Merge 460473a87f into 877537228f

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).
 
-_1503 TILs and counting..._
+_1508 TILs and counting..._
 
 ---
 
@@ -297,6 +297,7 @@ _1503 TILs and counting..._
 - [Configure Global gitignore File](git/configure-global-gitignore-file.md)
 - [Configuring The Pager](git/configuring-the-pager.md)
 - [Copy A File From Another Branch](git/copy-a-file-from-another-branch.md)
+- [Count All Files Of Specific Type Tracked By Git](git/count-all-files-of-specific-type-tracked-by-git.md)
 - [Create A New Branch With Git Switch](git/create-a-new-branch-with-git-switch.md)
 - [Delete All Untracked Files](git/delete-all-untracked-files.md)
 - [Determine The Hash Id For A Blob](git/determine-the-hash-id-for-a-blob.md)
@@ -886,6 +887,7 @@ _1503 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)
 - [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)
@@ -940,6 +942,7 @@ _1503 TILs and counting..._
 - [Demodulize A Class Name](rails/demodulize-a-class-name.md)
 - [Different Ways To Add A Foreign Key Reference](rails/different-ways-to-add-a-foreign-key-reference.md)
 - [Disambiguate Where In A Joined Relation](rails/disambiguate-where-in-a-joined-relation.md)
+- [Empty find_by Returns First Record](rails/empty-find-by-returns-first-record.md)
 - [Ensure A Rake Task Cannot Write Data](rails/ensure-a-rake-task-cannot-write-data.md)
 - [Ensure Migrations Use The Latest Schema](rails/ensure-migrations-use-the-latest-schema.md)
 - [Ensure Record Saved With after_commit Callback](rails/ensure-record-saved-with-after-commit-callback.md)
@@ -1208,6 +1211,7 @@ _1503 TILs and counting..._
 - [Enumerate A Pairing Of Every Two Sequential Items](ruby/enumerate-a-pairing-of-every-two-sequential-items.md)
 - [Evaluating One-Off Commands](ruby/evaluating-one-off-commands.md)
 - [Exclude Values From An Array](ruby/exclude-values-from-an-array.md)
+- [Execute Several Commands With Backtick Heredoc](ruby/execute-several-commands-with-backtick-heredoc.md)
 - [Exit A Process With An Error Message](ruby/exit-a-process-with-an-error-message.md)
 - [Expect A Method To Be Called And Actually Call It](ruby/expect-a-method-to-be-called-and-actually-call-it.md)
 - [Extract A Column Of Data From A CSV File](ruby/extract-a-column-of-data-from-a-csv-file.md)
@@ -1461,6 +1465,7 @@ _1503 TILs and counting..._
 - [Global Substitution On The Previous Command](unix/global-substitution-on-the-previous-command.md)
 - [Globbing For All Directories In Zsh](unix/globbing-for-all-directories-in-zsh.md)
 - [Globbing For Filenames In Zsh](unix/globbing-for-filenames-in-zsh.md)
+- [Gracefully Exit A Script With Trap](unix/gracefully-exit-a-script-with-trap.md)
 - [Grep For Files Without A Match](unix/grep-for-files-without-a-match.md)
 - [Grep For Files With Multiple Matches](unix/grep-for-files-with-multiple-matches.md)
 - [Grep For Multiple Patterns](unix/grep-for-multiple-patterns.md)

git/count-all-files-of-specific-type-tracked-by-git.md

@@ -0,0 +1,27 @@
+# Count All Files Of Specific Type Tracked By Git
+
+I want to get a count of all the markdown files in my [TIL
+repo](https://github.com/jbranchaud/til). Since all the files I care about are
+tracked by `git`, I can use `git ls-files` to get a listing of all files. That
+command on its own lists all files tracked by your git repository. Though there
+are many other flags we can apply, that will do for my purposes.
+
+By giving `git ls-files` a pattern to match against, I can turn up just, for
+instance, markdown files (`*.md`). I can pipe that to `wc -l` to get a count
+rather than exploding my terminal with a list of file names.
+
+```bash
+❯ git ls-files '*.md' | wc -l
+    1503
+```
+
+That command includes `README.md` and `CONTRIBUTING.md`, but really I only want
+to count the markdown files that constitute a TIL. Those all happen to be
+nested under a single directory. So I can tweak the glob pattern like so:
+
+```bash
+❯ git ls-files '*/*.md' | wc -l
+    1501
+```
+
+See `man git-ls-files` for more details.

python/dunder-methods.md

@@ -0,0 +1,22 @@
+# Dunder Methods
+
+Python has all kinds of special, or rather, _magic_ methods that allow for
+customizing all kinds of class behavior. There is `__init__()`, `__bool__()`,
+and so many others.
+
+The thing they all have in common is that their names are wrapped in _double
+underscores_. This is why they are called _dunder methods_.
+
+Some of these are used every single day, like the `__init__()` method for
+defining how a class should create an object. Others, used from time to time,
+are for overriding how comparisons or conversions happen. E.g. you may want to
+override `__bool__()` or `__len__()` to customize the truthiness of a custom
+class.
+
+There are so many others, ones you probably haven't even heard of. To see a
+full listing, check out this [cheat sheet of every dunder
+method](https://www.pythonmorsels.com/every-dunder-method/#cheat-sheet).
+
+Note: these are not to be confused with _dunder attributes_ which are things
+like `__name__`, `__file__`, and `__version__` which correspond to a value that
+you can access in a specific context rather than behavior to override.

rails/empty-find-by-returns-first-record.md

@@ -0,0 +1,49 @@
+# Empty find_by Returns First Record
+
+During a RubyConf 2024 talk, a speaker mentioned that if you pass `nil` to
+[ActiveRecord's `#find_by`
+method](https://api.rubyonrails.org/classes/ActiveRecord/FinderMethods.html#method-i-find_by),
+it will return the first record from the database. This is a bit unintuitive,
+so lets look at an example and then I'll show you why.
+
+```ruby
+> Book.first
+#=> #<Book:0x00000001142e4c48 id: 13, title: "The Secret History", ... >
+
+> Book.find_by(nil)
+#=> #<Book:0x00000001142ca3c0 id: 13, title: "The Secret History", ... >
+```
+
+So, that is the same object in both cases, but why?
+
+Our first hint is in the SQL that gets constructed when making that method
+call.
+
+```ruby
+Book Load (2.5ms)  SELECT "books".* FROM "books" LIMIT $1  [["LIMIT", 1]]
+```
+
+It's grabbing all books and limiting to _one_ result.
+
+Lets look at the underlying implementation of the `#find_by` method.
+
+```ruby
+# File activerecord/lib/active_record/relation/finder_methods.rb, line 111
+def find_by(arg, *args)
+  where(arg, *args).take
+end
+```
+
+Sure enough, the implementation is a `#where` followed by a `#take`. Since the
+`#where` is receiving `nil` as its `arg`, there are no conditions _filtering_
+the query. And the `#take` corresponds to the `limit 1`.
+
+Knowing that, we can understand that we will also get the first record from the
+database if we call `#find_by` with `{}`. Again, no conditions to filter on, so
+give me all books limited to one.
+
+One small caveat: notice how there is no `order by` clause in the above SQL
+output. This differs from `Books.first` which implicitly does an order on the
+`id` column. Though these method are likely to return the same result, the
+ordering of `#find_by` is not guaranteed to be the same without an `order by`
+clause.

ruby/execute-several-commands-with-backtick-heredoc.md

@@ -0,0 +1,38 @@
+# Execute Several Commands With Backtick Heredoc
+
+A fun feature of Ruby is that we can execute a command in a subprocess just by
+wrapping it in backticks.
+
+For instance, we might shell out to `git` to check if a file is tracked:
+
+```ruby
+`git ls-files --error-unmatch #{file_path} 2>/dev/null`
+$?.success?
+```
+
+But what if we need to execute several commands? Perhaps they depend on one
+another. We want them to run in the same subprocess.
+
+For this, we can use the backtick version of a heredoc. That is a special
+version of a heredoc where the delimiter is wrapped in backticks.
+
+```ruby
+puts <<`SHELL`
+  # Set up trap
+  trap 'echo "Cleaning up temp files"; rm -f *.tmp' EXIT
+  
+  # Create temporary file
+  echo "test data" > work.tmp
+  
+  # Do some work
+  cat work.tmp
+  
+  # Trap will clean up on exit
+SHELL
+```
+
+Here we set up a `trap` for file cleanup on exit, then create a file, then do
+something with the file, and that's it, the process exits (triggering the
+trap).
+
+[source](https://ruby-doc.org/3.3.6/syntax/literals_rdoc.html#label-Here+Document+Literals)

unix/gracefully-exit-a-script-with-trap.md

@@ -0,0 +1,29 @@
+# Gracefully Exit A Script With Trap
+
+With `trap` you can intercept signals that would cause your script to exit and
+then inject some additional behavior. Perhaps you want to make sure the script
+cleans up after itself before it exists.
+
+During this script's execution, it creates a file in the filesystem. It would
+be nice to make sure that no matter path this script ends up down that it will
+clean up after itself as it exits. We set up a `trap` that looks for the `EXIT`
+signal to do this.
+
+```bash
+# Set up trap
+trap 'echo "Cleaning up temp files"; rm -f *.tmp' EXIT
+
+# Create temporary file
+echo "test data" > work.tmp
+
+# Do some work
+cat work.tmp
+
+# Trap will clean up on exit
+```
+
+Whatever is in quotes is what the trap will execute when it is triggered. The
+following one or more signals are what the trap listens for, in this case
+`EXIT`.
+
+[source](https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_12_02.html)