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----------------------- Kitchen.text.converters ----------------------- .. automodule:: kitchen.text.converters Byte Strings and Unicode in Python2 =================================== Python2 has two string types, :class:`str` and :class:`unicode`. :class:`unicode` represents an abstract sequence of text characters. It can hold any character that is present in the unicode standard. :class:`str` can hold any byte of data. The operating system and python work together to display these bytes as characters in many cases but you should always keep in mind that the information is really a sequence of bytes, not a sequence of characters. In python2 these types are interchangeable a large amount of the time. They are one of the few pairs of types that automatically convert when used in equality:: >>> # string is converted to unicode and then compared >>> "I am a string" == u"I am a string" True >>> # Other types, like int, don't have this special treatment >>> 5 == "5" False However, this automatic conversion tends to lull people into a false sense of security. As long as you're dealing with :term:`ASCII` characters the automatic conversion will save you from seeing any differences. Once you start using characters that are not in :term:`ASCII`, you will start getting :exc:`UnicodeError` and :exc:`UnicodeWarning` as the automatic conversions between the types fail:: >>> "I am an ñ" == u"I am an ñ" __main__:1: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal False Why do these conversions fail? The reason is that the python2 :class:`unicode` type represents an abstract sequence of unicode text known as :term:`code points`. :class:`str`, on the other hand, really represents a sequence of bytes. Those bytes are converted by your operating system to appear as characters on your screen using a particular encoding (usually with a default defined by the operating system and customizable by the individual user.) Although :term:`ASCII` characters are fairly standard in what bytes represent each character, the bytes outside of the :term:`ASCII` range are not. In general, each encoding will map a different character to a particular byte. Newer encodings map individual characters to multiple bytes (which the older encodings will instead treat as multiple characters). In the face of these differences, python refuses to guess at an encoding and instead issues a warning or exception and refuses to convert. .. seealso:: :ref:`overcoming-frustration` For a longer introduction on this subject. Strategy for Explicit Conversion ================================ So what is the best method of dealing with this weltering babble of incoherent encodings? The basic strategy is to explicitly turn everything into :class:`unicode` when it first enters your program. Then, when you send it to output, you can transform the unicode back into bytes. Doing this allows you to control the encodings that are used and avoid getting tracebacks due to :exc:`UnicodeError`. Using the functions defined in this module, that looks something like this: .. code-block:: pycon :linenos: >>> from kitchen.text.converters import to_unicode, to_bytes >>> name = raw_input('Enter your name: ') Enter your name: Toshio くらとみ >>> name 'Toshio \xe3\x81\x8f\xe3\x82\x89\xe3\x81\xa8\xe3\x81\xbf' >>> type(name) <type 'str'> >>> unicode_name = to_unicode(name) >>> type(unicode_name) <type 'unicode'> >>> unicode_name u'Toshio \u304f\u3089\u3068\u307f' >>> # Do a lot of other things before needing to save/output again: >>> output = open('datafile', 'w') >>> output.write(to_bytes(u'Name: %s\\n' % unicode_name)) A few notes: Looking at line 6, you'll notice that the input we took from the user was a byte :class:`str`. In general, anytime we're getting a value from outside of python (The filesystem, reading data from the network, interacting with an external command, reading values from the environment) we are interacting with something that will want to give us a byte :class:`str`. Some |stdlib|_ modules and third party libraries will automatically attempt to convert a byte :class:`str` to :class:`unicode` strings for you. This is both a boon and a curse. If the library can guess correctly about the encoding that the data is in, it will return :class:`unicode` objects to you without you having to convert. However, if it can't guess correctly, you may end up with one of several problems: :exc:`UnicodeError` The library attempted to decode a byte :class:`str` into a :class:`unicode`, string failed, and raises an exception. Garbled data If the library returns the data after decoding it with the wrong encoding, the characters you see in the :exc:`unicode` string won't be the ones that you expect. A byte :class:`str` instead of :class:`unicode` string Some libraries will return a :class:`unicode` string when they're able to decode the data and a byte :class:`str` when they can't. This is generally the hardest problem to debug when it occurs. Avoid it in your own code and try to avoid or open bugs against upstreams that do this. See :ref:`DesigningUnicodeAwareAPIs` for strategies to do this properly. On line 8, we convert from a byte :class:`str` to a :class:`unicode` string. :func:`~kitchen.text.converters.to_unicode` does this for us. It has some error handling and sane defaults that make this a nicer function to use than calling :meth:`str.decode` directly: * Instead of defaulting to the :term:`ASCII` encoding which fails with all but the simple American English characters, it defaults to :term:`UTF-8`. * Instead of raising an error if it cannot decode a value, it will replace the value with the unicode "Replacement character" symbol (``�``). * If you happen to call this method with something that is not a :class:`str` or :class:`unicode`, it will return an empty :class:`unicode` string. All three of these can be overridden using different keyword arguments to the function. See the :func:`to_unicode` documentation for more information. On line 15 we push the data back out to a file. Two things you should note here: 1. We deal with the strings as :class:`unicode` until the last instant. The string format that we're using is :class:`unicode` and the variable also holds :class:`unicode`. People sometimes get into trouble when they mix a byte :class:`str` format with a variable that holds a :class:`unicode` string (or vice versa) at this stage. 2. :func:`~kitchen.text.converters.to_bytes`, does the reverse of :func:`to_unicode`. In this case, we're using the default values which turn :class:`unicode` into a byte :class:`str` using :term:`UTF-8`. Any errors are replaced with a ``�`` and sending nonstring objects yield empty :class:`unicode` strings. Just like :func:`to_unicode`, you can look at the documentation for :func:`to_bytes` to find out how to override any of these defaults. When to use an alternate strategy --------------------------------- The default strategy of decoding to :class:`unicode` strings when you take data in and encoding to a byte :class:`str` when you send the data back out works great for most problems but there are a few times when you shouldn't: * The values aren't meant to be read as text * The values need to be byte-for-byte when you send them back out -- for instance if they are database keys or filenames. * You are transferring the data between several libraries that all expect byte :class:`str`. In each of these instances, there is a reason to keep around the byte :class:`str` version of a value. Here's a few hints to keep your sanity in these situations: 1. Keep your :class:`unicode` and :class:`str` values separate. Just like the pain caused when you have to use someone else's library that returns both :class:`unicode` and :class:`str` you can cause yourself pain if you have functions that can return both types or variables that could hold either type of value. 2. Name your variables so that you can tell whether you're storing byte :class:`str` or :class:`unicode` string. One of the first things you end up having to do when debugging is determine what type of string you have in a variable and what type of string you are expecting. Naming your variables consistently so that you can tell which type they are supposed to hold will save you from at least one of those steps. 3. When you get values initially, make sure that you're dealing with the type of value that you expect as you save it. You can use :func:`isinstance` or :func:`to_bytes` since :func:`to_bytes` doesn't do any modifications of the string if it's already a :class:`str`. When using :func:`to_bytes` for this purpose you might want to use:: try: b_input = to_bytes(input_should_be_bytes_already, errors='strict', nonstring='strict') except: handle_errors_somehow() The reason is that the default of :func:`to_bytes` will take characters that are illegal in the chosen encoding and transform them to replacement characters. Since the point of keeping this data as a byte :class:`str` is to keep the exact same bytes when you send it outside of your code, changing things to replacement characters should be rasing red flags that something is wrong. Setting :attr:`errors` to ``strict`` will raise an exception which gives you an opportunity to fail gracefully. 4. Sometimes you will want to print out the values that you have in your byte :class:`str`. When you do this you will need to make sure that you transform :class:`unicode` to :class:`str` before combining them. Also be sure that any other function calls (including :mod:`gettext`) are going to give you strings that are the same type. For instance:: print to_bytes(_('Username: %(user)s'), 'utf-8') % {'user': b_username} Gotchas and how to avoid them ============================= Even when you have a good conceptual understanding of how python2 treats :class:`unicode` and :class:`str` there are still some things that can surprise you. In most cases this is because, as noted earlier, python or one of the python libraries you depend on is trying to convert a value automatically and failing. Explicit conversion at the appropriate place usually solves that. str(obj) -------- One common idiom for getting a simple, string representation of an object is to use:: str(obj) Unfortunately, this is not safe. Sometimes str(obj) will return :class:`unicode`. Sometimes it will return a byte :class:`str`. Sometimes, it will attempt to convert from a :class:`unicode` string to a byte :class:`str`, fail, and throw a :exc:`UnicodeError`. To be safe from all of these, first decide whether you need :class:`unicode` or :class:`str` to be returned. Then use :func:`to_unicode` or :func:`to_bytes` to get the simple representation like this:: u_representation = to_unicode(obj, nonstring='simplerepr') b_representation = to_bytes(obj, nonstring='simplerepr') print ----- python has a builtin :func:`print` statement that outputs strings to the terminal. This originated in a time when python only dealt with byte :class:`str`. When :class:`unicode` strings came about, some enhancements were made to the :func:`print` statement so that it could print those as well. The enhancements make :func:`print` work most of the time. However, the times when it doesn't work tend to make for cryptic debugging. The basic issue is that :func:`print` has to figure out what encoding to use when it prints a :class:`unicode` string to the terminal. When python is attached to your terminal (ie, you're running the interpreter or running a script that prints to the screen) python is able to take the encoding value from your locale settings :envvar:`LC_ALL` or :envvar:`LC_CTYPE` and print the characters allowed by that encoding. On most modern Unix systems, the encoding is :term:`utf-8` which means that you can print any :class:`unicode` character without problem. There are two common cases of things going wrong: 1. Someone has a locale set that does not accept all valid unicode characters. For instance:: $ LC_ALL=C python >>> print u'\ufffd' Traceback (most recent call last): File "<stdin>", line 1, in <module> UnicodeEncodeError: 'ascii' codec can't encode character u'\ufffd' in position 0: ordinal not in range(128) This often happens when a script that you've written and debugged from the terminal is run from an automated environment like :program:`cron`. It also occurs when you have written a script using a :term:`utf-8` aware locale and released it for consumption by people all over the internet. Inevitably, someone is running with a locale that can't handle all unicode characters and you get a traceback reported. 2. You redirect output to a file. Python isn't using the values in :envvar:`LC_ALL` unconditionally to decide what encoding to use. Instead it is using the encoding set for the terminal you are printing to which is set to accept different encodings by :envvar:`LC_ALL`. If you redirect to a file, you are no longer printing to the terminal so :envvar:`LC_ALL` won't have any effect. At this point, python will decide it can't find an encoding and fallback to :term:`ASCII` which will likely lead to :exc:`UnicodeError` being raised. You can see this in a short script:: #! /usr/bin/python -tt print u'\ufffd' And then look at the difference between running it normally and redirecting to a file: .. code-block:: console $ ./test.py � $ ./test.py > t Traceback (most recent call last): File "test.py", line 3, in <module> print u'\ufffd' UnicodeEncodeError: 'ascii' codec can't encode character u'\ufffd' in position 0: ordinal not in range(128) The short answer to dealing with this is to always use bytes when writing output. You can do this by explicitly converting to bytes like this:: from kitchen.text.converters import to_bytes u_string = u'\ufffd' print to_bytes(u_string) or you can wrap stdout and stderr with a :class:`~codecs.StreamWriter`. A :class:`~codecs.StreamWriter` is convenient in that you can assign it to encode for :data:`sys.stdout` or :data:`sys.stderr` and then have output automatically converted but it has the drawback of still being able to throw :exc:`UnicodeError` if the writer can't encode all possible unicode codepoints. Kitchen provides an alternate version which can be retrieved with :func:`kitchen.text.converters.getwriter` which will not traceback in its standard configuration. .. _unicode-and-dict-keys: Unicode, str, and dict keys --------------------------- The :func:`hash` of the :term:`ASCII` characters is the same for :class:`unicode` and byte :class:`str`. When you use them in :class:`dict` keys, they evaluate to the same dictionary slot:: >>> u_string = u'a' >>> b_string = 'a' >>> hash(u_string), hash(b_string) (12416037344, 12416037344) >>> d = {} >>> d[u_string] = 'unicode' >>> d[b_string] = 'bytes' >>> d {u'a': 'bytes'} When you deal with key values outside of :term:`ASCII`, :class:`unicode` and byte :class:`str` evaluate unequally no matter what their character content or hash value:: >>> u_string = u'ñ' >>> b_string = u_string.encode('utf-8') >>> print u_string ñ >>> print b_string ñ >>> d = {} >>> d[u_string] = 'unicode' >>> d[b_string] = 'bytes' >>> d {u'\\xf1': 'unicode', '\\xc3\\xb1': 'bytes'} >>> b_string2 = '\\xf1' >>> hash(u_string), hash(b_string2) (30848092528, 30848092528) >>> d = {} >>> d[u_string] = 'unicode' >>> d[b_string2] = 'bytes' {u'\\xf1': 'unicode', '\\xf1': 'bytes'} How do you work with this one? Remember rule #1: Keep your :class:`unicode` and byte :class:`str` values separate. That goes for keys in a dictionary just like anything else. * For any given dictionary, make sure that all your keys are either :class:`unicode` or :class:`str`. **Do not mix the two.** If you're being given both :class:`unicode` and :class:`str` but you don't need to preserve separate keys for each, I recommend using :func:`to_unicode` or :func:`to_bytes` to convert all keys to one type or the other like this:: >>> from kitchen.text.converters import to_unicode >>> u_string = u'one' >>> b_string = 'two' >>> d = {} >>> d[to_unicode(u_string)] = 1 >>> d[to_unicode(b_string)] = 2 >>> d {u'two': 2, u'one': 1} * These issues also apply to using dicts with tuple keys that contain a mixture of :class:`unicode` and :class:`str`. Once again the best fix is to standardise on either :class:`str` or :class:`unicode`. * If you absolutely need to store values in a dictionary where the keys could be either :class:`unicode` or :class:`str` you can use :class:`~kitchen.collections.strictdict.StrictDict` which has separate entries for all :class:`unicode` and byte :class:`str` and deals correctly with any :class:`tuple` containing mixed :class:`unicode` and byte :class:`str`. --------- Functions --------- Unicode and byte str conversion =============================== .. autofunction:: kitchen.text.converters.to_unicode .. autofunction:: kitchen.text.converters.to_bytes .. autofunction:: kitchen.text.converters.getwriter .. autofunction:: kitchen.text.converters.to_str .. autofunction:: kitchen.text.converters.to_utf8 Transformation to XML ===================== .. autofunction:: kitchen.text.converters.unicode_to_xml .. autofunction:: kitchen.text.converters.xml_to_unicode .. autofunction:: kitchen.text.converters.byte_string_to_xml .. autofunction:: kitchen.text.converters.xml_to_byte_string .. autofunction:: kitchen.text.converters.bytes_to_xml .. autofunction:: kitchen.text.converters.xml_to_bytes .. autofunction:: kitchen.text.converters.guess_encoding_to_xml .. autofunction:: kitchen.text.converters.to_xml Working with exception messages =============================== .. autodata:: kitchen.text.converters.EXCEPTION_CONVERTERS .. autodata:: kitchen.text.converters.BYTE_EXCEPTION_CONVERTERS .. autofunction:: kitchen.text.converters.exception_to_unicode .. autofunction:: kitchen.text.converters.exception_to_bytes
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