How to force a redraw of a panel in the 3D_View

Question

I have a panel in 3D_View that shows the current file and allows the selection of a new file. When the file is selected, the panel needs to redraw. How do I force a redraw?

In Operator.py file

class OT_TestOpenFilebrowser(Operator, ImportHelper): 
    #https://sinestesia.co/blog/tutorials/using-blenders-filebrowser-with-python/
    bl_idname = "file.open_filebrowser" 
    bl_label = "Open the file browser (yay)" 
    filter_glob: StringProperty( default='*.xml', options={'HIDDEN'} )
    run=0
    def execute(self, context): 
        if not 'PMTfileName' in globals():
            global PMTfileName
            PMTfileName = "None2"        
            self.report({'INFO'}, PMTfileName+ " created in op") 
        PMTfileName, extension = os.path.splitext(self.filepath)
        head_tail = os.path.split(self.filepath) 
        PMTfileName = head_tail[1]
        print(PMTfileName)
        self.report({'INFO'}, PMTfileName)
        self.report({'INFO'}, extension)
        self.report({'INFO'}, self.filepath)
        context.area.tag_redraw()
        self.__class__.run += 1
        """Do something with the selected file(s).""" 
        return {'FINISHED'}
bpy.utils.register_class(OT_TestOpenFilebrowser)

If Panel file:

class PMT_PT_Pannel(Panel):
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_label = "PMT Builder"
    bl_category = "21Geo Builder"
def draw(self, context):
    global PMTfileName
    layout = self.layout

    #2 Columns with Buttons
    box = layout.box()
    box.label(text="Open File: "+PMTfileName) 
    row = layout.row()
    row2 = layout.row()

    col3 = row.column()
    col3.operator("file.open_filebrowser", text="Select File" )


    col = row2.column()
    col.operator("object.apply_all_mods", text="Apply all")

    col2 = row2.column()
    col2.operator("export.some_data", text="Export")

Answer 1

I suggest start here: How to create a custom UI?

---

Blender will take care of (re-)drawing the layout properly without doing anything if you add a dedicated StringProperty for your filepath to the Scene, Object or whatever makes sense in your case eg. bpy.types.Scene.my_path = StringProperty(name="My Path") and assign self.filepath attribute from the ImportHelper class to your path property:

Code example based on the famous Operator File Import template that comes with Blender:

import bpy
import os
ImportHelper is a helper class, defines filename and
invoke() function which calls the file selector.
from bpy_extras.io_utils import ImportHelper
from bpy.props import StringProperty, BoolProperty, EnumProperty
from bpy.types import Operator, Panel
class ImportSomeData(Operator, ImportHelper):
    """This appears in the tooltip of the operator and in the generated docs"""
    bl_idname = "import_test.some_data"  # important since its how bpy.ops.import_test.some_data is constructed
    bl_label = "Import XML Data"
# ImportHelper mixin class uses this
filename_ext = ".xml"

filter_glob: StringProperty(
    default="*.xml",
    options={'HIDDEN'},
    maxlen=255,  # Max internal buffer length, longer would be clamped.
)

# List of operator properties, the attributes will be assigned
# to the class instance from the operator settings before calling.
use_setting: BoolProperty(
    name="Example Boolean",
    description="Example Tooltip",
    default=True,
)

def execute(self, context):
    folder, file = os.path.split(self.filepath)
    context.scene.my_path = self.filepath
    print (folder, file)
    return {'FINISHED'}



class OBJECT_PT_CustomPanel(Panel):
    bl_label = "My Panel"
    bl_idname = "OBJECT_PT_custom_panel"
    bl_space_type = "VIEW_3D"

    bl_region_type = "UI"
    bl_category = "Tools"
    bl_context = "objectmode"
    '''
    @classmethod
    def poll(self,context):
        return context.object is not None
    '''
    def draw(self, context):
        layout = self.layout
        scene = context.scene
        layout.prop(scene, "my_path", text="File")
        layout.operator(ImportSomeData.bl_idname)
        layout.separator()
def register():
    bpy.utils.register_class(ImportSomeData)
    bpy.utils.register_class(OBJECT_PT_CustomPanel)
    bpy.types.Scene.my_path = StringProperty(name="My Path")
def unregister():
    bpy.utils.unregister_class(OBJECT_PT_CustomPanel)
    bpy.utils.unregister_class(ImportSomeData)
    del bpy.types.Scene.my_path
if name == "main":
    register()
# test call
#bpy.ops.import_test.some_data('INVOKE_DEFAULT')

Answer 2

bpy.data.screens['Layout'].areas[3].regions[3].tag_redraw()

works in v3.3.

More generally, you can figure out the area and region index you need by typing / iterating though

bpy.data.screens[<screen/tab name>].areas[x].type

bpy.data.screens[<screen/tab name>].areas[x].regions[y].type

in the interactive console since this displays the name of that area/region as they aren't keyed like the screens are.