Python QGIS (PyQGIS) visibility preset manipulation functions

vispres.py

def activate_visibility_preset(preset_name, *, project=None, coll=None):
  """Activates a visibility preset by its name.
  Return a boolean indicating success."""
  # https://github.com/qgis/QGIS/blob/f044c95fd8927d86967ce8af3930bdc7523095fa/src/app/qgsmapthemes.cpp#L138
  if project is None:
    project = qgis.core.QgsProject.instance()
  if coll is None:
    coll = project.mapThemeCollection()
  if not coll.hasMapTheme(preset_name):
    return False
  root = project.layerTreeRoot()
  model = qgis.utils.iface.layerTreeView().model()
  coll.applyTheme(preset_name, root, model)
  return True

def active_visibility_preset(*, project=None, coll=None):
  """Return the name of the active visibility preset or
  None if no active visibility preset."""
  # https://github.com/qgis/QGIS/blob/f044c95fd8927d86967ce8af3930bdc7523095fa/src/app/qgsmapthemes.cpp#L210
  if project is None:
    project = qgis.core.QgsProject.instance()
  if coll is None:
    coll = project.mapThemeCollection()
  root = project.layerTreeRoot()
  model = qgis.utils.iface.layerTreeView().model()
  curr_mapthemecollection = coll.createThemeFromCurrentState(root, model)
  for preset_name in visibility_presets(project=project, coll=coll):
    checking_preset = coll.mapThemeState(preset_name)
    if checking_preset == curr_mapthemecollection:
      return preset_name

def visibility_presets(*, project=None, coll=None):
  """Return the names of the visibility presets in a list."""
  # https://github.com/qgis/QGIS/blob/f044c95fd8927d86967ce8af3930bdc7523095fa/src/app/qgsmapthemes.cpp#L149
  if project is None:
    project = qgis.core.QgsProject.instance()
  if coll is None:
    coll = project.mapThemeCollection()
  preset_names = coll.mapThemes()
  return preset_names

Usage:

>>> visibility_presets()
['Test-01']
>>> activate_visibility_preset('Test-01')
True
>>> active_visibility_preset()
'Test-01'

Tested on QGIS version 3.10.14

Code on GIS.StackExchange (if you star my code here I would appreciate an upvote there too)

Note that you might want to refresh the canvas after changing visibility preset:

qgis.utils.iface.mapCanvas().refreshAllLayers()