Add unit tests for calculator functions

tests/test_calc.py

@@ -0,0 +1,34 @@
+import pytest
+import sys
+import os
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
+
+from calculator import add, subtract, multiply, divide, is_even
+
+def test_add():
+    assert add(2, 3) == 5
+    assert add(-1, 1) == 0
+    assert add(0, 0) == 0
+
+def test_subtract():
+    assert subtract(5, 3) == 2
+    assert subtract(0, 5) == -5
+    assert subtract(-2, -3) == 1
+
+def test_multiply():
+    assert multiply(3, 4) == 12
+    assert multiply(-2, 3) == -6
+    assert multiply(0, 5) == 0
+
+def test_divide():
+    assert divide(10, 2) == 5
+    assert divide(7, 2) == 3.5
+    
+    with pytest.raises(ValueError):
+        divide(5, 0)
+
+def test_is_even():
+    assert is_even(4) == True
+    assert is_even(3) == False
+    assert is_even(0) == True