Unicode

Since early Python 2 days unicode was part of all default Python builds. It allows developers to write applications that deal with non-ASCII characters in a straightforward way. But working with unicode requires a basic knowledge about that matter, especially when working with libraries that do not support it.

Werkzeug uses unicode internally everywhere text data is assumed, even if the HTTP standard is not unicode aware as it. Basically all incoming data is decoded from the charset specified (per default utf-8) so that you don’t operate on bytestrings any more. Outgoing unicode data is then encoded into the target charset again.

Unicode in Python

In Python 2 there are two basic string types: str and unicode. str may carry encoded unicode data but it’s always represented in bytes whereas the unicode type does not contain bytes but charpoints. What does this mean? Imagine you have the German Umlaut ö. In ASCII you cannot represent that character, but in the latin-1 and utf-8 character sets you can represent it, but they look differently when encoded:

>>> u'ö'.encode('latin1')
'\xf6'
>>> u'ö'.encode('utf-8')
'\xc3\xb6'

So an ö might look totally different depending on the encoding which makes it hard to work with it. The solution is using the unicode type (as we did above, note the u prefix before the string). The unicode type does not store the bytes for ö but the information, that this is a LATIN SMALL LETTER O WITH DIAERESIS.

Doing len(u'ö') will always give us the expected “1” but len('ö') might give different results depending on the encoding of 'ö'.

Unicode in HTTP

The problem with unicode is that HTTP does not know what unicode is. HTTP is limited to bytes but this is not a big problem as Werkzeug decodes and encodes for us automatically all incoming and outgoing data. Basically what this means is that data sent from the browser to the web application is per default decoded from an utf-8 bytestring into a unicode string. Data sent from the application back to the browser that is not yet a bytestring is then encoded back to utf-8.

Usually this “just works” and we don’t have to worry about it, but there are situations where this behavior is problematic. For example the Python 2 IO layer is not unicode aware. This means that whenever you work with data from the file system you have to properly decode it. The correct way to load a text file from the file system looks like this:

f = file('/path/to/the_file.txt', 'r')
try:
    text = f.decode('utf-8')    # assuming the file is utf-8 encoded
finally:
    f.close()

There is also the codecs module which provides an open function that decodes automatically from the given encoding.

Error Handling

Functions that do internal encoding or decoding accept an errors keyword argument that is passed to str.decode() and str.encode(). The default is 'replace' so that errors are easy to spot. It might be useful to set it to 'strict' in order to catch the error and report the bad data to the client.

Request and Response Objects

As request and response objects usually are the central entities of Werkzeug powered applications you can change the default encoding Werkzeug operates on by subclassing these two classes. For example you can easily set the application to utf-7 and strict error handling:

from werkzeug.wrappers import BaseRequest, BaseResponse

class Request(BaseRequest):
    charset = 'utf-7'
    encoding_errors = 'strict'

class Response(BaseResponse):
    charset = 'utf-7'

Keep in mind that the error handling is only customizable for all decoding but not encoding. If Werkzeug encounters an encoding error it will raise a UnicodeEncodeError. It’s your responsibility to not create data that is not present in the target charset (a non issue with all unicode encodings such as utf-8).

The Filesystem

Up until version 0.11, Werkzeug used Python’s stdlib functionality to detect the filesystem encoding. However, several bug reports against Werkzeug have shown that the value of sys.getfilesystemencoding() cannot be trusted under traditional UNIX systems. The usual problems come from misconfigured systems, where LANG and similar environment variables are not set. In such cases, Python would default to ASCII as filesystem encoding, a very conservative default that is usually wrong and causes more problems than it avoids.

Therefore Werkzeug will force the filesystem encoding to UTF-8 and issue a warning whenever it detects that it is running under BSD or Linux, and sys.getfilesystemencoding() is returning an ASCII encoding.

See also werkzeug.filesystem.