[Walrus Operator] Add concept

Author: colinleach

concepts/walrus-operator/.meta/config.json

@@ -1,5 +1,5 @@
 {
-  "blurb": "TODO: add blurb for this concept",
-  "authors": ["bethanyg", "cmccandless"],
+  "blurb": "Properly called assignment expressions, the walrus operator assigns values to variables as part of a larger expression.",
+  "authors": ["BethanyG", "colinleach"],
   "contributors": []
 }

concepts/walrus-operator/about.md

@@ -1,2 +1,101 @@
-#TODO: Add about for this concept.
+# About
 
+Added in Python 3.8, assignment expressions use the `:=` operator, popularly called the ["walrus operator"][whats-new].
+
+The central idea is that a variable can be assigned within the context of another statement.
+A simple example is when something tested in a condition is needed again within the body of an `if` statement:
+
+```python
+# without walrus operator
+a = 'short'
+if len(a) < 10:
+    print(f"{len(a)} characters is too short")
+
+#  => "5 characters is too short"
+
+# with walrus operator 
+# Note the parentheses around the assignment (necessary)
+if (n := len(a)) < 10:
+    print(f"{n} characters is too short")
+
+#  => "5 characters is too short"
+```
+
+Thus, setting the `n` variable saves calculating `len(a)` twice.
+The saving is fairly trivial in this case, but becomes significant for a more expensive operation such as file I/O:
+
+```python
+# Loop over fixed length blocks
+while (block := f.read(256)) != '':
+    process(block)
+```
+
+## Use cases
+
+The walrus operator was [controversial][controversy] in the Python community when [first introduced][pep-572] in 2018.
+It is now an established part of the core language, simplifying code in some particular situations, but should not be over-used.
+As so often in programming, we should consider if use of a feature increases or decreases the readability of the code.
+
+Some applications of the walrus operator are listed below.
+
+### Comprehensions
+
+List comprehensions, dictionary comprehensions, generator expressions, _etc_, all allow Python to do a lot in a terse piece of code.
+
+Including a walrus operator can _sometimes_ make such code shorter and clearer, especially if a value calculated in the `if` clause is also needed in other parts of the comprehension.
+
+```python
+[clean_name.title() for name in names
+ if (clean_name := name.strip().lower()) in allowed_names]
+```
+
+Note that the calculated value `clean_name` is only in scope within the comprehension.
+It cannot be used in other parts of the program.
+
+### RegEx matching
+
+Use of the walrus operator can simplify nested `if...else` statements.
+Handling regular expression matches is [an example][walrus-blog] of this:
+
+```python
+import re
+
+test = "Something we want to search"
+pattern1 = r"^.*(thing).*"
+pattern2 = r"^.*(missing).*"
+
+# without walrus
+m = re.match(pattern1, test)
+if m:
+    print(f"Matched the 1st pattern: {m.group(1)}")
+else:
+    m = re.match(pattern2, test)
+    if m:
+        print(f"Matched the 2nd pattern: {m.group(1)}")
+# => "Matched 1st pattern: 'thing'"       
+
+# Cleaner with walrus
+if m := (re.match(pattern1, test)):
+    print(f"Matched 1st pattern: '{m.group(1)}'")
+elif m := (re.match(pattern2, test)):
+    print(f"Matched 2nd pattern: '{m.group(1)}'")
+# => "Matched 1st pattern: 'thing'"       
+```
+
+### Inside f-strings
+
+This is not a particularly common use of walrus operators, but it can sometimes simplify code.
+As usual, the assignment must be within parentheses.
+
+```python
+from datetime import datetime
+
+print(f"Today is: {(today:=datetime.today()):%Y-%m-%d}, which is {today:%A}")
+
+#  => "Today is: 2024-01-06, which is Saturday"
+```
+
+[whats-new]: https://docs.python.org/3/whatsnew/3.8.html#assignment-expressions
+[walrus-blog]: https://martinheinz.dev/blog/79
+[pep-572]: https://peps.python.org/pep-0572/
+[controversy]: https://dev.to/renegadecoder94/the-controversy-behind-the-walrus-operator-in-python-4k4e

concepts/walrus-operator/links.json

@@ -1,18 +1,18 @@
 [
   {
-    "url": "http://example.com/",
-    "description": "TODO:  add new link (above) and write a short description here of the resource."
+    "url": "https://docs.python.org/3/whatsnew/3.8.html#assignment-expressions/",
+    "description": "A What's New description for Python 3.8."
   },
   {
-    "url": "http://example.com/",
-    "description": "TODO:  add new link (above) and write a short description here of the resource."
+    "url": "https://peps.python.org/pep-0572/",
+    "description": "PEP 572 describes the walrus operator and its rationale."
   },
   {
-    "url": "http://example.com/",
-    "description": "TODO:  add new link (above) and write a short description here of the resource."
+    "url": "https://martinheinz.dev/blog/79/",
+    "description": "A thoughtful blog article from Martin Heinz."
   },
   {
-    "url": "http://example.com/",
-    "description": "TODO:  add new link (above) and write a short description here of the resource."
+    "url": "https://dev.to/renegadecoder94/the-controversy-behind-the-walrus-operator-in-python-4k4e/",
+    "description": "A discussion of the controversy surrounding the walrus operator when it was introduced."
   }
 ]