Python: Create a default BuildSystem for Python targets

This makes sure the project can be parsed even if we do not have a valid
build configuration for that target.

Amends 09e94ae4ac

Change-Id: I92214474f581af228bd5c2aaf2f3e4b620ffc9d5
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/python/pythonbuildsystem.cpp

@@ -129,6 +129,16 @@ PythonBuildSystem::PythonBuildSystem(PythonBuildConfiguration *buildConfig)
     requestParse();
 }
 
+PythonBuildSystem::PythonBuildSystem(ProjectExplorer::Target *target)
+    : BuildSystem(target)
+{
+    connect(project(),
+            &Project::projectFileIsDirty,
+            this,
+            &PythonBuildSystem::requestDelayedParse);
+    requestParse();
+}
+
 bool PythonBuildSystem::supportsAction(Node *context, ProjectAction action, const Node *node) const
 {
     if (node->asFileNode())  {

src/plugins/python/pythonbuildsystem.h

@@ -13,6 +13,7 @@ class PythonBuildSystem : public ProjectExplorer::BuildSystem
 {
 public:
     explicit PythonBuildSystem(PythonBuildConfiguration *buildConfig);
+    explicit PythonBuildSystem(ProjectExplorer::Target *target);
 
     bool supportsAction(ProjectExplorer::Node *context,
                         ProjectExplorer::ProjectAction action,

src/plugins/python/pythonproject.cpp

@@ -23,6 +23,8 @@ PythonProject::PythonProject(const FilePath &fileName)
     setId(PythonProjectId);
     setProjectLanguages(Context(ProjectExplorer::Constants::PYTHON_LANGUAGE_ID));
     setDisplayName(fileName.completeBaseName());
+
+    setBuildSystemCreator([](Target *t) { return new PythonBuildSystem(t); });
 }
 
 Project::RestoreResult PythonProject::fromMap(const Store &map, QString *errorMessage)