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Convert utf-8 string to cp950 encoding in python

I'm handling an encoding problem.
My input is a unicode string, such as:
>>> s
u'\xa6\xe8\xac\xc9'
Actually it is encoded in cp950. I want to decode it: (notice there's no "u")
>>> print unicode('\xa6\xe8\xac\xc9', 'cp950')
西界
However, I don't know how to get rid of that "u".
Direct conversion is not working:
>>> str(s)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)
The result of using encode() is not what I wanted:
>>> s.encode('utf8')
'\xc2\xa6\xc3\xa8\xc2\xac\xc3\x89'
what I want is '\xa6\xe8\xac\xc9'

This is a bit of an abuse of the unicode type. Characters in a unicode string are expected to be Unicode codepoints (e.g. u'\u897f\u754c'), and thus are encoding-agnostic. They are not supposed to be bytes from a specific encoding (Python 3 makes this distinction very clear by separating Unicode strings str, from byte strings bytes).
Since you want to just interpret each codepoint as bytes, you can do
u'\xa6\xe8\xac\xc9'.encode('iso-8859-1')
since the first 256 codepoints of Unicode are defined to be equal to the codepoints of ISO-8859-1. However, please try to fix the issue that gave you this incorrect Unicode string in the first place.

So let's get this straight: you have a sequence of bytes that were read in as Unicode codepoints, and you need them to be interpreted as cp950 instead?
>>> ''.join(chr(ord(c)) for c in s)
'\xa6\xe8\xac\xc9'
>>> print ''.join(chr(ord(c)) for c in s).decode('cp950')
西界

What's the difference between these method to deal with Unicode strings in Python?

I tried print a_str.decode("utf-8"), print uni_str, print uni_str.decode("utf-8"),print uni_str.encode("utf-8")..
But only the first one works.
>>> print '\xe8\xb7\xb3'.decode("utf-8")
跳
>>> print u'\xe8\xb7\xb3\xe8'
è·³è
>>> print u'\xe8\xb7\xb3\xe8'.decode("utf-8")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/encodings/utf_8.py", line 16, in decode
return codecs.utf_8_decode(input, errors, True)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)
>>> print u'\xe8\xb7\xb3\xe8'.encode("utf-8")
è·³è
I'm really confused with how to display a Unicode string normally. If I have a string like this:
a=u'\xe8\xb7\xb3\xe8', how can I print a?

'\xe8\xb7\xb3' is a Chinese character encoded with utf8, so '\xe8\xb7\xb3'.decode('utf-8') works fine, which returns the unicode value of 跳, u'\u8df3'. But u'\xe8\xb7\xb3' is a literal unicode String, which is not same with the unicode of 跳. And a unicode string cannot be decoded, it's unicode.
At last,a=u'\xe8\xb7\xb3\xe8' is really not a valid unicode string[1].
Where the u'\xe8\xb7\xb3' comes from? Another function?
[1]Check out the first comment.

If you have a string like that then it's broken. You'll need to encode it as Latin-1 to get it to a bytestring with the same byte values, and then decode as UTF-8.

The unicode string u'\xe8\xb7\xb3\xe8' is equivalent to u'\u00e8\u00b7\u00b3\u00e8'. What you want is u'\u8df3' which can be encoded in utf8 as '\xe8\xb7\xb3'.
In Python, unicode is a UCS-2 string (build option). So, u'\xe8\xb7\xb3\xe8' is a string of 4 16bit Unicode characters.
If you got a utf-8 string (8bit string) incorrectly presented as Unicode (16bit string), you have to convert it to 8bit string first:
>>> ''.join([chr(ord(a)) for a in u'\xe8\xb7\xb3']).decode('utf8')
u'\u8df3'
Note that '\xe8\xb7\xb3\xe8' is not valid utf8 string as the last byte '\xe8' is a first character of a two byte sequence and cannot terminate a utf8 string.

string encoding and decoding?

Here are my attempts with error messages. What am I doing wrong?
string.decode("ascii", "ignore")
UnicodeEncodeError: 'ascii' codec can't encode character u'\xa0' in
position 37: ordinal not in range(128)
string.encode('utf-8', "ignore")
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position
37: ordinal not in range(128)

You can't decode a unicode, and you can't encode a str. Try doing it the other way around.

Guessing at all the things omitted from the original question, but, assuming Python 2.x the key is to read the error messages carefully: in particular where you call 'encode' but the message says 'decode' and vice versa, but also the types of the values included in the messages.
In the first example string is of type unicode and you attempted to decode it which is an operation converting a byte string to unicode. Python helpfully attempted to convert the unicode value to str using the default 'ascii' encoding but since your string contained a non-ascii character you got the error which says that Python was unable to encode a unicode value. Here's an example which shows the type of the input string:
>>> u"\xa0".decode("ascii", "ignore")
Traceback (most recent call last):
File "<pyshell#7>", line 1, in <module>
u"\xa0".decode("ascii", "ignore")
UnicodeEncodeError: 'ascii' codec can't encode character u'\xa0' in position 0: ordinal not in range(128)
In the second case you do the reverse attempting to encode a byte string. Encoding is an operation that converts unicode to a byte string so Python helpfully attempts to convert your byte string to unicode first and, since you didn't give it an ascii string the default ascii decoder fails:
>>> "\xc2".encode("ascii", "ignore")
Traceback (most recent call last):
File "<pyshell#6>", line 1, in <module>
"\xc2".encode("ascii", "ignore")
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 0: ordinal not in range(128)

Aside from getting decode and encode backwards, I think part of the answer here is actually don't use the ascii encoding. It's probably not what you want.
To begin with, think of str like you would a plain text file. It's just a bunch of bytes with no encoding actually attached to it. How it's interpreted is up to whatever piece of code is reading it. If you don't know what this paragraph is talking about, go read Joel's The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets right now before you go any further.
Naturally, we're all aware of the mess that created. The answer is to, at least within memory, have a standard encoding for all strings. That's where unicode comes in. I'm having trouble tracking down exactly what encoding Python uses internally for sure, but it doesn't really matter just for this. The point is that you know it's a sequence of bytes that are interpreted a certain way. So you only need to think about the characters themselves, and not the bytes.
The problem is that in practice, you run into both. Some libraries give you a str, and some expect a str. Certainly that makes sense whenever you're streaming a series of bytes (such as to or from disk or over a web request). So you need to be able to translate back and forth.
Enter codecs: it's the translation library between these two data types. You use encode to generate a sequence of bytes (str) from a text string (unicode), and you use decode to get a text string (unicode) from a sequence of bytes (str).
For example:
>>> s = "I look like a string, but I'm actually a sequence of bytes. \xe2\x9d\xa4"
>>> codecs.decode(s, 'utf-8')
u"I look like a string, but I'm actually a sequence of bytes. \u2764"
What happened here? I gave Python a sequence of bytes, and then I told it, "Give me the unicode version of this, given that this sequence of bytes is in 'utf-8'." It did as I asked, and those bytes (a heart character) are now treated as a whole, represented by their Unicode codepoint.
Let's go the other way around:
>>> u = u"I'm a string! Really! \u2764"
>>> codecs.encode(u, 'utf-8')
"I'm a string! Really! \xe2\x9d\xa4"
I gave Python a Unicode string, and I asked it to translate the string into a sequence of bytes using the 'utf-8' encoding. So it did, and now the heart is just a bunch of bytes it can't print as ASCII; so it shows me the hexadecimal instead.
We can work with other encodings, too, of course:
>>> s = "I have a section \xa7"
>>> codecs.decode(s, 'latin1')
u'I have a section \xa7'
>>> codecs.decode(s, 'latin1')[-1] == u'\u00A7'
True
>>> u = u"I have a section \u00a7"
>>> u
u'I have a section \xa7'
>>> codecs.encode(u, 'latin1')
'I have a section \xa7'
('\xa7' is the section character, in both
Unicode and Latin-1.)
So for your question, you first need to figure out what encoding your str is in.
Did it come from a file? From a web request? From your database? Then the source determines the encoding. Find out the encoding of the source and use that to translate it into a unicode.
s = [get from external source]
u = codecs.decode(s, 'utf-8') # Replace utf-8 with the actual input encoding
Or maybe you're trying to write it out somewhere. What encoding does the destination expect? Use that to translate it into a str. UTF-8 is a good choice for plain text documents; most things can read it.
u = u'My string'
s = codecs.encode(u, 'utf-8') # Replace utf-8 with the actual output encoding
[Write s out somewhere]
Are you just translating back and forth in memory for interoperability or something? Then just pick an encoding and stick with it; 'utf-8' is probably the best choice for that:
u = u'My string'
s = codecs.encode(u, 'utf-8')
newu = codecs.decode(s, 'utf-8')
In modern programming, you probably never want to use the 'ascii' encoding for any of this. It's an extremely small subset of all possible characters, and no system I know of uses it by default or anything.
Python 3 does its best to make this immensely clearer simply by changing the names. In Python 3, str was replaced with bytes, and unicode was replaced with str.

That's because your input string can’t be converted according to the encoding rules (strict by default).
I don't know, but I always encoded using directly unicode() constructor, at least that's the ways at the official documentation:
unicode(your_str, errors="ignore")

Convert hash.digest() to unicode

import hashlib
string1 = u'test'
hashstring = hashlib.md5()
hashstring.update(string1)
string2 = hashstring.digest()
unicode(string2)
UnicodeDecodeError: 'ascii' codec can't decode byte 0x8f in position 1: ordinal
not in range(128)
The string HAS to be unicode for it to be any use to me, can this be done?
Using python 2.7 if that helps...

Ignacio just gave the perfect answer. Just a complement: when you convert some string from an encoding which has chars not found in ASCII to unicode, you have to pass the encoding as a parameter:
>>> unicode("órgão")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128)
>>> unicode("órgão", "UTF-8")
u'\xf3rg\xe3o'
If you cannot say what is the original encoding (UTF-8 in my example) you really cannot convert to Unicode. It is a signal that something is not pretty correct in your intentions.
Last but not least, encodings are pretty confusing stuff. This comprehensive text about them can make them clear.

The result of .digest() is a bytestring¹, so converting it to Unicode is pointless. Use .hexdigest() if you want a readable representation.
¹ Some bytestrings can be converted to Unicode, but the bytestrings returned by .digest() do not contain textual data. They can contain any byte including the null byte: they're usually not printable without using escape sequences.

Convert or strip out "illegal" Unicode characters

I've got a database in MSSQL that I'm porting to SQLite/Django. I'm using pymssql to connect to the database and save a text field to the local SQLite database.
However for some characters, it explodes. I get complaints like this:
UnicodeDecodeError: 'ascii' codec can't decode byte 0x97 in position 1916: ordinal not in range(128)
Is there some way I can convert the chars to proper unicode versions? Or strip them out?

Once you have the string of bytes s, instead of using it as a unicode obj directly, convert it explicitly with the right codec, e.g.:
u = s.decode('latin-1')
and use u instead of s in the code that follows this point (presumably the part that writes to sqlite). That's assuming latin-1 is the encoding that was used to make the byte string originally -- it's impossible for us to guess, so try to find out;-).
As a general rule, I suggest: don't process in your applications any text as encoded byte strings -- decode them to unicode objects right after input, and, if necessary, encode them back to byte strings right before output.

When you decode, just pass 'ignore' to strip those characters
there is some more way of stripping / converting those are
'replace': replace malformed data with a suitable replacement marker, such as '?' or '\ufffd'
'ignore': ignore malformed data and continue without further notice
'backslashreplace': replace with backslashed escape sequences (for encoding only)
Test
>>> "abcd\x97".decode("ascii")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0x97 in position 4: ordinal not in range(128)
>>>
>>> "abcd\x97".decode("ascii","ignore")
u'abcd'