Python - 'ascii' codec can't decode byte

I'm really confused. I tried to encode but the error said can't decode....

>>> "你好".encode("utf8")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe4 in position 0: ordinal not in range(128)

I know how to avoid the error with "u" prefix on the string. I'm just wondering why the error is "can't decode" when encode was called. What is Python doing under the hood?

Solution 1:

"你好".encode('utf-8')

encode converts a unicode object to a string object. But here you have invoked it on a string object (because you don't have the u). So python has to convert the string to a unicode object first. So it does the equivalent of

"你好".decode().encode('utf-8')

But the decode fails because the string isn't valid ascii. That's why you get a complaint about not being able to decode.

Solution 2:

Always encode from unicode to bytes.
In this direction, you get to choose the encoding.

>>> u"你好".encode("utf8")
'\xe4\xbd\xa0\xe5\xa5\xbd'
>>> print _
你好

The other way is to decode from bytes to unicode.
In this direction, you have to know what the encoding is.

>>> bytes = '\xe4\xbd\xa0\xe5\xa5\xbd'
>>> print bytes
你好
>>> bytes.decode('utf-8')
u'\u4f60\u597d'
>>> print _
你好

This point can't be stressed enough. If you want to avoid playing unicode "whack-a-mole", it's important to understand what's happening at the data level. Here it is explained another way:

A unicode object is decoded already, you never want to call decode on it.
A bytestring object is encoded already, you never want to call encode on it.

Now, on seeing .encode on a byte string, Python 2 first tries to implicitly convert it to text (a unicode object). Similarly, on seeing .decode on a unicode string, Python 2 implicitly tries to convert it to bytes (a str object).

These implicit conversions are why you can get UnicodeDecodeError when you've called encode. It's because encoding usually accepts a parameter of type unicode; when receiving a str parameter, there's an implicit decoding into an object of type unicode before re-encoding it with another encoding. This conversion chooses a default 'ascii' decoder^†, giving you the decoding error inside an encoder.

In fact, in Python 3 the methods str.decode and bytes.encode don't even exist. Their removal was a [controversial] attempt to avoid this common confusion.

^†_{...or whatever coding sys.getdefaultencoding() mentions; usually this is 'ascii'}

Solution 3:

You can try this

import sys
reload(sys)
sys.setdefaultencoding("utf-8")

You can also try following

Add following line at top of your .py file.

# -*- coding: utf-8 -*-

Solution 4:

If you're using Python < 3, you'll need to tell the interpreter that your string literal is Unicode by prefixing it with a u:

Python 2.7.2 (default, Jan 14 2012, 23:14:09) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> "你好".encode("utf8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe4 in position 0: ordinal not in range(128)
>>> u"你好".encode("utf8")
'\xe4\xbd\xa0\xe5\xa5\xbd'

Further reading: Unicode HOWTO.

Solution 5:

You use u"你好".encode('utf8') to encode an unicode string. But if you want to represent "你好", you should decode it. Just like:

"你好".decode("utf8")

You will get what you want. Maybe you should learn more about encode & decode.