Create skip_if_no helper function to skip tests when missing a package (

#2883)

Add a helper testing function which returns a pytest.mark.skipif mark
that will skip certain unit tests when a package is not installed. This is
adapted from pandas' internal skip_if_no function at
https://github.com/pandas-dev/pandas/blob/v2.1.4/pandas/util/_test_decorators.py#L121.
Added a unit test to ensure that a correct pytest mark is returned.

---------

Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

pygmt/helpers/testing.py

@@ -1,10 +1,12 @@
 """
 Helper functions for testing.
 """
+import importlib
 import inspect
 import os
 import string
 
+import pytest
 from pygmt.exceptions import GMTImageComparisonFailure
 from pygmt.io import load_dataarray
 from pygmt.src import which
@@ -237,3 +239,42 @@ def load_static_earth_relief():
     """
     fname = which("@static_earth_relief.nc", download="c")
     return load_dataarray(fname)
+
+
+def skip_if_no(package):
+    """
+    Generic function to help skip tests when required packages are not present
+    on the testing system.
+
+    This function returns a pytest mark with a skip condition that will be
+    evaluated during test collection. An attempt will be made to import the
+    specified ``package``.
+
+    The mark can be used as either a decorator for a test class or to be
+    applied to parameters in pytest.mark.parametrize calls or parametrized
+    fixtures. Use pytest.importorskip if an imported moduled is later needed
+    or for test functions.
+
+    If the import is unsuccessful, then the test function (or test case when
+    used in conjunction with parametrization) will be skipped.
+
+    Adapted from
+    https://github.com/pandas-dev/pandas/blob/v2.1.4/pandas/util/_test_decorators.py#L121
+
+    Parameters
+    ----------
+    package : str
+        The name of the required package.
+
+    Returns
+    -------
+    pytest.MarkDecorator
+        A pytest.mark.skipif to use as either a test decorator or a
+        parametrization mark.
+    """
+    try:
+        _ = importlib.import_module(name=package)
+        has_package = True
+    except ImportError:
+        has_package = False
+    return pytest.mark.skipif(not has_package, reason=f"Could not import '{package}'")

pygmt/tests/test_helpers.py

@@ -15,7 +15,7 @@
     kwargs_to_strings,
     unique_name,
 )
-from pygmt.helpers.testing import load_static_earth_relief
+from pygmt.helpers.testing import load_static_earth_relief, skip_if_no
 
 
 def test_load_static_earth_relief():
@@ -147,3 +147,19 @@ def test_args_in_kwargs():
     # Failing list of arguments
     failing_args = ["D", "E", "F"]
     assert not args_in_kwargs(args=failing_args, kwargs=kwargs)
+
+
+def test_skip_if_no():
+    """
+    Test that the skip_if_no helper testing function returns a
+    pytest.mask.skipif mark decorator.
+    """
+    # Check pytest.mark with a dependency that can be imported
+    mark_decorator = skip_if_no(package="numpy")
+    assert mark_decorator.args[0] is False
+
+    # Check pytest.mark with a dependency that cannot be imported
+    mark_decorator = skip_if_no(package="nullpackage")
+    assert mark_decorator.args[0] is True
+    assert mark_decorator.kwargs["reason"] == "Could not import 'nullpackage'"
+    assert mark_decorator.markname == "skipif"