# Strings (Python)

An explanation of how to deal with strings in Python and GTK+.
Sebastian Pölsterl
sebp@k-d-w.org 2011
Marta Maria Casetti
mmcasetti@gmail.com 2012
Mario Blättermann
mario.blaettermann@gmail.com 2011, 2013, 2016, 2018

# Strings

GNOME strongly encourages the use of Python 3 for writing applications!

# Strings in Python 2

Python 2 comes with two different kinds of objects that can be used to represent strings, `str` and `unicode`. Instances of `unicode` are used to express Unicode strings, whereas instances of the `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_string = u"Fu\u00dfb\u00e4lle" >>> print unicode_string Fußbälle

Unicode strings can be converted to 8-bit strings with `unicode.encode()`. Python’s 8-bit strings have a `str.decode()` method that interprets the string using the given encoding (that is, it is the inverse of the `unicode.encode()`):

>>> type(unicode_string) <type 'unicode'> >>> unicode_string.encode("utf-8") 'Fu\xc3\x9fb\xc3\xa4lle' >>> utf8_string = unicode_string.encode("utf-8") >>> type(utf8_string) <type 'str'> >>> unicode_string == utf8_string.decode("utf-8") True

Unfortunately, Python 2.x allows you to mix `unicode` and `str` if the 8-bit string happened to contain only 7-bit (ASCII) bytes, but would get UnicodeDecodeError if it contained non-ASCII values.

# Strings in Python 3

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 bytes type. Conceptually, `str` refers to text, whereas bytes refers to data. Use `encode()` to go from `str` to `bytes`, and `decode()` to go from `bytes` to `str`.

In addition, it is no longer possible to mix Unicode strings with encoded strings, because it will result in a `TypeError`:

>>> 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'

# 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 `str` 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 convert any 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) <type 'str'>

Furthermore:

>>> txt == unicode_string

would return `False`, with the warning `__main__:1: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal` (`Gtk.Label.get_text()` will always return a `str` instance; therefore, `txt` and `unicode_string` are not equal).

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 `unicode` objects in GTK+ applications at all, and only use UTF-8 encoded `str` objects since GTK+ does not fully integrate with `unicode` objects.

String encoding is more consistent in 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 `str` only.

# Referenzen

How To Deal With Strings - The Python GTK+ 3 Tutorial