covariance v. contravariance in Callable

08-def-type-hints/callable/variance.py

@@ -0,0 +1,22 @@
+from collections.abc import Callable
+
+def update(  # <1>
+        probe: Callable[[], float],  # <2>
+        display: Callable[[float], None]  # <3>
+    ) -> None:
+    temperature = probe()
+    # imagine lots of control code here
+    display(temperature)
+
+def probe_ok() -> int:  # <4>
+    return 42
+
+def display_wrong(temperature: int) -> None:  # <5>
+    print(hex(temperature))
+
+update(probe_ok, display_wrong)  # type error  # <6>
+
+def display_ok(temperature: complex) -> None:  # <7>
+    print(temperature)
+
+update(probe_ok, display_ok)  # OK  # <8>