Adding syntax to the CPython interpreter (opens in new tab)

The linked, more detailed post in the article is far more interesting: https://miguendes.me/what-if-python-had-this-ruby-feature

In the bygone era of 2008, I had fun adding Perl/Ruby-style pattern matching.

  for line in open("python_yacc.py"):
      if line =~ m/def (\w+)/:
          print repr($1)

See http://www.dalkescientific.com/writings/diary/archive/2008/0...

I started by writing a Python grammar definition based on PLY (see http://www.dabeaz.com/ply/) , then tweaked it to handle the new feature and emit the right AST. It's when I discovered the following was valid Python:

  >>> a=b=c=d=e=1
  >>> del a, (b, (c, (((((d,e)))))))

I don't think PLY can handle the newest Python grammar, but I haven't looked into it.

For what it's worth, my Python grammar was based on an older project I did called GardenSnake, which still available as a PLY example, at https://github.com/dabeaz/ply/tree/master/example/GardenSnak... .

I've been told it's was one of the few examples of how to handle an indentation-based grammar using a yacc-esque parser generator.

You could argue that this is not merely adding syntax, but also adding the associated semantics.

Anyway, if you found this interesting, you might enjoy Eli Bendersky's blog post from nearly 15 years ago where he adds an "until ... do" statement to Python.

https://eli.thegreenplace.net/2010/06/30/python-internals-ad...

This is where Lisp-like languages excel with macros. An example in Clojure:

    (defmacro do* [body expr cond]
     `(~expr ~cond
       ~body))

     (do* (println "hello") if true)
     (do* (println "hello") when-not (> 1 2))

Great post!

I have worked on a similar series of CPython customization articles that you might find interesting:

https://github.com/pkolchanov/CustomizingPython

For all of the syntax features Python has been adding over the years, this would be a nice enhancement: making the "else None" optional in the ternary if-expression. E.g.,

    spam = eggs if bar
    # vs
    spam = eggs if bar else None

Great, now you have the dangling else in python, the one thing that actually was solved by significant white space.

This is a cool article. If you want to understand how to contribute to python there is a guide at https://devguide.python.org

>Condensed version of this cool blog post.

The effort is very much appreciated.

I never knew this syntax worked in python 2 and 3

  1 if True else 2

If only there was a language that let you modify the interpreter on the fly so you could do this as part of normal execution...

You can almost do it with `condition and value`, but you get `False` instead of `None` (and adding any more code makes this worse than `value if condition else None`. Interestingly lua like `<condition> and <true-expression> or <false-expression>` ternaries actually work in python, with the added footgun that <true-expression> must be truthy and people will despise you).

Rust for example has a solution for this in std, there's a `bool::and_some` method (and `bool::then` that takes a closure to avoid eagerly evaluating the value), but `if { ... }` isn't an expression like `if { ... } else { ... }` is, probably to avoid coupling the language to `Option`.