Improve how PyGMT finds the GMT library

.azure-pipelines.yml

@@ -179,8 +179,6 @@ jobs:
     CONDA_REQUIREMENTS: requirements.txt
     CONDA_REQUIREMENTS_DEV: requirements-dev.txt
     CONDA_INSTALL_EXTRA: "codecov gmt=6.0.0"
-    # ctypes.CDLL cannot find conda's libraries
-    GMT_LIBRARY_PATH: 'C:\Miniconda\envs\testing\Library\bin'
 
   strategy:
     matrix:

pygmt/clib/loading.py

@@ -7,24 +7,19 @@
 import os
 import sys
 import ctypes
+from ctypes.util import find_library
 
 from ..exceptions import GMTOSError, GMTCLibError, GMTCLibNotFoundError
 
 
-def load_libgmt(env=None):
+def load_libgmt():
     """
     Find and load ``libgmt`` as a :py:class:`ctypes.CDLL`.
 
     By default, will look for the shared library in the directory specified by
     the environment variable ``GMT_LIBRARY_PATH``. If it's not set, will let
     ctypes try to find the library.
 
-    Parameters
-    ----------
-    env : dict or None
-        A dictionary containing the environment variables. If ``None``, will
-        default to ``os.environ``.
-
     Returns
     -------
     :py:class:`ctypes.CDLL` object
@@ -37,22 +32,21 @@ def load_libgmt(env=None):
         couldn't access the functions).
 
     """
-    if env is None:
-        env = os.environ
-    libnames = clib_name(os_name=sys.platform)
-    libpath = env.get("GMT_LIBRARY_PATH", "")
+    lib_fullnames = clib_full_names()
     error = True
-    for libname in libnames:
+    for libname in lib_fullnames:
         try:
-            libgmt = ctypes.CDLL(os.path.join(libpath, libname))
+            libgmt = ctypes.CDLL(libname)
             check_libgmt(libgmt)
             error = False
             break
         except OSError as err:
             error = err
     if error:
         raise GMTCLibNotFoundError(
-            "Error loading the GMT shared library '{}':".format(", ".join(libnames))
+            "Error loading the GMT shared library '{}':".format(
+                ", ".join(lib_fullnames)
+            )
         )
     return libgmt
 
@@ -84,6 +78,37 @@ def clib_name(os_name):
     return libname
 
 
+def clib_full_names(env=None):
+    """
+    Return the full path of GMT's shared library for the current OS.
+
+    Parameters
+    ----------
+    env : dict or None
+        A dictionary containing the environment variables. If ``None``, will
+        default to ``os.environ``.
+
+    Returns
+    -------
+    lib_fullnames: list of str
+        List of possible full names of GMT's shared library.
+
+    """
+    if env is None:
+        env = os.environ
+    libnames = clib_name(os_name=sys.platform)  # e.g. libgmt.so, libgmt.dylib, gmt.dll
+    libpath = env.get("GMT_LIBRARY_PATH", "")  # e.g. $HOME/miniconda/envs/pygmt/lib
+
+    lib_fullnames = [os.path.join(libpath, libname) for libname in libnames]
+    # Search for DLLs in PATH if GMT_LIBRARY_PATH is not defined [Windows only]
+    if not libpath and sys.platform == "win32":
+        for libname in libnames:
+            libfullpath = find_library(libname)
+            if libfullpath:
+                lib_fullnames.append(libfullpath)
+    return lib_fullnames
+
+
 def check_libgmt(libgmt):
     """
     Make sure that libgmt was loaded correctly.

pygmt/tests/test_clib_loading.py

@@ -18,13 +18,6 @@ def test_load_libgmt():
     check_libgmt(load_libgmt())
 
 
-def test_load_libgmt_fail():
-    "Test that loading fails when given a bad library path."
-    env = {"GMT_LIBRARY_PATH": "not/a/real/path"}
-    with pytest.raises(GMTCLibNotFoundError):
-        load_libgmt(env=env)
-
-
 def test_clib_name():
     "Make sure we get the correct library name for different OS names"
     for linux in ["linux", "linux2", "linux3"]: