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FreeCAD Developers Handbook

A handbook about FreeCAD development

Writing Code for Translation #

How to write code with strings that should be translated.

In most cases, any user-visible text in FreeCAD should be made translatable. Exceptions to this general rule are:

  1. Text that only appears as log-level messages in the Report View
  2. Names of Python commands, etc.

Some general guidelines when constructing strings for translation:

Important technical details:

Qt Translation Process #

Qt translation takes place in four steps (only two of which developers need concern themselves with, steps 1 and 4 below):

  1. A source-code pre-processor called lupdate examines the source code, looking for specific strings that mark string literals to be translated. This is not a compiler or interpreter, and does not understand all of the details of C++ or Python code. Its only purpose is to generate a *.ts file that can be uploaded to our translation platform, CrowdIn. That upload is not currently automated, and must be done manually by a CrowdIn Manager. See
  2. Translators on CrowdIn are presented with these strings (plus some metadata about where they are in the code, and optional comments that developers can leave for them). For each language, translators work to develop appropriate translated strings.
  3. A CrowdIn Manager exports those strings, processes them to *.qm files, and commits them to the FreeCAD repository.
  4. When FreeCAD is compiled, those files become part of FreeCAD’s Qt resources and are available to the translation functions at runtime. The translation functions tr() and translate() do the actual work of looking up the string in the translation table and extracting the correct user-visible translation.

lupdate extraction #

The critical first step above is the use of lupdate to extract strings marked for translation. From a developer point of view, the most critical thing to note is that only string literals are extracted. A variable containing a string is not. There are many different functions that can be used to mark strings for extraction: the four most important are:

The most basic usage is:

QPushButton myButton(tr("Do great things"));

In this case, tr() functions both as a marker for the lupdate tool to extract the string literal “Do great things”, as well as an actual function call at runtime to look up the appropriate translated string. The string’s context is the name of the class that this code is exectuded from, provided automatically by the tr() function.

In some cases it’s necessary or useful to split up the string extraction and the translation calls, for example:

// The following vector ALWAYS contains the English language strings, regardless of the current
// language setting. The macro just serves as a marker for lupdate.
std::vector<std::string> optionsToDisplay {
    QT_TRANSLATE_NOOP("WidgetOptionDisplay", "Long bar"),
    QT_TRANSLATE_NOOP("WidgetOptionDisplay", "Short bar"),
    QT_TRANSLATE_NOOP("WidgetOptionDisplay", "Dots"),
    QT_TRANSLATE_NOOP("WidgetOptionDisplay", "Dashes")

// Then later in your code:
QComboBox *optionCombo = getOptionCombo();
for (const auto &option : optionsToDisplay) {
    // option is in English: ask the translate function to look it up in the database
    optionCombo->addItem(translate("WidgetOptionDisplay", option.c_str()));

In that case, the translate() function call cannot provide any information to lupdate because it is not being called on a string literal. Therefore a separate mechanism must be used to provide the string to translators, and translate() only serves to do the final database lookup at runtime.

C++ #

The main translation function for C++ code is tr(), automatically defined in any class derived from QObject. It automatically sets the “context” of the translation to the name of the class, providing disambiguation between the same string in different parts of FreeCAD, where the meaning might be different. It generates a QString out of a string literal:

void SpecialWidget::DoThings()
    this->labelThing.setText(tr("This will get translated")); // Context is "SpecialWidget"

If your class is not derived from QObject, include the Q_DECLARE_TR_FUNCTIONS macro when defining your class to gain the tr() function:

#include <QCoreApplication>

class NotAWidget


Technically tr() can take an additional string: a “comment” that is displayed to translators, but is not itself translated. This is rarely used.

To do string replacement use QString::arg(), for example:

label.setText(tr("Item %1 of %2 confabulated. Please wait...").arg(i).arg(total));

The string “Item %1 of %2 confabulated. Please wait…” will be presented to translators, but CrowdIn will prevent them from removing the placeholders accidentally.

Note that in many places in the FreeCAD source code you will see uses of QT_TR_NOOP - this is only used in cases where the string is later passed through a separate call to tr(). If you aren’t sure, check with a Maintainer, and when in doubt, tr() is probably what you need.

Python #

In your Python code, import FreeCAD, then create a function called translate with the following code:

translate = FreeCAD.Qt.translate

It then gets used similarly to tr() in C++, but with a manually-specified context:

print(translate("MyMod", "This will get translated"))

Note that “translate” is not a true function: it is really a tag that the lupdate utility uses to identify strings for translation. You cannot rename it, or use it with non-literal strings, etc. You also cannot use f-strings:

print(translate(modNameInAVariable, someOtherVariable)) # NO!
print(translate("MyMod", f"This won't {work}")) # NO!

To do argument replacement use the format function of Python’s string class:

print(translate("MyMod", "There are {} widgets present, out of {} total").format(present, total))

(Note that the string sent to translators is “There are {} widgets present, out of {} total” – the format function is called on the string that is returned from the translate function).

Special Cases #

There are several places in FreeCAD where specific procedures must be followed to ensure translations are correct:

Preference Pages #

Whether using addPreferencePage in Python, or new Gui::PrefPageProducer in C++, the “Group” of the page must use the NOOP-functions and the QObject context.

In Python this is:

FreeCADGui.addPreferencePage("somePage.ui", QT_TRANSLATE_NOOP("QObject", "SomeGroupName"))

and in C++,

new Gui::PrefPageProducer<SomeClass> (QT_TRANSLATE_NOOP("QObject","SomeGroupName"));

Exceptions #

C++ exceptions that may end up displayed to the user should be put inside a QT_TRANSLATE_NOOP with the Exception context, e.g.

throw MyException(QT_TRANSLATE_NOOP("Exception", "Something bad happened"));

Commands #

When creating a new command, use the following template:

    sAppModule    = "MyWorkbench";
    sGroup        = "MyWorkbench";
    sMenuText     = QT_TR_NOOP("Do a thing");
    sToolTipText  = QT_TR_NOOP("A longer explanation of doing a thing");
    sWhatsThis    = "MyWorkbench_DoAThing";
    sStatusTip    = sToolTipText;
    sPixmap       = "MyWorkbench_DoAThing";

Note that sGroup should not be translated here (you will still find many counterexamples in older code, however).

If for any reason you need to manually set the name of the command’s menu item or tooltip, you will need to explicitly (and manually) set the context to the name of the class, e.g.:

action->setText(QCoreApplication::translate("MyCommandClass", "Some text to be translated"));

UI Files #

For the most part, all strings in UI files are automatically subject to translation. In some circumstances you may want to disable that translation. Using Qt Designer, uncheck the “translatable” checkbox on the widget that you want to disable translation for.

Translating plurals #

Many languages have complex pluralization rules. To support this, you can write strings that are designed for pluralization, including in English, by specifying a parameter that indicates how many of a thing there are. The translation system will dynamically select the correct translation (assuming translators wrote one), even for English. For example:

int nErrors = getNumberOfErrors(); // Might be zero, might be one, might be some other number.
tr("There were %n errors.", "", nErrors);

Translators will be given the opportunity to translate this string in several variants (depending on the language they are translating for). For example, in English we would have:

There were 0 errors.
There was 1 error.
There were 2 errors.

In other languages there might be more than two variants, the sentence structure might change completely, etc. Qt and the CrowdIn translation platform support this structure.

Testing your strings #

The translation process has several steps, each of which you can examine during the development process.

String extraction #

To determine whether your strings are being correctly extracted, you can directly run the lupdate command on the file you are working on, using a dummy TS output file. lupdate is included in your Qt installation (note that internally we use lupdate from Qt 6.4 – this is particularly important if you are testing Python code, as several major problems were address by a patch we submitted to Qt 6.4).

lupdate MySourceCodeFile.cpp --ts test.ts

will generate the file test.ts – you can examine it to ensure your string appears with the expected context in that file. In particular, make sure that each string makes sense alone: otherwise, without context, translators will not know what they should replace it with.

String replacement #

To determine whether the string is being replaced correctly, you can create a dummy language file from the above test.ts file, and inject it into your FreeCAD instance. To do this you must know the name of the file that the finished translations will be put into. In general this is the name of the module you are working on, an underscore, the language code, and “.ts”. For example, “PartDesign_es-ES.ts” is the Spanish (Spain) translation file for the Part Design workbench. An exception is the code in src/Gui, which is FreeCAD_xx.ts (where xx is the language code).

For testing purposes, choose a language you are familiar with (or at least, can navigate well enough to deactivate when you are done testing!), and copy the file generated by lupdate into the appropriate filename. Locate the string you want to test:

        <location filename="../CommandDoc.cpp" line="1649"/>
        <source>Activates or Deactivates the selected object&apos;s edit mode</source>
        <translation type="unfinished"></translation>

Edit this to eliminate the “unfinished” and to add a translation, e.g.

      <location filename="../CommandDoc.cpp" line="1649"/>
      <source>Activates or Deactivates the selected object's edit mode</source>
      <translation>Activa o desactiva el modo de edición del objeto seleccionado</translation>

Next, this file must be converted to a *.qm file using the lrelease tool (distributed with Qt). Finally, place the generated QM file in the same location as your user.cfg file, in a subdirectory called “translations”, then switch FreeCAD to the language you chose to test with. Your translated string should appear.

Synchronizing Translations with CrowdIn #

This section is for CrowdIn administrators and FreeCAD Maintainers. Approximately once per week, FreeCAD’s source tree should be synchronized with CrowdIn. This is a multi-step process that may be partially automated in the future. For the time being:

  1. Ensure you have at least Qt 6.4 available on your system, and use the script in src/Tools to extract the translatable strings from the FreeCAD source code. You do not need to be compiling FreeCAD with this version of Qt, but you must use it to extract the strings, it addresses many longstanding bugs with earlier extractors. Alternatively, you can also launch gather which will under the hood run update.ts.
  2. Examine the output from that script to ensure there were no errors that prevented some of the workbenches from being extracted.
  3. If all sections of the code completed appropriately, use the in src/Tools with the update subcommand to send the new TS files to CrowdIn. You will need a CrowdIn API key in your home directory for this to work: see the comments at the top of that script for details.
  4. Next, using build (or manually on the CrowdIn website) trigger a build of the current translation files. This process will take a few minutes. You can check the status with build-status, or by visiting the CrowdIn website.
  5. Use download to get a new copy of the translations.
  6. Create a new PR branch using git.
  7. Use apply to move the TS files into place, and to generate the QM files for the parts of the code where that is not yet part of the build process (the Python workbenches).
  8. Create a PR with the changed files.