class_docstr: A Formula is a model for a mean in a regression model.
  
  It is often given by a sequence of sympy expressions, with the mean
  model being the sum of each term multiplied by a linear regression
  coefficient.
  
  The expressions may depend on additional Symbol instances, giving a
  non-linear regression model.
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code: def _getdtype(self):
      vnames = [str(s) for s in self.design_expr]
      return np.dtype([(n, np.float64) for n in vnames])
  
imports: import itertools
  import warnings
  from string import ascii_letters, digits
  import numpy
  import sympy
  from scipy.linalg import pinv
  from sympy import Dummy, default_sort_key
  from sympy.utilities.lambdify import implemented_function, lambdify
  from nipy.algorithms.utils.matrices import full_rank, matrix_rank
  from nipy.utils import VisibleDeprecationWarning, _NoValue
label_desc: Use image node_id 6 for calling the Formula obj's underlying member method code with example usage: obj._getdtype() and returns: np