Use explicit finite difference instead of scipy.misc.derivative in pvsyst_temperature_coeff (#1674)

* use explicit finite difference instead of scipy.misc.derivative

* whatsnew

* private function in tools.py

Co-Authored-By: Mark Mikofski <bwana.marko@yahoo.com>

---------

Co-authored-by: Mark Mikofski <bwana.marko@yahoo.com>

Author: kandersolar

docs/sphinx/source/whatsnew/v0.9.5.rst

@@ -32,6 +32,8 @@ Bug fixes
 * When using ``utc_time_range`` with :py:func:`pvlib.iotools.read_ecmwf_macc`,
   the time index subset is now selected with ``nearest`` instead of ``before``
   and ``after`` for consistency with ``cftime>=1.6.0``. (:issue:`1609`, :pull:`1656`)
+* :py:func:`~pvlib.ivtools.sdm.pvsyst_temperature_coeff` no longer raises
+  a scipy deprecation warning (and is slightly more accurate) (:issue:`1644`, :pull:`1674`)
 
 
 Testing

pvlib/ivtools/sdm.py

@@ -11,14 +11,15 @@
 from scipy import constants
 from scipy import optimize
 from scipy.special import lambertw
-from scipy.misc import derivative
 
 from pvlib.pvsystem import calcparams_pvsyst, singlediode, v_from_i
 from pvlib.singlediode import bishop88_mpp
 
 from pvlib.ivtools.utils import rectify_iv_curve, _numdiff
 from pvlib.ivtools.sde import _fit_sandia_cocontent
 
+from pvlib.tools import _first_order_centered_difference
+
 
 CONSTANTS = {'E0': 1000.0, 'T0': 25.0, 'k': constants.k, 'q': constants.e}
 
@@ -1344,5 +1345,6 @@ def maxp(temp_cell, irrad_ref, alpha_sc, gamma_ref, mu_gamma, I_L_ref,
             I_o_ref, R_sh_ref, R_sh_0, R_s, cells_in_series, R_sh_exp, EgRef,
             temp_ref)
     pmp = maxp(temp_ref, *args)
-    gamma_pdc = derivative(maxp, temp_ref, args=args)
+    gamma_pdc = _first_order_centered_difference(maxp, x0=temp_ref, args=args)
+
     return gamma_pdc / pmp

pvlib/tools.py

@@ -458,3 +458,14 @@ def _degrees_to_index(degrees, coordinate):
         index = int(np.around(index))
 
     return index
+
+
+EPS = np.finfo('float64').eps  # machine precision NumPy-1.20
+DX = EPS**(1/3)  # optimal differential element
+
+
+def _first_order_centered_difference(f, x0, dx=DX, args=()):
+    # simple replacement for scipy.misc.derivative, which is scheduled for
+    # removal in scipy 1.12.0
+    df = f(x0+dx, *args) - f(x0-dx, *args)
+    return df / 2 / dx