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Translation Strings

While you write your software, you can insert specialized markup into your Python code that makes it possible for the system to translate text values into the languages used by your application's users. This markup generates a :term:`translation string`. A translation string is an object that behave mostly like a normal Unicode object, except that it also carries around extra information related to its job as part of a higher-level system's translation machinery.


Using a translation string can be thought of as equivalent to using a "lazy string" object in other i18n systems.

Using The TranslationString Class

The most primitive way to create a translation string is to use the :class:`translationstring.TranslationString` callable:

This creates a Unicode-like object that is a :class:`translationstring.TranslationString`.


For people familiar with Zope internationalization, a TranslationString is a lot like a zope.i18nmessageid.Message object. It is not a subclass, however.

The first argument to :class:`translationstring.TranslationString` is the msgid; it is required. It represents the key into the translation mappings provided by a particular localization. The msgid argument must be a Unicode object or an ASCII string. The msgid may optionally contain replacement markers. For instance:

Within the string above, ${stuff} is a replacement marker. It will be replaced by whatever is in the mapping for a translation string when the :meth:`translationstring.TranslationString.interpolate` method is called. The mapping may be supplied at the same time as the replacement marker itself:

You can also create a new translation string instance with a mapping using the standard python %-operator:

You may interpolate a translation string with a mapping:

The above result will be Add 1.

Any number of replacement markers can be present in the msgid value, any number of times. Only markers which can be replaced by the values in the mapping will be replaced at translation time. The others will not be interpolated and will be output literally.

Replacement markers may also be spelled without squiggly braces:

The Add $number msgid above is equivalent to Add ${number}.

A translation string should also usually carry a domain. The domain represents a translation category to disambiguate it from other translations of the same msgid, in case they conflict.

The above translation string named a domain of form. A translator function (see :ref:`translation_chapter`) will often use the domain to locate the right translator file on the filesystem which contains translations for a given domain. In this case, if it were trying to translate to our msgid to German, it might try to find a translation from a :term:`gettext` file within a :term:`translation directory` like this one:


In other words, it would want to take translations from the translation file in the German language.

Finally, the TranslationString constructor accepts a default argument. If a default argument is supplied, it replaces usages of the msgid as the default value for the translation string. When default is None, the msgid value passed to a TranslationString is used as an implicit message identifier. Message identifiers are matched with translations in translation files, so it is often useful to create translation strings with "opaque" message identifiers unrelated to their default text:

When a default value is used, the default may contain replacement markers and the msgid should not contain replacement markers.

Using the TranslationStringFactory Class

Another way to generate a translation string is to use the :attr:`translationstring.TranslationStringFactory` object. This object is a translation string factory. Basically a translation string factory presets the domain value of any :term:`translation string` generated by using it. For example:


We assigned the translation string factory to the name _. This is a convention which will be supported by translation file generation tools.

After assigning _ to the result of a :func:`translationstring.TranslationStringFactory`, the subsequent result of calling _ will be a :class:`translationstring.TranslationString` instance. Even though a domain value was not passed to _ (as would have been necessary if the :class:`translationstring.TranslationString` constructor were used instead of a translation string factory), the domain attribute of the resulting translation string will be bfg. As a result, the previous code example is completely equivalent (except for spelling) to:

You can set up your own translation string factory much like the one provided above by using the :class:`translationstring.TranslationStringFactory` class. For example, if you'd like to create a translation string factory which presets the domain value of generated translation strings to form, you'd do something like this:


For people familiar with Zope internationalization, a TranslationStringFactory is a lot like a zope.i18nmessageid.MessageFactoy object. It is not a subclass, however.


Translation strings may be pickled and unpickled.

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