Merge 5615da920f into b492a9d765

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1509 TILs and counting..._
+_1510 TILs and counting..._
 
 ---
 
@@ -190,7 +190,7 @@ _1509 TILs and counting..._
 - [Aliasing An Ansible Host](devops/aliasing-an-ansible-host.md)
 - [Allow Cross-Origin Requests To Include Cookies](devops/allow-cross-origin-requests-to-include-cookies.md)
 - [Allow HTTPS Through Your UFW Firewall](devops/allow-https-through-your-ufw-firewall.md)
-- [Check For Cached Site Assocation File For iOS](devops/check-for-cached-site-association-file-for-ios.md)
+- [Check For Cached Site Association File For iOS](devops/check-for-cached-site-association-file-for-ios.md)
 - [Check The Status of All Services](devops/check-the-status-of-all-services.md)
 - [Check The Syntax Of nginx Files](devops/check-the-syntax-of-nginx-files.md)
 - [Connect To An RDS PostgreSQL Database](devops/connect-to-an-rds-postgresql-database.md)
@@ -723,7 +723,7 @@ _1509 TILs and counting..._
 - [Check If Clusters Are Upgrade Compatible](postgres/check-if-clusters-are-upgrade-compatible.md)
 - [Check If The Local Server Is Running](postgres/check-if-the-local-server-is-running.md)
 - [Check If User Role Exists For Database](postgres/check-if-user-role-exists-for-database.md)
-- [Check Table For Any Oprhaned Records](postgres/check-table-for-any-orphaned-records.md)
+- [Check Table For Any Orphaned Records](postgres/check-table-for-any-orphaned-records.md)
 - [Checking Inequality](postgres/checking-inequality.md)
 - [Checking The Type Of A Value](postgres/checking-the-type-of-a-value.md)
 - [Clear The Screen In psql](postgres/clear-the-screen-in-psql.md)
@@ -888,6 +888,7 @@ _1509 TILs and counting..._
 - [Access Instance Variables](python/access-instance-variables.md)
 - [Create A Dummy DataFrame In Pandas](python/create-a-dummy-dataframe-in-pandas.md)
 - [Dunder Methods](python/dunder-methods.md)
+- [Override The Boolean Context Of A Class](python/override-the-boolean-context-of-a-class.md)
 - [Store And Access Immutable Data In A Tuple](python/store-and-access-immutable-data-in-a-tuple.md)
 - [Test A Function With Pytest](python/test-a-function-with-pytest.md)
 - [Use pipx To Install End User Apps](python/use-pipx-to-install-end-user-apps.md)

python/override-the-boolean-context-of-a-class.md

@@ -0,0 +1,86 @@
+# Override The Boolean Context Of A Class
+
+Everything in Python has a truthiness that can be checked with `bool()`. An
+empty list (`[]`) is falsy. A non-empty list (`[1,2,3]`) is truthy. Similar
+with numbers:
+
+```python
+>>> bool(0)
+False
+>>> bool(1)
+True
+```
+
+Any instance of an object is going to be truthy by default. If you want to
+control in what context an instance is considered truthy or falsy, you can
+override
+[`__bool__()`](https://docs.python.org/3/reference/datamodel.html#object.__bool__).
+If that's not implemented, but
+[`__len__()`](https://docs.python.org/3/reference/datamodel.html#object.__len__)
+is, then it will fallback to that.
+
+Let's look at a few example classes:
+
+```python
+class CartZero:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+class CartBool:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+    def __bool__(self):
+        print("__bool__() override")
+        return bool(self.items)
+
+class CartLen:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+    def __len__(self):
+        print("__len__() override")
+        return len(self.items)
+
+class CartBoolAndLen:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+    def __len__(self):
+        print("__len__() override")
+        return len(self.items)
+
+    def __bool__(self):
+        print("__bool__() override")
+        return bool(self.items)
+
+cart1 = CartZero()
+cart2 = CartBool()
+cart3 = CartLen()
+cart4 = CartBoolAndLen()
+
+print("CartZero() -> %s" %(bool(cart1)))
+print('')
+print("CartBool() -> %s" %(bool(cart2)))
+print('')
+print("CartLen() -> %s" %(bool(cart3)))
+print('')
+print("CartBoolAndLen() -> %s" %(bool(cart4)))
+```
+
+An 'empty' `Cart` be default is truthy. However, we can override some
+combination of `__bool__()` or `__len__()` to give it a boolean context that
+goes `false` when "empty".
+
+```
+CartZero() -> True
+
+__bool__() override
+CartBool() -> False
+
+__len__() override
+CartLen() -> False
+
+__bool__() override
+CartBoolAndLen() -> False
+```