary data to plain text. To do this, the encoder stores each group of three binary bytes as a group of four characters from the following set:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz
0123456789+/

In addition, the = character is used for padding at the end of the data stream.

Example 4-18 shows how the encode and decode functions work on file objects.

Example 4-18. Using the base64 Module to Encode Files

File: base64-example-1.py

import base64

MESSAGE = "life of brian"

file = open("out.txt", "w")
file.write(MESSAGE)
file.close()

base64.encode(open("out.txt"), open("out.b64", "w"))
base64.decode(open("out.b64"), open("out.txt", "w"))

print "original:", repr(MESSAGE)
print "encoded message:", repr(open("out.b64").read())
print "decoded message:", repr(open("out.txt").read())

original:  'life of brian'
encoded message: 'bGlmZSBvZiBicmlhbg==\012'
decoded message: 'life of brian'

Example 4-19 shows the encodestring and decodestring functions converting between strings. The functions are currently implemented as wrappers on top of encode and decode, using StringIO objects for input and output.

Example 4-19. Using the base64 Module to Encode Strings

File: base64-example-2.py

import base64

MESSAGE = "life of brian"

data = base64.encodestring(MESSAGE)

original_data = base64.decodestring(data)

print "original:", repr(MESSAGE)
print "encoded data:", repr(data)
print "decoded data:", repr(original_data)

original: 'life of brian'
encoded data: 'bGlmZSBvZiBicmlhbg==\012'
decoded data: 'life of brian'

Example 4-20 shows how to convert a username and a password to an HTTP basic authentication string. (Note that you don’t really have to work for the NSA to be able to decode this format.)

Example 4-20. Using the base64 Module for Basic Authentication

File: base64-example-3.py

import base64

def getbasic(user, password):
    # basic authentication (according to HTTP)
    return base64.encodestring(user + ":" + password)    

print getbasic("Aladdin", "open sesame")

'QWxhZGRpbjpvcGVuIHNlc2FtZQ=='

Finally, Example 4-21 shows a small utility that converts a GIF image to a Python script, for use with the Tkinter library.

Example 4-21. Using the base64 Module to Wrap GIF Images for Tkinter

File: base64-example-4.py

import base64, sys

if not sys.argv[1:]:
    print "Usage: gif2tk.py giffile >pyfile"
    sys.exit(1)

data = open(sys.argv[1], "rb").read()

if data[:4] != "GIF8":
    print sys.argv[1], "is not a GIF file"
    sys.exit(1)

print '# generated from', sys.argv[1], 'by gif2tk.py'
print
print 'from Tkinter import PhotoImage' 
print
print 'image = PhotoImage(data="""'
print base64.encodestring(data),
print '""")'

# generated from samples/sample.gif by gif2tk.py

from Tkinter import PhotoImage

image = PhotoImage(data="""
R0lGODlhoAB4APcAAAAAAIAAAACAAICAAAAAgIAAgACAgICAgAQEBIwEBIyMBJRUlISE/LRUBAQE
...
AjmQBFmQBnmQCJmQCrmQDNmQDvmQEBmREnkRAQEAOw==
""")