• PrintFails

Issue

If you try to print a unicode string to console and get a message like this one:

>>> print u"\u03A9"
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\Python24\lib\encodings\cp866.py", line 18, in encode
    return codecs.charmap_encode(input,errors,encoding_map)
UnicodeEncodeError: 'charmap' codec can't encode character u'\u1234' in position
 0: character maps to <undefined>

This means that the python console app can't write the given character to the console's encoding.

More specifically, the python console app created a _io.TextIOWrapperd instance with an encoding that cannot represent the given character.

sys.stdout --> _io.TextIOWrapperd --> (your console)

To understand it more clearly, look at:

• sys.stdout

• sys.stdout.encoding -- This seems to work on one of my computers (Vista,) but not on another of my computers (XP.) I haven't looked into differences of situation in detail.

Windows

By default, the console in Microsoft Windows only displays 256 characters (cp437, of "Code page 437", the original IBM-PC 1981 extended ASCII character set.)

If you try to print an unprintable character you will get UnicodeEncodeError.

Setting the PYTHONIOENCODING environment variable as described above can be used to suppress the error messages. Setting to "utf-8" is not recommended as this produces an inaccurate, garbled representation of the output to the console. For best results, use your console's correct default codepage and a suitable error handler other than "strict".

Various UNIX consoles

There is no standard way to query UNIX console for find out what characters it supports but fortunately there is a way to find out what characters are considered to be printable. Locale category LC_CTYPE defines what characters are printable. To find out its value type at python prompt:

   1 >>> import locale
   2 >>> locale.getdefaultlocale()[1]
   3 'utf-8'

If you got any other value you won't be able to print all unicode characters. As soon as you try to print a unprintable character you will get UnicodeEncodeError. To fix this situation you need to set the environment variable LANG to one of supported by your system unicode locales. To get the full list of locales use command "locale -a", look for locales that end with string ".utf-8". If you have set LANG variable but now instead of UnicodeEncodeError you see garbage on your screen you need to set up your terminal to use font unicode font. Consult terminal manual on how to do it.

print, write and Unicode in pre-3.0 Python

Because file operations are 8-bit clean, reading data from the original stdin will return str's containing data in the input character set. Writing these str's to stdout without any codecs will result in the output identical to the input.

  $ echo $LANG
  en_CA.utf8

  $ python -c 'import sys; line = sys.stdin.readline(); print type(line), len(line); print line;'
  [TYPING: абв ENTER]
  <type 'str'> 7
  абв

  $ echo "абв" | python -c 'import sys; line = sys.stdin.readline(); print type(line), len(line); print line;'
  <type 'str'> 7
  абв
  $ echo "абв" | python -c 'import sys; line = sys.stdin.readline(); print type(line), len(line); print line;' | cat
  <type 'str'> 7
  абв

Since programmers need to display unicode strings, the designers of the print statement built the required transformation into it.

• When Python finds its output attached to a terminal, it sets the sys.stdout.encoding attribute to the terminal's encoding. The print statement's handler will automatically encode unicode arguments into str output.

    $ python -c 'import sys; print sys.stdout.encoding; print u"\u0411\n"'
    UTF-8
    Б

• When Python does not detect the desired character set of the output, it sets sys.stdout.encoding to None, and print will invoke the "ascii" codec.

    $ python -c 'import sys; print sys.stdout.encoding; print u"\u0411\n"' 2>&1 | cat
    None
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    UnicodeEncodeError: 'ascii' codec can't encode character u'\u0411' in position 0: ordinal not in range(128)

I (IL) understand the implementation of Python2.5's print statement as follows.

   1     # At Python startup.
   2     sys.stdout.encoding = tty_enc
   3     if tty_enc is not None:
   4       class_tty_enc_sw = codecs.getstreamwriter(tty_enc)
   5     else
   6       class_tty_enc_sw = None
   7 
   8     def print(*args):
   9       if class_tty_enc_sw is not None:
  10         eout = class_tty_enc_sw(sys.stdout)
  11       else:
  12         eout = None
  13       for arg in args:
  14          sarg = stringify_to_str_or_unicode(arg)
  15          if type(sarg) == str or eout is None:
  16            # Avoid coercion to unicode in eout.write().
  17            sys.stdout.write(sarg)
  18          else:
  19            eout.write(sarg)

• At startup, Python will detect the encoding of the standard output and, probably, store the respective StreamWriter class definition. The print statement stringifies all its arguments to narrow str and wide unicode strings based on the width of the original arguments. Then print passes narrow strings to sys.stdout directly and wide strings to the instance of StreamWriter wrapped around sys.stdout.

• If the user does not replace sys.stdout as shown below and Python does not detect an output encoding, the write method will coerce unicode values to str by invoking the ASCII codec (DefaultEncoding).

Python file's write and read methods do not invoke codecs internally. Python2.5's file open built-in sets the .encoding attribute of the resulting instance to None.

Wrapping sys.stdout into an instance of StreamWriter will allow writing unicode data with sys.stdout.write() and print.

  $ python -c 'import sys, codecs, locale; print sys.stdout.encoding; \
    sys.stdout = codecs.getwriter(locale.getpreferredencoding())(sys.stdout); \
    line = u"\u0411\n"; print type(line), len(line); \
    sys.stdout.write(line); print line'
  UTF-8
  <type 'unicode'> 2
  Б
  Б

  $ python -c 'import sys, codecs, locale; print sys.stdout.encoding; \
    sys.stdout = codecs.getwriter(locale.getpreferredencoding())(sys.stdout); \
    line = u"\u0411\n"; print type(line), len(line); \
    sys.stdout.write(line); print line' | cat
  None
  <type 'unicode'> 2
  Б
  Б

The write call executes StreamWriter.write which in turn invokes codec-specific encode and passes the result to the underlying file. It appears that the print statement will not fail due to the argument type coercion when sys.stdout is wrapped. My (IL's) understanding of print's implementation above agrees with that.

read and Unicode in pre-3.0 Python

I (IL) believe reading from stdin does not involve coercion at all because the existing ways to read from stdin such as "for line in sys.stdin" do not convey the expected type of the returned value to the stdin handler. A function that would complement the print statement might look like this:

  line = typed_read(unicode)   # Generally, a list of input data types along with an optional parsing format line.

print statement encodes unicode strings to str strings. One can complement this with decoding of str input data into unicode strings in sys.stdin.read/readline. For this, we will wrap sys.stdin into a StreamReader instance:

  $ python -c 'import sys, codecs, locale; \
    print sys.stdin.encoding; \
    sys.stdin = codecs.getreader(locale.getpreferredencoding())(sys.stdin); \
    line = sys.stdin.readline(); print type(line), len(line)' 2>&1
  UTF-8
  [TYPING: абв ENTER]
  <type 'unicode'> 4
  $ echo "абв" | python -c 'import sys, codecs, locale; \
    print sys.stdin.encoding; \
    sys.stdin = codecs.getreader(locale.getpreferredencoding())(sys.stdin); \
    line = sys.stdin.readline(); print type(line), len(line)'
  None
  <type 'unicode'> 4

See also: Unicode

CategoryUnicode