Improve performance by avoiding loading the GMT library repeatedly (#2930)

Co-authored-by: Wei Ji <[email protected]>

Author: seisman

pygmt/clib/session.py

@@ -87,6 +87,9 @@
     np.datetime64: "GMT_DATETIME",
 }
 
+# Load the GMT library outside the Session class to avoid repeated loading.
+_libgmt = load_libgmt()
+
 
 class Session:
     """
@@ -308,7 +311,7 @@ def get_libgmt_func(self, name, argtypes=None, restype=None):
         <class 'ctypes.CDLL.__init__.<locals>._FuncPtr'>
         """
         if not hasattr(self, "_libgmt"):
-            self._libgmt = load_libgmt()
+            self._libgmt = _libgmt
         function = getattr(self._libgmt, name)
         if argtypes is not None:
             function.argtypes = argtypes

pygmt/tests/test_clib_loading.py

@@ -11,6 +11,7 @@
 
 import pytest
 from pygmt.clib.loading import check_libgmt, clib_full_names, clib_names, load_libgmt
+from pygmt.clib.session import Session
 from pygmt.exceptions import GMTCLibError, GMTCLibNotFoundError, GMTOSError
 
 
@@ -207,6 +208,44 @@ def test_brokenlib_brokenlib_workinglib(self):
         assert check_libgmt(load_libgmt(lib_fullnames=lib_fullnames)) is None
 
 
+class TestLibgmtCount:
+    """
+    Test that the GMT library is not repeatedly loaded in every session.
+    """
+
+    loaded_libgmt = load_libgmt()  # Load the GMT library and reuse it when necessary
+    counter = 0  # Global counter for how many times ctypes.CDLL is called
+
+    def _mock_ctypes_cdll_return(self, libname):  # noqa: ARG002
+        """
+        Mock ctypes.CDLL to count how many times the function is called.
+
+        If ctypes.CDLL is called, the counter increases by one.
+        """
+        self.counter += 1  # Increase the counter
+        return self.loaded_libgmt
+
+    def test_libgmt_load_counter(self, monkeypatch):
+        """
+        Make sure that the GMT library is not loaded in every session.
+        """
+        # Monkeypatch the ctypes.CDLL function
+        monkeypatch.setattr(ctypes, "CDLL", self._mock_ctypes_cdll_return)
+
+        # Create two sessions and check the global counter
+        with Session() as lib:
+            _ = lib
+        with Session() as lib:
+            _ = lib
+        assert self.counter == 0  # ctypes.CDLL is not called after two sessions.
+
+        # Explicitly calling load_libgmt to make sure the mock function is correct
+        load_libgmt()
+        assert self.counter == 1
+        load_libgmt()
+        assert self.counter == 2
+
+
 ###############################################################################
 # Test clib_full_names
 @pytest.fixture(scope="module", name="gmt_lib_names")