Allow python to be initialised without interface or error hook

src/app/qgisapp.cpp

@@ -10511,7 +10511,7 @@ void QgisApp::loadPythonSupport()
   mPythonUtils = pythonlib_inst();
   if ( mPythonUtils )
   {
-    mPythonUtils->initPython( mQgisInterface );
+    mPythonUtils->initPython( mQgisInterface, true );
   }
 
   if ( mPythonUtils && mPythonUtils->isEnabled() )

src/python/qgspythonutils.h

@@ -50,8 +50,15 @@ class PYTHON_EXPORT QgsPythonUtils
     //! returns TRUE if Python support is ready to use (must be inited first)
     virtual bool isEnabled() = 0;
 
-    //! initialize Python and import bindings
-    virtual void initPython( QgisInterface *iface ) = 0;
+    /**
+     * Initialize Python and import bindings.
+     *
+     * The \a iface argument should be set to an instance of the QGIS interface, or
+     * NULLPTR if no interface is available.
+     *
+     * If \a installErrorHook is true then the custom QGIS GUI error hook will be used.
+     */
+    virtual void initPython( QgisInterface *iface, bool installErrorHook ) = 0;
 
 #ifdef HAVE_SERVER_PYTHON_PLUGINS
     //! initialize Python and import server bindings

src/python/qgspythonutilsimpl.cpp

@@ -39,9 +39,6 @@ PyThreadState *_mainState = nullptr;
 
 QgsPythonUtilsImpl::QgsPythonUtilsImpl()
 {
-  mMainModule = nullptr;
-  mMainDict = nullptr;
-  mPythonEnabled = false;
 }
 
 QgsPythonUtilsImpl::~QgsPythonUtilsImpl()
@@ -208,23 +205,29 @@ void QgsPythonUtilsImpl::doCustomImports()
   }
 }
 
-void QgsPythonUtilsImpl::initPython( QgisInterface *interface )
+void QgsPythonUtilsImpl::initPython( QgisInterface *interface, const bool installErrorHook )
 {
   init();
   if ( !checkSystemImports() )
   {
     exitPython();
     return;
   }
-  // initialize 'iface' object
-  runString( "qgis.utils.initInterface(" + QString::number( ( quint64 ) interface ) + ')' );
+
+  if ( interface )
+  {
+    // initialize 'iface' object
+    runString( "qgis.utils.initInterface(" + QString::number( ( quint64 ) interface ) + ')' );
+  }
+
   if ( !checkQgisUser() )
   {
     exitPython();
     return;
   }
   doCustomImports();
-  installErrorHook();
+  if ( installErrorHook )
+    QgsPythonUtilsImpl::installErrorHook();
   finish();
 }
 
@@ -265,7 +268,8 @@ bool QgsPythonUtilsImpl::startServerPlugin( QString packageName )
 
 void QgsPythonUtilsImpl::exitPython()
 {
-  uninstallErrorHook();
+  if ( mErrorHookInstalled )
+    uninstallErrorHook();
   Py_Finalize();
   mMainModule = nullptr;
   mMainDict = nullptr;
@@ -281,6 +285,7 @@ bool QgsPythonUtilsImpl::isEnabled()
 void QgsPythonUtilsImpl::installErrorHook()
 {
   runString( QStringLiteral( "qgis.utils.installErrorHook()" ) );
+  mErrorHookInstalled = true;
 }
 
 void QgsPythonUtilsImpl::uninstallErrorHook()

src/python/qgspythonutilsimpl.h

@@ -36,8 +36,7 @@ class QgsPythonUtilsImpl : public QgsPythonUtils
 
     /* general purpose functions */
 
-    //! initialize Python and import bindings
-    void initPython( QgisInterface *interface ) override;
+    void initPython( QgisInterface *interface, bool installErrorHook ) override;
 
 #ifdef HAVE_SERVER_PYTHON_PLUGINS
     //! initialize Python for server and import bindings
@@ -156,7 +155,11 @@ class QgsPythonUtilsImpl : public QgsPythonUtils
     PyObject *mMainDict = nullptr;
 
     //! flag determining that Python support is enabled
-    bool mPythonEnabled;
+    bool mPythonEnabled = false;
+
+  private:
+
+    bool mErrorHookInstalled = false;
 };
 
 #endif