code: def test_delete_default_template(self):
      pio.templates.default = "test_template"
      self.assertEqual(pio.templates.default, "test_template")
  
      del pio.templates["test_template"]
      self.assertIsNone(pio.templates.default)
  
imports: import copy
  from unittest import TestCase
  import pytest
  import plotly.io
  import plotly.graph_objs
  from plotly.tests.utils import TestCaseNoTemplate
label_desc: Use image node_id 17 for calling the TemplateTest obj's underlying member method code with example usage: obj.test_delete_default_template() without return types