4. How to Deal With Strings¶
This section explains how strings are represented in Python 2.x, Python 3.x and GTK+ and discusses common errors that arise when working with strings.
Conceptionally, a string is a list of characters such as ‘A’, ‘B’, ‘C’ or ‘É’. Characters are abstract representations and their meaning depends on the language and context they are used in. The Unicode standard describes how characters are represented by code points. For example the characters above are represented with the code points U+0041, U+0042, U+0043, and U+00C9, respectively. Basically, code points are numbers in the range from 0 to 0x10FFFF.
As mentioned earlier, the representation of a string as a list of code points is abstract. In order to convert this abstract representation into a sequence of bytes the Unicode string must be encoded. The simplest form of encoding is ASCII and is performed as follows:
- If the code point is < 128, each byte is the same as the value of the code point.
- If the code point is 128 or greater, the Unicode string can’t be represented
in this encoding. (Python raises a
UnicodeEncodeErrorexception in this case.)
Although ASCII encoding is simple to apply it can only encode for 128 different characters which is hardly enough. One of the most commonly used encodings that addresses this problem is UTF-8 (it can handle any Unicode code point). UTF stands for “Unicode Transformation Format”, and the ‘8’ means that 8-bit numbers are used in the encoding.
4.2. Python 2¶
4.2.1. Python 2.x’s Unicode Support¶
Python 2 comes with two different kinds of objects that can be used to represent strings,
Instances of the latter are used to express Unicode strings, whereas instances of
str type are byte representations (the encoded string).
Under the hood, Python represents Unicode strings as either 16-
or 32-bit integers, depending on how the Python interpreter was compiled.
Unicode strings can be converted to 8-bit strings
>>> unicode_string = u"Fu\u00dfb\u00e4lle" >>> print unicode_string Fußbälle >>> type(unicode_string) <type 'unicode'> >>> unicode_string.encode("utf-8") 'Fu\xc3\x9fb\xc3\xa4lle'
Python’s 8-bit strings have a
str.decode() method that
interprets the string using the given encoding:
>>> utf8_string = unicode_string.encode("utf-8") >>> type(utf8_string) <type 'str'> >>> u2 = utf8_string.decode("utf-8") >>> unicode_string == u2 True
Unfortunately, Python 2.x allows you to mix
if the 8-bit string happened to contain only 7-bit (ASCII) bytes, but would
UnicodeDecodeError if it contained non-ASCII values:
>>> utf8_string = " sind rund" >>> unicode_string + utf8_string u'Fu\xdfb\xe4lle sind rund' >>> utf8_string = " k\xc3\xb6nnten rund sein" >>> print utf8_string könnten rund sein >>> unicode_string + utf8_string Traceback (most recent call last): File "<stdin>", line 1, in <module> UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 2: ordinal not in range(128)
4.2.2. Unicode in GTK+¶
GTK+ uses UTF-8 encoded strings for all text. This means that if you call a
method that returns a string you will always obtain an instance of the
type. The same applies to methods that expect one or more strings as parameter,
they must be UTF-8 encoded. However, for convenience PyGObject will automatically
unicode instance to
str if supplied as argument:
>>> from gi.repository import Gtk >>> label = Gtk.Label() >>> unicode_string = u"Fu\u00dfb\u00e4lle" >>> label.set_text(unicode_string) >>> txt = label.get_text() >>> type(txt), txt (<type 'str'>, 'Fu\xc3\x9fb\xc3\xa4lle') >>> txt == unicode_string __main__:1: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal False
Note the warning at the end. Although we called
unicode instance as argument,
Gtk.Label.get_text() will always
str instance. Accordingly,
This is especially important if you want to internationalize your
program using gettext. You
have to make sure that gettext will return UTF-8 encoded 8-bit strings for all
languages. In general it is recommended to not use
in GTK+ applications at all and only use UTF-8 encoded
str objects since
GTK+ does not fully integrate with
unicode objects. Otherwise, you would
have to decode the return values to Unicode strings each time you call a GTK+ method:
>>> txt = label.get_text().decode("utf-8") >>> txt == unicode_string True
4.3. Python 3¶
4.3.1. Python 3.x’s Unicode support¶
Since Python 3.0, all strings are stored as Unicode in an instance of the
str type. Encoded strings on the other hand are represented as
binary data in the form of instances of the
str refers to text, whereas
bytes refers to
str.encode() to go from
bytes.decode() to go from
In addition, it is no longer possible to mix Unicode strings with encoded strings,
because it will result in a
>>> text = "Fu\u00dfb\u00e4lle" >>> data = b" sind rund" >>> text + data Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: Can't convert 'bytes' object to str implicitly >>> text + data.decode("utf-8") 'Fußbälle sind rund' >>> text.encode("utf-8") + data b'Fu\xc3\x9fb\xc3\xa4lle sind rund'
4.3.2. Unicode in GTK+¶
As a consequence, things are much cleaner and consistent with Python 3.x, because
PyGObject will automatically encode/decode to/from UTF-8 if you pass a string to
a method or a method returns a string. Strings, or text, will always be represented
as instances of
>>> from gi.repository import Gtk >>> label = Gtk.Label() >>> text = "Fu\u00dfb\u00e4lle" >>> label.set_text(text) >>> txt = label.get_text() >>> type(txt), txt (<class 'str'>, 'Fußbälle') >>> txt == text True
What’s new in Python 3.0 describes the new concepts that clearly distinguish between text and data.
The Unicode HOWTO discusses Python 2.x’s support for Unicode, and explains various problems that people commonly encounter when trying to work with Unicode.
The Unicode HOWTO for Python 3.x discusses Unicode support in Python 3.x.
UTF-8 encoding table and Unicode characters contains a list of Unicode code points and their respective UTF-8 encoding.