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Replace interp1d #2394 #2741

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8773810
Replace interp1d #2394
jason-rpkt Apr 23, 2026
ab6c74f
Replace CubicSpline by make_interp_spline k=3
jason-rpkt Apr 24, 2026
c658199
clean up/ simplify/move import to top
jason-rpkt Apr 27, 2026
486cf38
remove list input
jason-rpkt May 5, 2026
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8 changes: 4 additions & 4 deletions docs/examples/shading/plot_partial_module_shading_simple.py
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Expand Up @@ -38,7 +38,7 @@
from pvlib import pvsystem, singlediode
import pandas as pd
import numpy as np
from scipy.interpolate import interp1d
from scipy.interpolate import make_interp_spline
import matplotlib.pyplot as plt

from scipy.constants import e as qe, k as kB
Expand Down Expand Up @@ -178,9 +178,9 @@ def plot_curves(dfs, labels, title):


def interpolate(df, i):
"""convenience wrapper around scipy.interpolate.interp1d"""
f_interp = interp1d(np.flipud(df['i']), np.flipud(df['v']), kind='linear',
fill_value='extrapolate')
"""convenience wrapper around scipy.interpolate"""
f_interp = make_interp_spline(np.flipud(df['i']), np.flipud(df['v']), k=1)

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@adriesse adriesse May 6, 2026

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This is one where performance is likely important. Maybe check if np.interp is faster?

return f_interp(i)


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25 changes: 18 additions & 7 deletions pvlib/iam.py
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Expand Up @@ -13,6 +13,7 @@
import functools
from scipy.optimize import minimize
from pvlib.tools import cosd, sind, acosd
from scipy.interpolate import make_interp_spline

# a dict of required parameter names for each IAM model
# keys are the function names for the IAM models
Expand Down Expand Up @@ -440,7 +441,7 @@ def interp(aoi, theta_ref, iam_ref, method='linear', normalize=True):
method : str, default 'linear'
Specifies the interpolation method.
Useful options are: 'linear', 'quadratic', 'cubic'.
See scipy.interpolate.interp1d for more options.
See scipy.interpolate for more options.
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@kandersolar kandersolar Apr 24, 2026

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This line may need to be edited, depending on https://github.com/pvlib/pvlib-python/pull/2741/changes#r3137773820


normalize : boolean, default True
When true, the interpolated values are divided by the interpolated
Expand Down Expand Up @@ -469,9 +470,10 @@ def interp(aoi, theta_ref, iam_ref, method='linear', normalize=True):
pvlib.iam.sapm
'''
# Contributed by Anton Driesse (@adriesse), PV Performance Labs. July, 2019

from scipy.interpolate import interp1d

if isinstance(theta_ref, list):
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@cwhanse cwhanse May 5, 2026

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We normally don't check the type of input. If a user inputs a non-allowed type e.g. a list, make_interp_spline will emit an error. That's OK.

raise TypeError("theta_ref cannot be a list")
if isinstance(iam_ref, list):
raise TypeError("iam_ref cannot be a list")
# Scipy doesn't give the clearest feedback, so check number of points here.
MIN_REF_VALS = {'linear': 2, 'quadratic': 3, 'cubic': 4, 1: 2, 2: 3, 3: 4}
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@adriesse adriesse May 6, 2026

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You could add a dictionary with names and k values, or something like that, to use below.


Expand All @@ -483,10 +485,19 @@ def interp(aoi, theta_ref, iam_ref, method='linear', normalize=True):
raise ValueError("Negative value(s) found in 'iam_ref'. "
"This is not physically possible.")

interpolator = interp1d(theta_ref, iam_ref, kind=method,
fill_value='extrapolate')
aoi_input = aoi
if method == "linear":
interpolator = make_interp_spline(theta_ref, iam_ref, k=1)

elif method == "quadratic":
interpolator = make_interp_spline(theta_ref, iam_ref, k=2)

elif method == "cubic":
interpolator = make_interp_spline(theta_ref, iam_ref, k=3)

else:
raise ValueError(f"Invalid interpolation method '{method}'.")
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@kandersolar kandersolar Apr 24, 2026

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This is, technically, a breaking change, since the interp1d way also supported 'nearest', 'nearest-up', 'zero', 'slinear', 'previous', and 'next'. I doubt these got much use, if any. Any thoughts on how to handle that?

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@jason-rpkt jason-rpkt Apr 27, 2026

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I'm not sure I know how to update to support all of those.
Is it OK to update the error message?

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@cwhanse cwhanse May 5, 2026

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Since interp1d is legacy but not intended for removal, it is still available and we could fall back:

elif method in {'nearest', 'nearest-up', 'zero', 'slinear', 'previous', 'next'}:
    interpolator = interp1d(theta_ref, iam_ref, kind=method,
                               fill_value='extrapolate')

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@kandersolar kandersolar May 6, 2026

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I think that's reasonable, as long as we deprecate these weird methods too. If someone really wants these special interpolations, they can do it themselves and provide a pre-calculated input.

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@adriesse adriesse May 6, 2026

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None of those options really make any sense here. I would vote for dropping them immediately.

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@cwhanse cwhanse May 5, 2026

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Suggested change
raise ValueError(f"Invalid interpolation method '{method}'.")
raise ValueError(f"Interpolation method '{method}' is not supported in pvlib-python.")


aoi_input = aoi
aoi = np.asanyarray(aoi)
aoi = np.abs(aoi)
iam = interpolator(aoi)
Expand Down
16 changes: 7 additions & 9 deletions pvlib/spectrum/response.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@
import numpy as np
import pandas as pd
import scipy.constants
from scipy.interpolate import interp1d
from scipy.interpolate import make_interp_spline


_PLANCK_BY_LIGHT_SPEED_OVER_ELEMENTAL_CHARGE_BY_BILLION = (
Expand Down Expand Up @@ -66,16 +66,14 @@ def get_example_spectral_response(wavelength=None):
if wavelength is None:
resolution = 5.0
wavelength = np.arange(280, 1200 + resolution, resolution)
x = SR_DATA[0]
y = SR_DATA[1]
spline = make_interp_spline(x, y, k=3)

interpolator = interp1d(SR_DATA[0], SR_DATA[1],
kind='cubic',
bounds_error=False,
fill_value=0.0,
copy=False,
assume_sorted=True)

sr = pd.Series(data=interpolator(wavelength), index=wavelength)
values = spline(wavelength)
values[(wavelength < x[0]) | (wavelength > x[-1])] = 0.0

sr = pd.Series(data=values, index=wavelength)
sr.index.name = 'wavelength'
sr.name = 'spectral_response'

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46 changes: 40 additions & 6 deletions tests/test_iam.py
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Expand Up @@ -171,8 +171,8 @@ def test_martin_ruiz_diffuse():

def test_iam_interp():

aoi_meas = [0.0, 45.0, 65.0, 75.0]
iam_meas = [1.0, 0.9, 0.8, 0.6]
aoi_meas = np.array([0.0, 45.0, 65.0, 75.0])
iam_meas = np.array([1.0, 0.9, 0.8, 0.6])

# simple default linear method
aoi = 55.0
Expand Down Expand Up @@ -200,18 +200,52 @@ def test_iam_interp():
assert_series_equal(iam, expected)

# check beyond reference values
aoi = [-45, 0, 45, 85, 90, 95, 100, 105, 110]
expected = [0.9, 1.0, 0.9, 0.4, 0.3, 0.2, 0.1, 0.0, 0.0]
aoi = np.array([-45, 0, 45, 85, 90, 95, 100, 105, 110])
expected = np.array([0.9, 1.0, 0.9, 0.4, 0.3, 0.2, 0.1, 0.0, 0.0])
iam = _iam.interp(aoi, aoi_meas, iam_meas)
assert_allclose(iam, expected)

# check exception clause
with pytest.raises(ValueError):
_iam.interp(0.0, [0], [1])
_iam.interp(0.0, np.array([0]), np.array([1]))

# check exception clause
with pytest.raises(ValueError):
_iam.interp(0.0, [0, 90], [1, -1])
_iam.interp(0.0, np.array([0, 90]), np.array([1, -1]))

# check linear after updating interp1d
theta_ref = np.array([0, 60, 90])
iam_ref = np.array([1.0, 0.8, 0.0])

aoi = np.array([0, 30, 60])
iam = _iam.interp(
aoi, theta_ref, iam_ref,
method="linear", normalize=False)
expected = np.array([1.0, 0.9, 0.8])
np.testing.assert_allclose(iam, expected)

# check quadratic
theta_ref = np.array([0, 30, 60, 90])
iam_ref = 1.0 - 1e-4 * theta_ref**2
aoi = np.array([15, 45, 75])
iam = _iam.interp(
aoi,
theta_ref,
iam_ref,
method="quadratic",
normalize=False
)

expected = 1.0 - 1e-4 * aoi**2
np.testing.assert_allclose(iam, expected, rtol=1e-12)

# check exception clause - list input for theta_ref
with pytest.raises(TypeError):
_iam.interp(0.0, [0, 60, 90], np.array([1.0, 0.8, 0.0]))

# check exception clause - list input for iam_ref
with pytest.raises(TypeError):
_iam.interp(0.0, np.array([0, 60, 90]), [1.0, 0.8, 0.0])


@pytest.mark.parametrize('aoi,expected', [
Expand Down
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