Best output type and coding practices for __repr__() functions?

Question

Lately, I've had lots of trouble with __repr__(), format(), and encodings. Should the output of __repr__() be encoded or be a unicode string? Is there a best encoding for the result of __repr__() in Python? What I want to output does have non-ASCII characters.

I use Python 2.x, and want to write code that can easily be adapted to Python 3. The program thus uses

# -*- coding: utf-8 -*-
from __future__ import unicode_literals, print_function  # The 'Hello' literal represents a Unicode object

Here are some problems that have been bothering me, and I'm looking for a solution that solves them:

1. Printing to an UTF-8 terminal should work (I have sys.stdout.encoding set to UTF-8, but it would be best if other cases worked too).
2. Piping the output to a file (encoded in UTF-8) should work (in this case, sys.stdout.encoding is None).
3. My code for many __repr__() functions currently has many return ….encode('utf-8'), and that's heavy. Is there anything robust and lighter?
4. In some cases, I even have ugly beasts like return ('<{}>'.format(repr(x).decode('utf-8'))).encode('utf-8'), i.e., the representation of objects is decoded, put into a formatting string, and then re-encoded. I would like to avoid such convoluted transformations.

What would you recommend to do in order to write simple __repr__() functions that behave nicely with respect to these encoding questions?

Answer 1

In Python2, __repr__ (and __str__) must return a string object, not a unicode object. In Python3, the situation is reversed, __repr__ and __str__ must return unicode objects, not byte (née string) objects:

class Foo(object):
    def __repr__(self):
        return u'\N{WHITE SMILING FACE}' 

class Bar(object):
    def __repr__(self):
        return u'\N{WHITE SMILING FACE}'.encode('utf8')

repr(Bar())
# ☺
repr(Foo())
# UnicodeEncodeError: 'ascii' codec can't encode character u'\u263a' in position 0: ordinal not in range(128)

In Python2, you don't really have a choice. You have to pick an encoding for the return value of __repr__.

By the way, have you read the PrintFails wiki? It may not directly answer your other questions, but I did find it helpful in illuminating why certain errors occur.

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When using from __future__ import unicode_literals,

'<{}>'.format(repr(x).decode('utf-8'))).encode('utf-8')

can be more simply written as

str('<{}>').format(repr(x))

assuming str encodes to utf-8 on your system.

Without from __future__ import unicode_literals, the expression can be written as:

'<{}>'.format(repr(x))