```python
v1 = py5.Py5Vector(3, 4)
print("magnitude =", v1.mag)
# magnitude = 5.0
print("magnitude squared =", v1.mag_sq)
# magnitude squared = 25.0
v1.mag_sq = 100
print("magnitude =", v1.mag)
# magnitude = 10.0
print("magnitude squared =", v1.mag_sq)
# magnitude squared = 100.0
```