TST: add unit tests to cover the savetxt method

tests/conftest.py

@@ -1166,3 +1166,18 @@ def func_2d_from_csv():
         source="tests/fixtures/function/2d.csv",
     )
     return func
+
+
+@pytest.fixture
+def lambda_quad_func():
+    """Create a lambda function based on a string.
+
+    Returns
+    -------
+    Function
+        A lambda function based on a string.
+    """
+    func = lambda x: x**2
+    return Function(
+        source=func,
+    )

tests/unit/test_function.py

@@ -1,3 +1,9 @@
+"""Unit tests for the Function class. Each method in tis module tests an 
+individual method of the Function class. The tests are made on both the
+expected behaviour and the return instances."""
+
+import os
+
 import numpy as np
 import pytest
 
@@ -158,3 +164,63 @@ def test_get_value_opt(x, y, z):
     func = Function(source, interpolation="shepard", extrapolation="natural")
     assert isinstance(func.get_value_opt(x, y), float)
     assert np.isclose(func.get_value_opt(x, y), z, atol=1e-6)
+
+
+@pytest.mark.parametrize(
+    "func",
+    [
+        "linearly_interpolated_func",
+        "spline_interpolated_func",
+        "func_2d_from_csv",
+        "lambda_quad_func",
+    ],
+)
+def test_savetxt(request, func):
+    """Test the savetxt method of various Function objects.
+
+    This test function verifies that the `savetxt` method correctly writes the
+    function's data to a CSV file and that a new function object created from
+    this file has the same data as the original function object.
+
+    Notes
+    -----
+    The test performs the following steps:
+    1. It invokes the `savetxt` method of the given function object.
+    2. It then reads this file to create a new function object.
+    3. The test asserts that the data of the new function matches the original.
+    4. Finally, the test cleans up by removing the created CSV file.
+
+    Raises
+    ------
+    AssertionError
+        If the `savetxt` method fails to save the file, or if the data of the
+        newly read function does not match the data of the original function.
+    """
+    func = request.getfixturevalue(func)
+    assert (
+        func.savetxt(
+            filename="test_func.csv",
+            lower=0,
+            upper=9,
+            samples=10,
+            fmt="%.6f",
+            delimiter=",",
+            newline="\n",
+            encoding=None,
+        )
+        is None
+    ), "Couldn't save the file using the Function.savetxt method."
+
+    read_func = Function(
+        "test_func.csv", interpolation="linear", extrapolation="natural"
+    )
+    if callable(func.source):
+        source = np.column_stack(
+            (np.linspace(0, 9, 10), func.source(np.linspace(0, 9, 10)))
+        )
+        assert np.allclose(source, read_func.source)
+    else:
+        assert np.allclose(func.source, read_func.source)
+
+    # clean up the file
+    os.remove("test_func.csv")