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+# python
+
+## Decorator [[run][run-decorator]]
+Some decorator examples with type annotation.
+
+```python
+from typing import Callable
+
+def log(f: Callable[[int], None]) -> Callable[[int], None]:
+    def inner(x: int):
+        print(f"log::inner f={f.__name__} x={x}")
+        f(x)
+    return inner
+
+@log
+def some_fn(x: int):
+    print(f"some_fn x={x}")
+
+
+def log_tag(tag: str) -> Callable[[Callable[[int], None]], Callable[[int], None]]:
+    def decorator(f: Callable[[int], None]) -> Callable[[int], None]:
+        def inner(x: int):
+            print(f"log_tag::inner f={f.__name__} tag={tag} x={x}")
+            f(x)
+        return inner
+    return decorator
+
+@log_tag("some_tag")
+def some_fn2(x: int):
+    print(f"some_fn2 x={x}")
+```
+
+## Walrus operator [[run][run-walrus]]
+Walrus operator `:=` added since **python 3.8**.
+```python
+from typing import Optional
+
+# Example 1: if let statements
+
+def foo(ret: Optional[int]) -> Optional[int]:
+    return ret
+
+if r := foo(None):
+    print(f"foo(None) -> {r}")
+
+if r := foo(1337):
+    print(f"foo(1337) -> {r}")
+
+# Example 2: while let statements
+
+toks = iter(['a', 'b', 'c'])
+while tok := next(toks, None):
+    print(f"{tok}")
+
+# Example 3: list comprehension
+
+print([tok for t in ["  a", "  ", " b "] if (tok := t.strip())])
+```
+
+## [Unittest][doc-unittest] [[run][run-unittest]]
+Run unittests directly from the command line as \
+`python3 -m unittest -v test`
+
+Optionally pass `-k <patter>` to only run subset of tests.
+```python
+# file: test.py
+
+import unittest
+
+class MyTest(unittest.TestCase):
+    def setUp(self):
+        pass
+    def tearDown(self):
+        pass
+    # Tests need to start with the prefix 'test'.
+    def test_foo(self):
+        self.assertEqual(1 + 2, 3)
+    def test_bar(self):
+        with self.assertRaises(IndexError):
+            list()[0]
+```
+
+## [Doctest][doc-doctest] [[run][run-doctest]]
+Run doctests directly from the command line as \
+`python -m doctest -v test.py`
+
+```python
+# file: test.py
+
+def sum(a: int, b: int) -> int:
+    """Sum a and b.
+
+    >>> sum(1, 2)
+    3
+
+    >>> sum(10, 20)
+    30
+    """
+    return a + b
+```
+
+## [timeit][doc-timeit]
+Micro benchmarking.
+```bash
+python -m timeit '[x.strip() for x in ["a ", " b"]]'
+```
+
+[run-decorator]: https://www.online-python.com/IDdiE0gpYU
+[run-walrus]: https://www.online-python.com/9T12PvmKVy
+[doc-unittest]: https://docs.python.org/3/library/unittest.html
+[run-unittest]: https://www.online-python.com/2fit4UcbzI
+[doc-doctest]: https://docs.python.org/3/library/doctest.html
+[run-doctest]: https://www.online-python.com/LZst51UNIH
+[doc-timeit]: https://docs.python.org/3/library/timeit.html