Setting the correct encoding when piping stdout in Python

Your code works when run in an script because Python encodes the output to whatever encoding your terminal application is using. If you are piping you must encode it yourself.

A rule of thumb is: Always use Unicode internally. Decode what you receive, and encode what you send.

# -*- coding: utf-8 -*-
print u"åäö".encode('utf-8')

Another didactic example is a Python program to convert between ISO-8859-1 and UTF-8, making everything uppercase in between.

import sys
for line in sys.stdin:
    # Decode what you receive:
    line = line.decode('iso8859-1')

    # Work with Unicode internally:
    line = line.upper()

    # Encode what you send:
    line = line.encode('utf-8')
    sys.stdout.write(line)

Setting the system default encoding is a bad idea, because some modules and libraries you use can rely on the fact it is ASCII. Don't do it.

First, regarding this solution:

# -*- coding: utf-8 -*-
print u"åäö".encode('utf-8')

It's not practical to explicitly print with a given encoding every time. That would be repetitive and error-prone.

A better solution is to change sys.stdout at the start of your program, to encode with a selected encoding. Here is one solution I found on Python: How is sys.stdout.encoding chosen?, in particular a comment by "toka":

import sys
import codecs
sys.stdout = codecs.getwriter('utf8')(sys.stdout)

I'm surprised this answer has not been posted here yet

Since Python 3.7 you can change the encoding of standard streams with reconfigure():

sys.stdout.reconfigure(encoding='utf-8')

You can also modify how encoding errors are handled by adding an errors parameter.

https://stackoverflow.com/a/52372390/15675011

I could "automate" it with a call to:

def __fix_io_encoding(last_resort_default='UTF-8'):
  import sys
  if [x for x in (sys.stdin,sys.stdout,sys.stderr) if x.encoding is None] :
      import os
      defEnc = None
      if defEnc is None :
        try:
          import locale
          defEnc = locale.getpreferredencoding()
        except: pass
      if defEnc is None :
        try: defEnc = sys.getfilesystemencoding()
        except: pass
      if defEnc is None :
        try: defEnc = sys.stdin.encoding
        except: pass
      if defEnc is None :
        defEnc = last_resort_default
      os.environ['PYTHONIOENCODING'] = os.environ.get("PYTHONIOENCODING",defEnc)
      os.execvpe(sys.argv[0],sys.argv,os.environ)
__fix_io_encoding() ; del __fix_io_encoding

Yes, it's possible to get an infinite loop here if this "setenv" fails.

On Windows, I had this problem very often when running a Python code from an editor (like Sublime Text), but not if running it from command-line.

In this case, check your editor's parameters. In the case of SublimeText, this Python.sublime-build solved it:

{
  "cmd": ["python", "-u", "$file"],
  "file_regex": "^[ ]*File \"(...*?)\", line ([0-9]*)",
  "selector": "source.python",
  "encoding": "utf8",
  "env": {"PYTHONIOENCODING": "utf-8", "LANG": "en_US.UTF-8"}
}

Since Python 3.7, we can use Python UTF-8 Mode, by using command line option -X utf8:

 python -X utf8 testzh.py

The script testzh.py contains

print("Content-type: text/html; charset=UTF-8\n") 
print("地球你好!")

To set Windows 10 Internet Service IIS as CGI Script handler,

We set Executable as this:

"C:\Program Files\Python39\python.exe" -X utf8 %s

This works for Chinese Ideograms as expected on Browser Microsoft.Edge like this screenshot: Otherwise, error occurs.

Please see https://docs.python.org/3/library/os.html#utf8-mode