Example of how to cache Maya outliner (and other maya resources) icons to use in a Qt widget or view.

get_maya_outliner_icons.py

import os
from maya import cmds
from PySide2 import QtWidgets, QtGui, QtCore


class IconCache(object):
    """Simple Icon cache for Maya resources"""
    def __init__(self):
    
        self._paths_cache = {}
        self._icons_cache = {}
        
    def __get_key(self, fname):
        return os.path.splitext(fname)[0]
        
    def refresh(self):
        self._paths_cache.clear()
        self._icons_cache.clear()
        
        # Add internal Maya resources
        for resource in cmds.resourceManager(nameFilter="*"):
            key = self.__get_key(resource)
            if key in self._paths_cache:
                continue
                
            self._paths_cache[key] = ":" + key
            
        # Add icons on XBMLANGPATH
        for root in os.environ.get("XBMLANGPATH", "").split(os.pathsep):
            if not os.path.exists(root):
                continue
            
            _, _, filenames = next(os.walk(root))
            for fname in filenames:
                key = self.__get_key(fname)
                if key in self._paths_cache:
                    continue
                
                self._paths_cache[key] = os.path.join(root, fname)
            
    def get_path(self, name):
        return self._paths_cache.get(name, None)
        
    def get_icon(self, name):
        icon = self._icons_cache.get(name, None)
        if not icon:
            path = self.get_path(name)
            if path:
                icon = QtGui.QIcon(path)
                self._icons_cache[name] = icon
        return icon
        
    def get_outliner_icon(self, node_type):
        """Helper function to get the outliner icon for a node type.
        
        Outliner icons are named "out_{nodetype}" so we search for those
        resources instead. When not found, fall back to "out_default".
        
        """
        
        icon = self.get_icon("out_{}".format(node_type))
        if icon:
            return icon
        else:
            # Default icon
            return self.get_icon("out_default")

        
# Example usage
cache = IconCache()
cache.refresh()

widget = QtWidgets.QListWidget()
widget.setIconSize(QtCore.QSize(20, 20)) # Maya outliner default sizes
for node_type in sorted(cmds.allNodeTypes()):
    
    icon = cache.get_outliner_icon(node_type)
    item = QtWidgets.QListWidgetItem(icon, node_type)
    widget.addItem(item)
    
widget.show()