code: def test_sigmoid():
      s = pd.Series([0, 1, 2, 3, -1])
      out = s.sigmoid()
      assert (out == expit(s)).all()
      assert (s.index == out.index).all()
  
imports: import pandas
  import pytest
  from scipy.special import expit
label_desc: Use image node_id 1 for calling a global function with example usage: test_sigmoid() without return types