Unicode Primer¶

CPython 2.x supports two types of strings for working with text data. Old-style str instances use a single 8-bit byte to represent each character of the string using its ASCII code. In contrast, unicode strings are managed internally as a sequence of Unicode code points. The code point values are saved as a sequence of 2 or 4 bytes each, depending on the options given when Python was compiled. Both unicode and str are derived from a common base class, and support a similar API.

When unicode strings are output, they are encoded using one of several standard schemes so that the sequence of bytes can be reconstructed as the same string later. The bytes of the encoded value are not necessarily the same as the code point values, and the encoding defines a way to translate between the two sets of values. Reading Unicode data also requires knowing the encoding so that the incoming bytes can be converted to the internal representation used by the unicode class.

The most common encodings for Western languages are UTF-8 and UTF-16, which use sequences of one and two byte values respectively to represent each character. Other encodings can be more efficient for storing languages where most of the characters are represented by code points that do not fit into two bytes.

import binascii

def to_hex(t, nbytes):
    "Format text t as a sequence of nbyte long values separated by spaces."
    chars_per_item = nbytes * 2
    hex_version = binascii.hexlify(t)
    num_chunks = len(hex_version) / chars_per_item
    def chunkify():
        for start in xrange(0, len(hex_version), chars_per_item):
            yield hex_version[start:start + chars_per_item]
    return ' '.join(chunkify())

if __name__ == '__main__':
    print to_hex('abcdef', 1)
    print to_hex('abcdef', 2)
    

$ python codecs_to_hex.py
61 62 63 64 65 66
6162 6364 6566

from codecs_to_hex import to_hex

text = u'pi: π'

print 'Raw   :', repr(text)
print 'UTF-8 :', to_hex(text.encode('utf-8'), 1)
print 'UTF-16:', to_hex(text.encode('utf-16'), 2)

$ python codecs_encodings.py
Raw   : u'pi: \u03c0'
UTF-8 : 70 69 3a 20 cf 80
UTF-16: fffe 7000 6900 3a00 2000 c003

from codecs_to_hex import to_hex

text = u'pi: π'
encoded = text.encode('utf-8')
decoded = encoded.decode('utf-8')

print 'Original :', repr(text)
print 'Encoded  :', to_hex(encoded, 1), type(encoded)
print 'Decoded  :', repr(decoded), type(decoded)

$ python codecs_decode.py
Original : u'pi: \u03c0'
Encoded  : 70 69 3a 20 cf 80 <type 'str'>
Decoded  : u'pi: \u03c0' <type 'unicode'>

Working with Files¶

Encoding and decoding strings is especially important when dealing with I/O operations. Whether you are writing to a file, socket, or other stream, you will want to ensure that the data is using the proper encoding. In general, all text data needs to be decoded from its byte representation as it is read, and encoded from the internal values to a specific representation as it is written. Your program can explicitly encode and decode data, but depending on the encoding used it can be non-trivial to determine whether you have read enough bytes in order to fully decode the data. codecs provides classes that manage the data encoding and decoding for you, so you don’t have to create your own.

The simplest interface provided by codecs is a replacement for the built-in open() function. The new version works just like the built-in, but adds two new arguments to specify the encoding and desired error handling technique.

from codecs_to_hex import to_hex

import codecs
import sys

encoding = sys.argv[1]
filename = encoding + '.txt'

print 'Writing to', filename
with codecs.open(filename, mode='wt', encoding=encoding) as f:
    f.write(u'pi: \u03c0')

# Determine the byte grouping to use for to_hex()
nbytes = { 'utf-8':1,
           'utf-16':2,
           'utf-32':4,
           }.get(encoding, 1) 

# Show the raw bytes in the file
print 'File contents:'
with open(filename, mode='rt') as f:
    print to_hex(f.read(), nbytes)

Starting with a unicode string with the code point for π, this example saves the text to a file using an encoding specified on the command line.

$ python codecs_open_write.py utf-8
Writing to utf-8.txt
File contents:
70 69 3a 20 cf 80

$ python codecs_open_write.py utf-16
Writing to utf-16.txt
File contents:
fffe 7000 6900 3a00 2000 c003

$ python codecs_open_write.py utf-32
Writing to utf-32.txt
File contents:
fffe0000 70000000 69000000 3a000000 20000000 c0030000

Reading the data with open() is straightforward, with one catch: you must know the encoding in advance, in order to set up the decoder correctly. Some data formats, such as XML, let you specify the encoding as part of the file, but usually it is up to the application to manage. codecs simply takes the encoding as an argument and assumes it is correct.

import codecs
import sys

encoding = sys.argv[1]
filename = encoding + '.txt'

print 'Reading from', filename
with codecs.open(filename, mode='rt', encoding=encoding) as f:
    print repr(f.read())

This example reads the files created by the previous program, and prints the representation of the resulting unicode object to the console.

$ python codecs_open_read.py utf-8
Reading from utf-8.txt
u'pi: \u03c0'

$ python codecs_open_read.py utf-16
Reading from utf-16.txt
u'pi: \u03c0'

$ python codecs_open_read.py utf-32
Reading from utf-32.txt
u'pi: \u03c0'

Byte Order¶

Multi-byte encodings such as UTF-16 and UTF-32 pose a problem when transferring the data between different computer systems, either by copying the file directly or with network communication. Different systems use different ordering of the high and low order bytes. This characteristic of the data, known as its endianness, depends on factors such as the hardware architecture and choices made by the operating system and application developer. There isn’t always a way to know in advance what byte order to use for a given set of data, so the multi-byte encodings include a byte-order marker (BOM) as the first few bytes of encoded output. For example, UTF-16 is defined in such a way that 0xFFFE and 0xFEFF are not valid characters, and can be used to indicate the byte order. codecs defines constants for the byte order markers used by UTF-16 and UTF-32.

import codecs
from codecs_to_hex import to_hex

for name in [ 'BOM', 'BOM_BE', 'BOM_LE',
              'BOM_UTF8',
              'BOM_UTF16', 'BOM_UTF16_BE', 'BOM_UTF16_LE',
              'BOM_UTF32', 'BOM_UTF32_BE', 'BOM_UTF32_LE',
              ]:
    print '{:12} : {}'.format(name, to_hex(getattr(codecs, name), 2))

BOM, BOM_UTF16, and BOM_UTF32 are automatically set to the appropriate big-endian or little-endian values depending on the current system’s native byte order.

$ python codecs_bom.py
BOM          : fffe
BOM_BE       : feff
BOM_LE       : fffe
BOM_UTF8     : efbb bf
BOM_UTF16    : fffe
BOM_UTF16_BE : feff
BOM_UTF16_LE : fffe
BOM_UTF32    : fffe 0000
BOM_UTF32_BE : 0000 feff
BOM_UTF32_LE : fffe 0000

Byte ordering is detected and handled automatically by the decoders in codecs, but you can also choose an explicit ordering for the encoding.

import codecs
from codecs_to_hex import to_hex

# Pick the non-native version of UTF-16 encoding
if codecs.BOM_UTF16 == codecs.BOM_UTF16_BE:
    bom = codecs.BOM_UTF16_LE
    encoding = 'utf_16_le'
else:
    bom = codecs.BOM_UTF16_BE
    encoding = 'utf_16_be'

print 'Native order  :', to_hex(codecs.BOM_UTF16, 2)
print 'Selected order:', to_hex(bom, 2)

# Encode the text.
encoded_text = u'pi: \u03c0'.encode(encoding)
print '{:14}: {}'.format(encoding, to_hex(encoded_text, 2))

with open('non-native-encoded.txt', mode='wb') as f:
    # Write the selected byte-order marker.  It is not included in the
    # encoded text because we were explicit about the byte order when
    # selecting the encoding.
    f.write(bom)
    # Write the byte string for the encoded text.
    f.write(encoded_text)

codecs_bom_create_file.py figures out the native byte ordering, then uses the alternate form explicitly so the next example can demonstrate auto-detection while reading.

$ python codecs_bom_create_file.py
Native order  : fffe
Selected order: feff
utf_16_be     : 0070 0069 003a 0020 03c0

codecs_bom_detection.py does not specify a byte order when opening the file, so the decoder uses the BOM value in the first two bytes of the file to determine it.

import codecs
from codecs_to_hex import to_hex

# Look at the raw data
with open('non-native-encoded.txt', mode='rb') as f:
    raw_bytes = f.read()

print 'Raw    :', to_hex(raw_bytes, 2)

# Re-open the file and let codecs detect the BOM
with codecs.open('non-native-encoded.txt', mode='rt', encoding='utf-16') as f:
    decoded_text = f.read()

print 'Decoded:', repr(decoded_text)

Since the first two bytes of the file are used for byte order detection, they are not included in the data returned by read().

$ python codecs_bom_detection.py
Raw    : feff 0070 0069 003a 0020 03c0
Decoded: u'pi: \u03c0'

Error Handling¶

The previous sections pointed out the need to know the encoding being used when reading and writing Unicode files. Setting the encoding correctly is important for two reasons. If the encoding is configured incorrectly while reading from a file, the data will be interpreted wrong and may be corrupted or simply fail to decode. Not all Unicode characters can be represented in all encodings, so if the wrong encoding is used while writing an error will be generated and data may be lost.

codecs uses the same five error handling options that are provided by the encode() method of unicode and the decode() method of str.

import codecs
import sys

error_handling = sys.argv[1]

text = u'pi: \u03c0'

try:
    # Save the data, encoded as ASCII, using the error
    # handling mode specified on the command line.
    with codecs.open('encode_error.txt', 'w',
                     encoding='ascii',
                     errors=error_handling) as f:
        f.write(text)
        
except UnicodeEncodeError, err:
    print 'ERROR:', err
    
else:
    # If there was no error writing to the file,
    # show what it contains.
    with open('encode_error.txt', 'rb') as f:
        print 'File contents:', repr(f.read())

$ python codecs_encode_error.py strict
ERROR: 'ascii' codec can't encode character u'\u03c0' in position 4: ordinal not in range(128)

$ python codecs_encode_error.py replace
File contents: 'pi: ?'

$ python codecs_encode_error.py ignore
File contents: 'pi: '

$ python codecs_encode_error.py xmlcharrefreplace
File contents: 'pi: π'

$ python codecs_encode_error.py backslashreplace
File contents: 'pi: \\u03c0'

import codecs
import sys

from codecs_to_hex import to_hex

error_handling = sys.argv[1]

text = u'pi: \u03c0'
print 'Original     :', repr(text)

# Save the data with one encoding
with codecs.open('decode_error.txt', 'w', encoding='utf-16') as f:
    f.write(text)

# Dump the bytes from the file
with open('decode_error.txt', 'rb') as f:
    print 'File contents:', to_hex(f.read(), 1)

# Try to read the data with the wrong encoding
with codecs.open('decode_error.txt', 'r',
                 encoding='utf-8',
                 errors=error_handling) as f:
    try:
        data = f.read()
    except UnicodeDecodeError, err:
        print 'ERROR:', err
    else:
        print 'Read         :', repr(data)

$ python codecs_decode_error.py strict
Original     : u'pi: \u03c0'
File contents: ff fe 70 00 69 00 3a 00 20 00 c0 03
ERROR: 'utf8' codec can't decode byte 0xff in position 0: invalid start byte

$ python codecs_decode_error.py ignore
Original     : u'pi: \u03c0'
File contents: ff fe 70 00 69 00 3a 00 20 00 c0 03
Read         : u'p\x00i\x00:\x00 \x00\x03'

$ python codecs_decode_error.py replace
Original     : u'pi: \u03c0'
File contents: ff fe 70 00 69 00 3a 00 20 00 c0 03
Read         : u'\ufffd\ufffdp\x00i\x00:\x00 \x00\ufffd\x03'

Standard Input and Output Streams¶

The most common cause of UnicodeEncodeError exceptions is code that tries to print unicode data to the console or a Unix pipeline when sys.stdout is not configured with an encoding.

import codecs
import sys

text = u'pi: π'

# Printing to stdout may cause an encoding error
print 'Default encoding:', sys.stdout.encoding
print 'TTY:', sys.stdout.isatty()
print text

Problems with the default encoding of the standard I/O channels can be difficult to debug because the program works as expected when the output goes to the console, but cause encoding errors when it is used as part of a pipeline and the output includes Unicode characters above the ASCII range. This difference in behavior is caused by Python’s initialization code, which sets the default encoding for each standard I/O channel only if the channel is connected to a terminal (isatty() returns True). If there is no terminal, Python assumes the program will configure the encoding explicitly, and leaves the I/O channel alone.

$ python codecs_stdout.py
Default encoding: utf-8
TTY: True
pi: π

$ python codecs_stdout.py | cat -
Default encoding: None
TTY: False
Traceback (most recent call last):
  File "codecs_stdout.py", line 18, in <module>
    print text
UnicodeEncodeError: 'ascii' codec can't encode character u'\u03c0' in
 position 4: ordinal not in range(128)

To explicitly set the encoding on the standard output channel, use getwriter() to get a stream encoder class for a specific encoding. Instantiate the class, passing sys.stdout as the only argument.

import codecs
import sys

text = u'pi: π'

# Wrap sys.stdout with a writer that knows how to handle encoding
# Unicode data.
wrapped_stdout = codecs.getwriter('UTF-8')(sys.stdout)
wrapped_stdout.write(u'Via write: ' + text + '\n')

# Replace sys.stdout with a writer
sys.stdout = wrapped_stdout

print u'Via print:', text

Writing to the wrapped version of sys.stdout passes the Unicode text through an encoder before sending the encoded bytes to stdout. Replacing sys.stdout means that any code used by your application that prints to standard output will be able to take advantage of the encoding writer.

$ python codecs_stdout_wrapped.py
Via write: pi: π
Via print: pi: π

The next problem to solve is how to know which encoding should be used. The proper encoding varies based on location, language, and user or system configuration, so hard-coding a fixed value is not a good idea. It would also be annoying for a user to need to pass explicit arguments to every program setting the input and output encodings. Fortunately, there is a global way to get a reasonable default encoding, using locale.

import codecs
import locale
import sys

text = u'pi: π'

# Configure locale from the user's environment settings.
locale.setlocale(locale.LC_ALL, '')

# Wrap stdout with an encoding-aware writer.
lang, encoding = locale.getdefaultlocale()
print 'Locale encoding    :', encoding
sys.stdout = codecs.getwriter(encoding)(sys.stdout)

print 'With wrapped stdout:', text

getdefaultlocale() returns the language and preferred encoding based on the system and user configuration settings in a form that can be used with getwriter().

$ python codecs_stdout_locale.py
Locale encoding    : UTF-8
With wrapped stdout: pi: π

The encoding also needs to be set up when working with sys.stdin. Use getreader() to get a reader capable of decoding the input bytes.

import codecs
import locale
import sys

# Configure locale from the user's environment settings.
locale.setlocale(locale.LC_ALL, '')

# Wrap stdin with an encoding-aware reader.
lang, encoding = locale.getdefaultlocale()
sys.stdin = codecs.getreader(encoding)(sys.stdin)

print 'From stdin:', repr(sys.stdin.read())

Reading from the wrapped handle returns unicode objects instead of str instances.

$ python codecs_stdout_locale.py | python codecs_stdin.py
From stdin: u'Locale encoding    : UTF-8\nWith wrapped stdout: pi: \u03c0\n'

Network Communication¶

Network sockets are also byte-streams, and so Unicode data must be encoded into bytes before it is written to a socket.

import sys
import SocketServer


class Echo(SocketServer.BaseRequestHandler):

    def handle(self):
        # Get some bytes and echo them back to the client.
        data = self.request.recv(1024)
        self.request.send(data)
        return


if __name__ == '__main__':
    import codecs
    import socket
    import threading

    address = ('localhost', 0) # let the kernel give us a port
    server = SocketServer.TCPServer(address, Echo)
    ip, port = server.server_address # find out what port we were given

    t = threading.Thread(target=server.serve_forever)
    t.setDaemon(True) # don't hang on exit
    t.start()

    # Connect to the server
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((ip, port))

    # Send the data
    text = u'pi: π'
    len_sent = s.send(text)

    # Receive a response
    response = s.recv(len_sent)
    print repr(response)

    # Clean up
    s.close()
    server.socket.close()

You could encode the data explicitly, before sending it, but miss one call to send() and your program would fail with an encoding error.

   $ python codecs_socket_fail.py
   Traceback (most recent call last):
     File "codecs_socket_fail.py", line 43, in <module>
       len_sent = s.send(text)
   UnicodeEncodeError: 'ascii' codec can't encode character u'\u03c0' in
position 4: ordinal not in range(128)

By using makefile() to get a file-like handle for the socket, and then wrapping that with a stream-based reader or writer, you will be able to pass Unicode strings and know they are encoded on the way in to and out of the socket.

import sys
import SocketServer


class Echo(SocketServer.BaseRequestHandler):

    def handle(self):
        # Get some bytes and echo them back to the client.  There is
        # no need to decode them, since they are not used.
        data = self.request.recv(1024)
        self.request.send(data)
        return


class PassThrough(object):

    def __init__(self, other):
        self.other = other

    def write(self, data):
        print 'Writing :', repr(data)
        return self.other.write(data)

    def read(self, size=-1):
        print 'Reading :',
        data = self.other.read(size)
        print repr(data)
        return data

    def flush(self):
        return self.other.flush()

    def close(self):
        return self.other.close()
    

if __name__ == '__main__':
    import codecs
    import socket
    import threading

    address = ('localhost', 0) # let the kernel give us a port
    server = SocketServer.TCPServer(address, Echo)
    ip, port = server.server_address # find out what port we were given

    t = threading.Thread(target=server.serve_forever)
    t.setDaemon(True) # don't hang on exit
    t.start()

    # Connect to the server
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((ip, port))

    # Wrap the socket with a reader and writer.
    incoming = codecs.getreader('utf-8')(PassThrough(s.makefile('r')))
    outgoing = codecs.getwriter('utf-8')(PassThrough(s.makefile('w')))

    # Send the data
    text = u'pi: π'
    print 'Sending :', repr(text)
    outgoing.write(text)
    outgoing.flush()

    # Receive a response
    response = incoming.read()
    print 'Received:', repr(response)

    # Clean up
    s.close()
    server.socket.close()

This example uses PassThrough to show that the data is encoded before being sent, and the response is decoded after it is received in the client.

$ python codecs_socket.py
Sending : u'pi: \u03c0'
Writing : 'pi: \xcf\x80'
Reading : 'pi: \xcf\x80'
Received: u'pi: \u03c0'

Encoding Translation¶

Although most applications will work with unicode data internally, decoding or encoding it as part of an I/O operation, there are times when changing a file’s encoding without holding on to that intermediate data format is useful. EncodedFile() takes an open file handle using one encoding and wraps it with a class that translates the data to another encoding as the I/O occurs.

from codecs_to_hex import to_hex

import codecs
from cStringIO import StringIO

# Raw version of the original data.
data = u'pi: \u03c0'

# Manually encode it as UTF-8.
utf8 = data.encode('utf-8')
print 'Start as UTF-8   :', to_hex(utf8, 1)

# Set up an output buffer, then wrap it as an EncodedFile.
output = StringIO()
encoded_file = codecs.EncodedFile(output, data_encoding='utf-8',
                                  file_encoding='utf-16')
encoded_file.write(utf8)

# Fetch the buffer contents as a UTF-16 encoded byte string
utf16 = output.getvalue()
print 'Encoded to UTF-16:', to_hex(utf16, 2)

# Set up another buffer with the UTF-16 data for reading,
# and wrap it with another EncodedFile.
buffer = StringIO(utf16)
encoded_file = codecs.EncodedFile(buffer, data_encoding='utf-8',
                                  file_encoding='utf-16')

# Read the UTF-8 encoded version of the data.
recoded = encoded_file.read()
print 'Back to UTF-8    :', to_hex(recoded, 1)

This example shows reading from and writing to separate handles returned by EncodedFile(). No matter whether the handle is used for reading or writing, the file_encoding always refers to the encoding in use by the open file handle passed as the first argument, and data_encoding value refers to the encoding in use by the data passing through the read() and write() calls.

$ python codecs_encodedfile.py
Start as UTF-8   : 70 69 3a 20 cf 80
Encoded to UTF-16: fffe 7000 6900 3a00 2000 c003
Back to UTF-8    : 70 69 3a 20 cf 80

Non-Unicode Encodings¶

Although most of the earlier examples use Unicode encodings, codecs can be used for many other data translations. For example, Python includes codecs for working with base-64, bzip2, ROT-13, ZIP, and other data formats.

import codecs
from cStringIO import StringIO

buffer = StringIO()
stream = codecs.getwriter('rot_13')(buffer)

text = 'abcdefghijklmnopqrstuvwxyz'

stream.write(text)
stream.flush()

print 'Original:', text
print 'ROT-13  :', buffer.getvalue()

Any transformation that can be expressed as a function taking a single input argument and returning a byte or Unicode string can be registered as a codec.

$ python codecs_rot13.py
Original: abcdefghijklmnopqrstuvwxyz
ROT-13  : nopqrstuvwxyzabcdefghijklm

Using codecs to wrap a data stream provides a simpler interface than working directly with zlib.

import codecs
from cStringIO import StringIO

from codecs_to_hex import to_hex

buffer = StringIO()
stream = codecs.getwriter('zlib')(buffer)

text = 'abcdefghijklmnopqrstuvwxyz\n' * 50

stream.write(text)
stream.flush()

print 'Original length :', len(text)
compressed_data = buffer.getvalue()
print 'ZIP compressed  :', len(compressed_data)

buffer = StringIO(compressed_data)
stream = codecs.getreader('zlib')(buffer)

first_line = stream.readline()
print 'Read first line :', repr(first_line)

uncompressed_data = first_line + stream.read()
print 'Uncompressed    :', len(uncompressed_data)
print 'Same            :', text == uncompressed_data

Not all of the compression or encoding systems support reading a portion of the data through the stream interface using readline() or read() because they need to find the end of a compressed segment to expand it. If your program cannot hold the entire uncompressed data set in memory, use the incremental access features of the compression library instead of codecs.

$ python codecs_zlib.py
Original length : 1350
ZIP compressed  : 48
Read first line : 'abcdefghijklmnopqrstuvwxyz\n'
Uncompressed    : 1350
Same            : True

Incremental Encoding¶

Some of the encodings provided, especially bz2 and zlib, may dramatically change the length of the data stream as they work on it. For large data sets, these encodings operate better incrementally, working on one small chunk of data at a time. The IncrementalEncoder and IncrementalDecoder API is designed for this purpose.

import codecs
import sys

from codecs_to_hex import to_hex

text = 'abcdefghijklmnopqrstuvwxyz\n'
repetitions = 50

print 'Text length :', len(text)
print 'Repetitions :', repetitions
print 'Expected len:', len(text) * repetitions

# Encode the text several times build up a large amount of data
encoder = codecs.getincrementalencoder('bz2')()
encoded = []

print
print 'Encoding:',
for i in range(repetitions):
    en_c = encoder.encode(text, final = (i==repetitions-1))
    if en_c:
        print '\nEncoded : {} bytes'.format(len(en_c))
        encoded.append(en_c)
    else:
        sys.stdout.write('.')
    
bytes = ''.join(encoded)
print
print 'Total encoded length:', len(bytes)
print

# Decode the byte string one byte at a time
decoder = codecs.getincrementaldecoder('bz2')()
decoded = []

print 'Decoding:',
for i, b in enumerate(bytes):
    final= (i+1) == len(text)
    c = decoder.decode(b, final)
    if c:
        print '\nDecoded : {} characters'.format(len(c))
        print 'Decoding:',
        decoded.append(c)
    else:
        sys.stdout.write('.')
print

restored = u''.join(decoded)

print
print 'Total uncompressed length:', len(restored)

Each time data is passed to the encoder or decoder its internal state is updated. When the state is consistent (as defined by the codec), data is returned and the state resets. Until that point, calls to encode() or decode() will not return any data. When the last bit of data is passed in, the argument final should be set to True so the codec knows to flush any remaining buffered data.

$ python codecs_incremental_bz2.py
Text length : 27
Repetitions : 50
Expected len: 1350
Encoding:.................................................
Encoded : 99 bytes
Total encoded length: 99
Decoding:............................................................
............................
Decoded : 1350 characters
Decoding:..........
Total uncompressed length: 1350

Defining Your Own Encoding¶

Since Python comes with a large number of standard codecs already, it is unlikely that you will need to define your own. If you do, there are several base classes in codecs to make the process easier.

The first step is to understand the nature of the transformation described by the encoding. For example, an “invertcaps” encoding converts uppercase letters to lowercase, and lowercase letters to uppercase. Here is a simple definition of an encoding function that performs this transformation on an input string:

import string

def invertcaps(text):
    """Return new string with the case of all letters switched.
    """
    return ''.join( c.upper() if c in string.ascii_lowercase
                    else c.lower() if c in string.ascii_uppercase
                    else c
                    for c in text
                    )

if __name__ == '__main__':
    print invertcaps('ABC.def')
    print invertcaps('abc.DEF')

In this case, the encoder and decoder are the same function (as with ROT-13).

$ python codecs_invertcaps.py
abc.DEF
ABC.def

Although it is easy to understand, this implementation is not efficient, especially for very large text strings. Fortunately, codecs includes some helper functions for creating character map based codecs such as invertcaps. A character map encoding is made up of two dictionaries. The encoding map converts character values from the input string to byte values in the output and the decoding map goes the other way. Create your decoding map first, and then use make_encoding_map() to convert it to an encoding map. The C functions charmap_encode() and charmap_decode() use the maps to convert their input data efficiently.

import codecs
import string

# Map every character to itself
decoding_map = codecs.make_identity_dict(range(256))

# Make a list of pairs of ordinal values for the lower and upper case
# letters
pairs = zip([ ord(c) for c in string.ascii_lowercase],
            [ ord(c) for c in string.ascii_uppercase])

# Modify the mapping to convert upper to lower and lower to upper.
decoding_map.update( dict( (upper, lower) for (lower, upper) in pairs) )
decoding_map.update( dict( (lower, upper) for (lower, upper) in pairs) )

# Create a separate encoding map.
encoding_map = codecs.make_encoding_map(decoding_map)

if __name__ == '__main__':
    print codecs.charmap_encode('abc.DEF', 'strict', encoding_map)
    print codecs.charmap_decode('abc.DEF', 'strict', decoding_map)
    print encoding_map == decoding_map
    

Although the encoding and decoding maps for invertcaps are the same, that may not always be the case. make_encoding_map() detects situations where more than one input character is encoded to the same output byte and replaces the encoding value with None to mark the encoding as undefined.

$ python codecs_invertcaps_charmap.py
('ABC.def', 7)
(u'ABC.def', 7)
True

The character map encoder and decoder support all of the standard error handling methods described earlier, so you do not need to do any extra work to comply with that part of the API.

import codecs
from codecs_invertcaps_charmap import encoding_map

text = u'pi: π'

for error in [ 'ignore', 'replace', 'strict' ]:
    try:
        encoded = codecs.charmap_encode(text, error, encoding_map)
    except UnicodeEncodeError, err:
        encoded = str(err)
    print '{:7}: {}'.format(error, encoded)

Because the Unicode code point for π is not in the encoding map, the strict error handling mode raises an exception.

$ python codecs_invertcaps_error.py
ignore : ('PI: ', 5)
replace: ('PI: ?', 5)
strict : 'charmap' codec can't encode character u'\u03c0' in position
 4: character maps to <undefined>

After that the encoding and decoding maps are defined, you need to set up a few additional classes and register the encoding. register() adds a search function to the registry so that when a user wants to use your encoding codecs can locate it. The search function must take a single string argument with the name of the encoding, and return a CodecInfo object if it knows the encoding, or None if it does not.

import codecs
import encodings

def search1(encoding):
    print 'search1: Searching for:', encoding
    return None

def search2(encoding):
    print 'search2: Searching for:', encoding
    return None

codecs.register(search1)
codecs.register(search2)

utf8 = codecs.lookup('utf-8')
print 'UTF-8:', utf8

try:
    unknown = codecs.lookup('no-such-encoding')
except LookupError, err:
    print 'ERROR:', err

You can register multiple search functions, and each will be called in turn until one returns a CodecInfo or the list is exhausted. The internal search function registered by codecs knows how to load the standard codecs such as UTF-8 from encodings, so those names will never be passed to your search function.

$ python codecs_register.py
UTF-8: <codecs.CodecInfo object for encoding utf-8 at 0x100439ce8>
search1: Searching for: no-such-encoding
search2: Searching for: no-such-encoding
ERROR: unknown encoding: no-such-encoding

The CodecInfo instance returned by the search function tells codecs how to encode and decode using all of the different mechanisms supported: stateless, incremental, and stream. codecs includes base classes that make setting up a character map encoding easy. This example puts all of the pieces together to register a search function that returns a CodecInfo instance configured for the invertcaps codec.

import codecs

from codecs_invertcaps_charmap import encoding_map, decoding_map

# Stateless encoder/decoder

class InvertCapsCodec(codecs.Codec):
    def encode(self, input, errors='strict'):
        return codecs.charmap_encode(input, errors, encoding_map)

    def decode(self, input, errors='strict'):
        return codecs.charmap_decode(input, errors, decoding_map)

# Incremental forms

class InvertCapsIncrementalEncoder(codecs.IncrementalEncoder):
    def encode(self, input, final=False):
        return codecs.charmap_encode(input, self.errors, encoding_map)[0]

class InvertCapsIncrementalDecoder(codecs.IncrementalDecoder):
    def decode(self, input, final=False):
        return codecs.charmap_decode(input, self.errors, decoding_map)[0]

# Stream reader and writer

class InvertCapsStreamReader(InvertCapsCodec, codecs.StreamReader):
    pass

class InvertCapsStreamWriter(InvertCapsCodec, codecs.StreamWriter):
    pass

# Register the codec search function

def find_invertcaps(encoding):
    """Return the codec for 'invertcaps'.
    """
    if encoding == 'invertcaps':
        return codecs.CodecInfo(
            name='invertcaps',
            encode=InvertCapsCodec().encode,
            decode=InvertCapsCodec().decode,
            incrementalencoder=InvertCapsIncrementalEncoder,
            incrementaldecoder=InvertCapsIncrementalDecoder,
            streamreader=InvertCapsStreamReader,
            streamwriter=InvertCapsStreamWriter,
            )
    return None

codecs.register(find_invertcaps)

if __name__ == '__main__':

    # Stateless encoder/decoder
    encoder = codecs.getencoder('invertcaps')
    text = 'abc.DEF'
    encoded_text, consumed = encoder(text)
    print 'Encoder converted "{}" to "{}", consuming {} characters'.format(
        text, encoded_text, consumed)

    # Stream writer
    import sys
    writer = codecs.getwriter('invertcaps')(sys.stdout)
    print 'StreamWriter for stdout: ',
    writer.write('abc.DEF')
    print

    # Incremental decoder
    decoder_factory = codecs.getincrementaldecoder('invertcaps')
    decoder = decoder_factory()
    decoded_text_parts = []
    for c in encoded_text:
        decoded_text_parts.append(decoder.decode(c, final=False))
    decoded_text_parts.append(decoder.decode('', final=True))
    decoded_text = ''.join(decoded_text_parts)
    print 'IncrementalDecoder converted "{}" to "{}"'.format(
        encoded_text, decoded_text)

The stateless encoder/decoder base class is Codec. Override encode() and decode() with your implementation (in this case, calling charmap_encode() and charmap_decode() respectively). Each method must return a tuple containing the transformed data and the number of the input bytes or characters consumed. Conveniently, charmap_encode() and charmap_decode() already return that information.

IncrementalEncoder and IncrementalDecoder serve as base classes for the incremental interfaces. The encode() and decode() methods of the incremental classes are defined in such a way that they only return the actual transformed data. Any information about buffering is maintained as internal state. The invertcaps encoding does not need to buffer data (it uses a one-to-one mapping). For encodings that produce a different amount of output depending on the data being processed, such as compression algorithms, BufferedIncrementalEncoder and BufferedIncrementalDecoder are more appropriate base classes, since they manage the unprocessed portion of the input for you.

StreamReader and StreamWriter need encode() and decode() methods, too, and since they are expected to return the same value as the version from Codec you can use multiple inheritance for the implementation.

$ python codecs_invertcaps_register.py
Encoder converted "abc.DEF" to "ABC.def", consuming 7 characters
StreamWriter for stdout: ABC.def
IncrementalDecoder converted "ABC.def" to "abc.DEF"