code: def register_return_func(func):
      """
      Registers the 'returns_func' that will be called for
      retrieving returns data.
  
      Parameters
      ----------
      func : function
          A function that returns a pandas Series of asset returns.
          The signature of the function must be as follows
  
          >>> func(symbol)
  
          Where symbol is an asset identifier
  
      Returns
      -------
      None
      """
  
      SETTINGS["returns_func"] = func
  
imports: from __future__ import division
  import warnings
  from itertools import cycle
  from matplotlib.pyplot import cm
  import numpy
  import pandas
  from IPython.display import display, HTML
  import empyrical.utils
  from .None import pos
  from .None import txn
label_desc: Use image node_id 15 for calling a global function with example usage: register_return_func(func) without return types