Setting the correct encoding when piping stdout in Python


When piping the output of a Python program, the Python interpreter gets confused about encoding and sets it to None. This means a program like this:

# -*- coding: utf-8 -*-
print u"åäö"

will work fine when run normally, but fail with:

UnicodeEncodeError: 'ascii' codec can't encode character u'\xa0' in position 0: ordinal not in range(128)

when used in a pipe sequence.

What is the best way to make this work when piping? Can I just tell it to use whatever encoding the shell/filesystem/whatever is using?

The suggestions I have seen thus far is to modify your directly, or hardcoding the defaultencoding using this hack:

# -*- coding: utf-8 -*-
import sys
print u"åäö"

Is there a better way to make piping work?

11/11/2014 6:28:09 PM

Accepted Answer

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')

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.

11/11/2014 6:30:18 PM

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)

Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow