Fix Windows DLL loading for Python 3.8+

[Copilot]

What problem does this PR address?

Python 3.8+ restricts DLL loading on Windows to specific directories for security. Extension module srreal_ext cannot find dependencies (libdiffpy.dll, libObjCryst.dll, Boost, GSL) installed in %CONDA_PREFIX%\Library\bin, causing import failures.

What should the reviewer(s) do?

Review the Windows-specific initialization added to __init__.py and verify:

• Logic only executes on Windows with Python 3.8+
• No impact on Linux/macOS platforms
• Test coverage is adequate

Changes:

• src/diffpy/srreal/__init__.py: Add os.add_dll_directory() call for %CONDA_PREFIX%\Library\bin before extension imports
• tests/test_dll_loading.py: Verify import succeeds and Windows DLL handling works across platforms

Example:

# Before: ImportError on Windowsfromdiffpy.srrealimportpdfcalculator# After: Imports successfullyfromdiffpy.srrealimportpdfcalculatorpc=PDFCalculator()

Original prompt

---

This section details on the original issue you should resolve

<issue_title>srreal DLL not loading properly on windows</issue_title>
<issue_description>### Problem
I am getting this when I try and run tests in pdfbl-sequential on windows:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
#!/usr/bin/env python
##############################################################################
#
# diffpy.srreal by DANSE Diffraction group
# Simon J. L. Billinge
# (c) 2010 The Trustees of Columbia University
# in the City of New York. All rights reserved.
#
# File coded by: Pavol Juhas
#
# See AUTHORS.txt for a list of people who contributed.
# See LICENSE_DANSE.txt for license information.
#
##############################################################################
"""
Top-level classes for PDF calculation:
DebyePDFCalculator -- simulate PDF by evaluating Debye sum in Q-space
PDFCalculator -- calculate PDF by peak summation in real space
"""
> from diffpy.srreal.srreal_ext import (
DebyePDFCalculator,
PDFCalculator,
fftftog,
fftgtof,
)
E ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
..\..\micromamba\envs\pdfbl\Lib\site-packages\diffpy\srreal\pdfcalculator.py:21: ImportError
====================================================== short test summary info ======================================================
FAILED tests/test_pdfadapter.py::test_pdfadapter - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
FAILED tests/test_sequential_cmi_runner.py::test_load_inputs_bad - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
FAILED tests/test_sequential_cmi_runner.py::test_run_sequential_cmi_runner - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
FAILED tests/test_sequential_cmi_runner.py::test_data_for_plot - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
========================================================= 4 failed in 7.15s =========================================================

Proposed solution

Figure out why and fix it. Here is the env list:

List of packages in environment: "C:\\Users\\simon\\micromamba\\envs\\pdfbl"
Name Version Build Channel
------------------------------------------------------------------------
arrow 1.4.0 py312haa95532_0 pkgs/main
bg-mpl-stylesheets 0.5.1 pyhff2d567_0 conda-forge
binaryornot 0.4.4 pyhd3eb1b0_1 pkgs/main
blas 1.0 mkl pkgs/main
brotlicffi 1.2.0.0 py312h885b0b7_0 pkgs/main
bzip2 1.0.8 h2bbff1b_6 pkgs/main
ca-certificates 2025.12.2 haa95532_0 pkgs/main
cairo 1.18.4 he9e932c_0 pkgs/main
certifi 2026.01.04 py312haa95532_0 pkgs/main
cffi 2.0.0 py312h02ab6af_1 pkgs/main
cfgv 3.5.0 py312haa95532_0 pkgs/main
chardet 5.2.0 py312haa95532_0 pkgs/main
charset-normalizer 3.4.4 py312haa95532_0 pkgs/main
click 8.2.1 py312haa95532_1 pkgs/main
codecov 2.1.11 pyhd3eb1b0_0 pkgs/main
colorama 0.4.6 py312haa95532_0 pkgs/main
contourpy 1.3.3 py312h214f63a_0 pkgs/main
cookiecutter 2.6.0 py312haa95532_1 pkgs/main
coverage 7.11.0 py312h630b2a1_1 pkgs/main
cycler 0.12.1 py312haa95532_0 pkgs/main
diffpy.cmi 3.1.2 pyhd8ed1ab_0 conda-forge
diffpy.srfit 3.2.0 pyhd8ed1ab_0 conda-forge
diffpy.srreal 1.4.0 py312h0e1366e_3 conda-forge
diffpy.structure 3.3.1 pyhd8ed1ab_0 conda-forge
diffpy.utils 3.7.1 pyhd8ed1ab_0 conda-forge
distlib 0.4.0 py312h02ab6af_0 pkgs/main
dlfcn-win32 1.4.2 hac47afa_0 conda-forge
expat 2.7.4 hd7fb8db_0 pkgs/main
filelock 3.20.3 py312haa95532_0 pkgs/main
flake8 7.3.0 py312haa95532_0 pkgs/main
fontconfig 2.15.0 hd211d86_0 pkgs/main
fonttools 4.61.0 py312h02ab6af_0 pkgs/main
freetype 2....
</details>
<!-- START COPILOT CODING AGENT SUFFIX -->
- Fixes diffpy/diffpy.srreal#81
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