Set encoding in Python 3 CGI scripts

Question

When writing a Python 3.1 CGI script, I run into horrible UnicodeDecodeErrors. However, when running the script on the command line, everything works.

Answer 1

Answering this for late-comers because I don't think that the posted answers get to the root of the problem, which is the lack of locale environment variables in a CGI context. I'm using Python 3.2.

1. open() opens file objects in text (string) or binary (bytes) mode for reading and/or writing; in text mode the encoding used to encode strings written to the file, and decode bytes read from the file, may be specified in the call; if it isn't then it is determined by locale.getpreferredencoding(), which on linux uses the encoding from your locale environment settings, which is normally utf-8 (from e.g. LANG=en_US.UTF-8)

   >>> f = open('foo', 'w')         # open file for writing in text mode
   >>> f.encoding
   'UTF-8'                          # encoding is from the environment
   >>> f.write('€')                 # write a Unicode string
   1
   >>> f.close()
   >>> exit()
   user@host:~$ hd foo
   00000000  e2 82 ac      |...|    # data is UTF-8 encoded
   

2. sys.stdout is in fact a file opened for writing in text mode with an encoding based on locale.getpreferredencoding(); you can write strings to it just fine and they'll be encoded to bytes based on sys.stdout's encoding; print() by default writes to sys.stdout - print() itself has no encoding, rather it's the file it writes to that has an encoding;

   >>> sys.stdout.encoding
   'UTF-8'                          # encoding is from the environment
   >>> exit()
   user@host:~$ python3 -c 'print("€")' > foo
   user@host:~$ hd foo
   00000000  e2 82 ac 0a   |....|   # data is UTF-8 encoded; \n is from print()
   

   ; you cannot write bytes to sys.stdout - use sys.stdout.buffer.write() for that; if you try to write bytes to sys.stdout using sys.stdout.write() then it will return an error, and if you try using print() then print() will simply turn the bytes object into a string object and an escape sequence like \xff will be treated as the four characters \, x, f, f

   user@host:~$ python3 -c 'print(b"\xe2\xf82\xac")' > foo
   user@host:~$ hd foo
   00000000  62 27 5c 78 65 32 5c 78  66 38 32 5c 78 61 63 27  |b'\xe2\xf82\xac'|
   00000010  0a                                                |.|
   

3. in a CGI script you need to write to sys.stdout and you can use print() to do it; but a CGI script process in Apache has no locale environment settings - they are not part of the CGI specification; therefore the sys.stdout encoding defaults to ANSI_X3.4-1968 - in other words, ASCII; if you try to print() a string that contain non-ASCII characters to sys.stdout you'll get "UnicodeEncodeError: 'ascii' codec can't encode character...: ordinal not in range(128)"

4. a simple solution is to pass the Apache process's LANG environment variable through to the CGI script using Apache's mod_env PassEnv command in the server or virtual host configuration: PassEnv LANG; on Debian/Ubuntu make sure that in /etc/apache2/envvars you have uncommented the line ". /etc/default/locale" so that Apache runs with the system default locale and not the C (Posix) locale (which is also ASCII encoding); the following CGI script should run without errors in Python 3.2:

   #!/usr/bin/env python3
   import sys
   print('Content-Type: text/html; charset=utf-8')
   print()
   print('
   ' + sys.stdout.encoding + '
   h€lló wörld')