Improve how PyGMT finds the GMT library (#440)

This PR improves how PyGMT finds the GMT library on Windows, using the `ctypes.util.find_library` function, which searches DLLs in the system search path, (i.e., the variable PATH).

Changes in this PR:

- Remove the variable **GMT_LIBRARY_PATH** from the CI settings
- Use `find_library` to search DLLs on Windows
- Fix function `clib_name` to `clib_names`
- Improve the tests

.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
@@ -37,27 +32,26 @@ 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
 
 
-def clib_name(os_name):
+def clib_names(os_name):
     """
     Return the name of GMT's shared library for the current OS.
 
@@ -68,20 +62,51 @@ def clib_name(os_name):
 
     Returns
     -------
-    libname : list of str
+    libnames : list of str
         List of possible names of GMT's shared library.
 
     """
     if os_name.startswith("linux"):
-        libname = ["libgmt.so"]
+        libnames = ["libgmt.so"]
     elif os_name == "darwin":
         # Darwin is macOS
-        libname = ["libgmt.dylib"]
+        libnames = ["libgmt.dylib"]
     elif os_name == "win32":
-        libname = ["gmt.dll", "gmt_w64.dll", "gmt_w32.dll"]
+        libnames = ["gmt.dll", "gmt_w64.dll", "gmt_w32.dll"]
     else:
         raise GMTOSError('Operating system "{}" not supported.'.format(sys.platform))
-    return libname
+    return libnames
+
+
+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_names(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):

pygmt/tests/test_clib_loading.py

@@ -1,9 +1,10 @@
 """
 Test the functions that load libgmt
 """
+import os
 import pytest
 
-from ..clib.loading import clib_name, load_libgmt, check_libgmt
+from ..clib.loading import clib_names, load_libgmt, check_libgmt
 from ..exceptions import GMTCLibError, GMTOSError, GMTCLibNotFoundError
 
 
@@ -20,16 +21,25 @@ def test_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"}
+    # save the old value (if any) before setting a fake "GMT_LIBRARY_PATH"
+    old_gmt_library_path = os.environ.get("GMT_LIBRARY_PATH")
+
+    os.environ["GMT_LIBRARY_PATH"] = "/not/a/real/path"
     with pytest.raises(GMTCLibNotFoundError):
-        load_libgmt(env=env)
+        load_libgmt()
+
+    # revert back to the original status (if any)
+    if old_gmt_library_path:
+        os.environ["GMT_LIBRARY_PATH"] = old_gmt_library_path
+    else:
+        del os.environ["GMT_LIBRARY_PATH"]
 
 
-def test_clib_name():
+def test_clib_names():
     "Make sure we get the correct library name for different OS names"
     for linux in ["linux", "linux2", "linux3"]:
-        assert clib_name(linux) == ["libgmt.so"]
-    assert clib_name("darwin") == ["libgmt.dylib"]
-    assert clib_name("win32") == ["gmt.dll", "gmt_w64.dll", "gmt_w32.dll"]
+        assert clib_names(linux) == ["libgmt.so"]
+    assert clib_names("darwin") == ["libgmt.dylib"]
+    assert clib_names("win32") == ["gmt.dll", "gmt_w64.dll", "gmt_w32.dll"]
     with pytest.raises(GMTOSError):
-        clib_name("meh")
+        clib_names("meh")