BUG: scipy.stats.theilslopes returns invalid data when input data is np.uint8

[korypostma]

Describe your issue.

``

Expected result:
linregress = LinregressResult(slope=-0.08117408906882585, intercept=212.77543859649109, rvalue=-0.14521616256311687, pvalue=0.3777456213641338, stderr=0.09092294431311998, intercept_stderr=182.21245386044583) theilslopes = (0.0, 52.0, -0.2, 0.1388888888888889) siegelslopes= (-0.07448275862068965, 201.11448275862068) linregress = LinregressResult(slope=0.7799595141700404, intercept=-1526.3721997300943, rvalue=0.648915022489458, pvalue=7.885128179731413e-06, stderr=0.1503448757858782, intercept_stderr=301.2958824556652) theilslopes = (0.6923076923076923, -1350.3846153846152, 0.41379310344827586, 1.2307692307692308) siegelslopes= (0.980072463768116, -1927.0851449275362) linregress = LinregressResult(slope=-1.130971659919028, intercept=2291.15951417004, rvalue=-0.7989778176023127, pvalue=1.0820717823130094e-09, stderr=0.13994295315037716, intercept_stderr=280.4501007599699) theilslopes = (-1.0434782608695652, 2123.1304347826085, -1.3333333333333333, -0.5) siegelslopes= (-1.0947293447293447, 2218.258547008547) linregress = LinregressResult(slope=0.02186234817813765, intercept=7.469905533063432, rvalue=0.07626971562528158, pvalue=0.6444538083463693, stderr=0.04698691899214995, intercept_stderr=94.16327059776272) theilslopes = (0.0, 52.0, 0.0, 0.0) siegelslopes= (0.0, 52.0)

Current result:
linregress = LinregressResult(slope=-0.08117408906882585, intercept=212.77543859649109, rvalue=-0.14521616256311687, pvalue=0.3777456213641338, stderr=0.09092294431311998, intercept_stderr=182.21245386044583) theilslopes = (5.5, -10970.0, 1.0909090909090908, 11.272727272727273) siegelslopes= (-0.027777777777777776, 107.36111111111111) linregress = LinregressResult(slope=0.7799595141700404, intercept=-1526.3721997300943, rvalue=0.648915022489458, pvalue=7.885128179731413e-06, stderr=0.1503448757858782, intercept_stderr=301.2958824556652) theilslopes = (1.6666666666666667, -3303.0, 1.2857142857142858, 2.1333333333333333) siegelslopes= (0.04999999999999999, -62.39999999999998) linregress = LinregressResult(slope=-1.130971659919028, intercept=2291.15951417004, rvalue=-0.7989778176023127, pvalue=1.0820717823130094e-09, stderr=0.13994295315037716, intercept_stderr=280.4501007599699) theilslopes = (11.333333333333334, -22680.0, 8.73076923076923, 14.125) siegelslopes= (-0.1539473684210526, 339.12499999999994) linregress = LinregressResult(slope=0.02186234817813765, intercept=7.469905533063432, rvalue=0.07626971562528158, pvalue=0.6444538083463693, stderr=0.04698691899214995, intercept_stderr=94.16327059776272) theilslopes = (0.3333333333333333, -616.0, 0.07142857142857142, 0.625) siegelslopes= (0.0, 52.0)

Reproducing Code Example

import numpy as np
from scipy import stats
import matplotlib.pyplot as plt

def plot_theil(xin, yin, png_path='out_theilslopes.png'):
    xin = xin.astype(np.int16)
    yin = yin.astype(np.uint8)
    #yin = yin.astype(float)
    lsq_res = stats.linregress(xin, yin)
    res = stats.theilslopes(yin, xin)
    sie_res = stats.siegelslopes(yin, xin)

    fig = plt.figure()
    ax = fig.add_subplot(111)
    ax.plot(xin, yin, 'b.')
    ax.plot(xin, res[1] + res[0] * xin, 'r-')
    ax.plot(xin, sie_res[1] + sie_res[0] * xin, 'b-')
    ax.plot(xin, lsq_res[1] + lsq_res[0] * xin, 'g-')

    print('linregress = ', lsq_res)
    print('theilslopes = ', res)
    print('siegelslopes= ', sie_res)

    #plt.show()
    plt.savefig(png_path, bbox_inches='tight')

xin = np.array(range(1985,2023+1))
yin = np.array([46,46,51,54,49,50,52,53,52,54,56,53,56,57,57,55,53,52,51,49,55,54,38,44,37,40,31,38,45,50,53,58,57,53,56,55,51,48,45])
plot_theil(xin, yin, 'out_theilslopes.png')

yin = np.array([34,41,38,37,35,41,25,38,17,29,14,13,14,13,21,15,27,25,36,32,42,33,38,33,31,29,43,53,53,53,53,53,53,53,53,53,53,53,53])
plot_theil(xin, yin, 'out_theilslopes_1.png')
yin = np.array([55,50,40,50,49,35,37,42,34,41,32,13,11,13,17,38,36,33,33,32,38,37,37,40,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8])
plot_theil(xin, yin, 'out_theilslopes_2.png')
yin = np.array([52,52,52,52,52,52,52,51,54,54,44,40,55,52,52,52,52,52,50,51,46,46,47,61,50,49,54,52,52,52,52,52,52,52,52,52,52,52,52])
plot_theil(xin, yin, 'out_theilslopes_3.png')

Error message

No error

SciPy/NumPy/Python version and system information

import sys, scipy, numpy; print(scipy.__version__, numpy.__version__, sys.version_info); scipy.show_config()
1.8.1 1.22.4 sys.version_info(major=3, minor=10, micro=4, releaselevel='final', serial=0)
lapack_mkl_info:
  NOT AVAILABLE
openblas_lapack_info:
  NOT AVAILABLE
openblas_clapack_info:
  NOT AVAILABLE
flame_info:
  NOT AVAILABLE
accelerate_info:
  NOT AVAILABLE
atlas_3_10_threads_info:
  NOT AVAILABLE
atlas_3_10_info:
  NOT AVAILABLE
atlas_threads_info:
  NOT AVAILABLE
atlas_info:
  NOT AVAILABLE
lapack_info:
    libraries = ['lapack', 'blas', 'lapack', 'blas']
    library_dirs = ['C:/Users/kpostma/Anaconda3/envs/rcmap\\Library\\lib']
    language = f77
blas_mkl_info:
  NOT AVAILABLE
blis_info:
  NOT AVAILABLE
openblas_info:
  NOT AVAILABLE
atlas_3_10_blas_threads_info:
  NOT AVAILABLE
atlas_3_10_blas_info:
  NOT AVAILABLE
atlas_blas_threads_info:
  NOT AVAILABLE
atlas_blas_info:
  NOT AVAILABLE
blas_info:
    libraries = ['cblas', 'blas', 'cblas', 'blas', 'cblas', 'blas']
    library_dirs = ['C:/Users/kpostma/Anaconda3/envs/rcmap\\Library\\lib']
    include_dirs = ['C:/Users/kpostma/Anaconda3/envs/rcmap\\Library\\include']
    language = f77
    define_macros = [('HAVE_CBLAS', None)]
blas_opt_info:
    define_macros = [('NO_ATLAS_INFO', 1), ('HAVE_CBLAS', None)]
    libraries = ['cblas', 'blas', 'cblas', 'blas', 'cblas', 'blas']
    library_dirs = ['C:/Users/kpostma/Anaconda3/envs/rcmap\\Library\\lib']
    include_dirs = ['C:/Users/kpostma/Anaconda3/envs/rcmap\\Library\\include']
    language = f77
lapack_opt_info:
    libraries = ['lapack', 'blas', 'lapack', 'blas', 'cblas', 'blas', 'cblas', 'blas', 'cblas', 'blas']
    library_dirs = ['C:/Users/kpostma/Anaconda3/envs/rcmap\\Library\\lib']
    language = f77
    define_macros = [('NO_ATLAS_INFO', 1), ('HAVE_CBLAS', None)]
    include_dirs = ['C:/Users/kpostma/Anaconda3/envs/rcmap\\Library\\include']
Supported SIMD extensions in this NumPy install:
    baseline = SSE,SSE2,SSE3
    found = SSSE3,SSE41,POPCNT,SSE42,AVX,F16C,FMA3,AVX2,AVX512F,AVX512CD,AVX512_SKX,AVX512_CLX,AVX512_CNL
    not found =