diff --git a/notes-other/pythonetc/assert.md b/notes-other/pythonetc/assert.md
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+Basically, `assert` could be a function:
+
+```python
+
+def assert_(test, *args):
+    if not test:
+        raise AssertionError(*args)
+
+assert_(2 + 2 == 4, 'the world is broken')
+```
+
+However, there are few advantages of assert as directive over assert as function:
+
+1. All asserts removed on the bytecode compilation step if [optimization is enabled](https://t.me/pythonetc/115).
+
+2. The message is lazy and executed only when needed:
+
+```python
+assert False, print("executed")
+# executed
+# AssertionError: None
+
+assert True, print("not executed")
+# (prints nothing)
+```
diff --git a/notes-other/pythonetc/cached-property.md b/notes-other/pythonetc/cached-property.md
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+Decorator `@cached_property` is an amazing way to simplify your code. It's like the regular `@property` but remembers the value after the first call:
+
+```python
+class C:
+  @cached_property
+  def p(self):
+    print('computating...')
+    return 1
+
+c = C()
+c.p
+# computating...
+# 1
+
+c.p
+# 1
+```
+
+The implementation is short and relatively simple:
+
+``` python
+class cached_property:
+  def __init__(self, func):
+    self.func = func
+
+  def __get__(self, obj, cls):
+    if obj is None:
+        return self
+    value = obj.__dict__[self.func.__name__] = self.func(obj)
+    return value
+```
+
+However, there are a few corner-cases, like async functions and threads. Luckily, from Python 3.8 it's a part of standard library ([functools.cached_property](https://docs.python.org/dev/library/functools.html#functools.cached_property)) and for older versions [cached-propery](https://github.com/pydanny/cached-property) library can be used.
diff --git a/notes-other/pythonetc/nan.md b/notes-other/pythonetc/nan.md
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+++ b/notes-other/pythonetc/nan.md
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+Python has [NaN](https://t.me/pythonetc/561) float value and it's a rule-breaking thing:
+
+```python
+import math
+
+sorted([5.0, math.nan, 10.0, 0.0])
+# [5.0, nan, 0.0, 10.0]
+
+3 < math.nan
+# False
+3 > math.nan
+# False
+
+min(3, math.nan)
+# 3
+min(math.nan, 3)
+# nan
+```
+
+Be careful. Use `math.isnan` to check if a value is NaN.