5 ms·

Adding syntax to the CPython interpreter

161 points by leontrolski 2y ago

Y_Y 2y ago
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...
- BiteCode_dev 2y ago
  Python can actually do this using "# coding:", albeit less elegantly than lisp. I would say it's a good thing, I don't want to see a hundred of half baked, badly tested and vaguely document DSL with no decent tooling support.
  - develatio 2y ago
    Can you provide some more info / links regarding the “# coding:” feature? I wasn’t able to find anything.
    - deleted 2y ago
      [deleted]
    - klibertp 2y ago
      If you place `# coding: utf-8` on the first line of a Python script, it'll try to interpret the raw bytes contained later in the file by passing them through a relevant codec[1]. Since the codec receives the raw source and transforms it before any interpretation happens, you can (here's the point I'm starting to guess) supply a codec that parses the code, transforms the AST, and dumps it back to the source for execution. It would be similar to how "parse transforms" work in Erlang, though there you're handed AST and not bytes. [1] https://docs.python.org/3/library/codecs.html https://docs.python.org/3/library/codecs.html
      - zahlman 2y ago
        >how "parse transforms" work in Erlang, though there you're handed AST and not bytes. Oh, I didn't know about this. Was thinking about doing something similar in my own language, so I'll have to check that out.
    - JackC 2y ago
      The idea is you can register a custom file encoding, analogous to built-in file encodings like utf-8, and use it to transform the source file before python loads it. An example is pyxl, kind of like jsx, where you put `# coding: pyxl` at the top and then it will transform bare `<html>` in the python file into calls to a template builder: https://github.com/gvanrossum/pyxl3 https://github.com/gvanrossum/pyxl3 Incidentally this turns out to be super hard to search for without asking an LLM, since "python coding" is so overloaded, and using the feature this way is intentionally undocumented because it's not really what it's for, and not something I think most python users really want to encourage. So, forbidden python knowledge!
      - kevindamm 2y ago
        It's specified in PEP 263 [0] which pulls up easier on search if you quote it and include the typical "special comment symbol" -*- around it. [0]: https://peps.python.org/pep-0263/ https://peps.python.org/pep-0263/
    - BiteCode_dev 2y ago
      Codecs (https://docs.python.org/3/library/codecs.html https://docs.python.org/3/library/codecs.html) can change the content of the file on the fly. You can abuse that to create new syntax, although it is evidently not the original intent. Let's say you hate significative spaces, here is a (very fragile) PoC for your pain: https://0bin.net/paste/42AQCTIC#dLEscW0rWQbE70cdnVCCiY72VuJwnSoLfJ0FlEGl82m https://0bin.net/paste/42AQCTIC#dLEscW0rWQbE70cdnVCCiY72VuJw... Import that into a *.pth file in your venv, and you can then do: # coding: braces_indent def main() { print("Hello, World!") if True { print("This is indented using braces.") } } You also can use import hooks (python ideas does that), bytecode manipulations (pytest does that) or use the ctypes module (forbiddenfruit does that). Still, I'm very happy it stays limited to super specific niches. Big power, big responsibilities, and all that.
  - pas 2y ago
    That's probably an argument for a language with good DSL support. When this comes up I usually link to the work of Alan Kay and others (the very mystical sounding STEPS project at VPRI) """ The big breakthrough is making it easy to create new DSLs for any situation. Every area of the OS has its own language (and you can just add more if you feel the need) so that the whole OS including networking and GUI is very compact, understandable, and hackable. This particular project focused on compactness, just to prove that it is quantitatively more expressive. """ comment by sp332 https://news.ycombinator.com/item?id=11687952 https://news.ycombinator.com/item?id=11687952 final report from 2016 https://news.ycombinator.com/item?id=11686325 https://news.ycombinator.com/item?id=11686325
- knighthack 2y ago
  Try Nim's macros.
- klibertp 2y ago
  You're Lisp-baiting, aren't you? ;) I'd add Elixir next to Nim (already mentioned); also Rust. Recently, also Scala. The reason we don't have such metaprogramming available everywhere is mostly because you have to subscribe to a particular ideology to allow it. If you think programmers are generally intelligent and responsible, you put macros and metaclasses in your language. If, on the other hand, you think most programmers are dumb code monkeys (with a few exceptions, maybe) your language becomes like early Java or early Go.
  - keybored 2y ago
    That dichotomy is interesting considering Guy Steele’s Growing a Language talk and his Scheme background. But maybe just mentioning Scheme is misleading here...
    - klibertp 2y ago
      Thanks for bringing it up. I had a good read. I agree entirely with Steele! It's ironic, however, that it's written in the context of Java - he mentions just three features that would make Java growable: generics, operator overloading, and value types. We all know how it went: generics were added sometime later, but there's still no operator overloading, and value types came to the language 20 years after the talk :)
  - Y_Y 2y ago
    (Isn't lisp-baiting the raison d'être of hn?) Since you mention it, Python does have a fairly elaborate metaclass system, but it seems like it's only really used for implementing the language and rarely if ever wielded by "users". I guess that's a reflection of the language ideology you're talking about. Also for what it's worth, I know myself to be a dumb code monkey, but being in the CRUD gutter doesn't preclude me from looking at the metasyntactic stars.
- radarsat1 2y ago
  Why would I want to extend the language syntax as part of normal execution?
nickpresta 2y ago
The linked, more detailed post in the article is far more interesting: https://miguendes.me/what-if-python-had-this-ruby-feature https://miguendes.me/what-if-python-had-this-ruby-feature
- sph 2y ago
  Did OP basically summarise another blog post and publish it on their own blog, even though the original is linked? Not a good look, also because it's "narrated" in the first person, as if they were the one to implement this change.
  - Wowfunhappy 2y ago
    ...it's not, although I have to say, I really appreciated the succinctness of OP's article versus the original.
nemoniac 2y ago
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-adding-a-new-statement-to-python/ https://eli.thegreenplace.net/2010/06/30/python-internals-ad...
- pansa2 2y ago
  That blog post seems a lot more involved - it adds code to the bytecode compiler as well as to the parser. I suspect that’s mostly because the `until` statement is more complex - but another factor seems to be Python’s change in parsing technology from LL(1) to PEG.
- asplake 2y ago
  I sometimes think it would be fun to do “do: ... where:/while/until”, wherein the “where” part does local bindings.
  - tpoacher 2y ago
    there is a nice "staticscope" package in python, which acts as a context manager, and does pretty much what you refer to. pretty neat. https://pypi.org/project/scoping/ https://pypi.org/project/scoping/
- deleted 2y ago
  [deleted]
pkolchanov 2y ago
Great post! I have worked on a similar series of CPython customization articles that you might find interesting: https://github.com/pkolchanov/CustomizingPython https://github.com/pkolchanov/CustomizingPython
tgv 2y ago
Great, now you have the dangling else in python, the one thing that actually was solved by significant white space.
dig1 2y ago
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))
- BoingBoomTschak 2y ago
  Yeah, this is the kind of material that shows that any attempt to prevent/hamper meta-programming or punt it to preprocessors is at best misguided. It also feeds the smug Lisp weenie within me. Above all the wonders of Lisp's pantheon stand its metalinguistic tools; by their grace have Lisp's acolytes been liberated from the rigid asceticism of lesser faiths. Thanks to Macro and kin, the jolly, complacent Lisp hacker can gaze through a fragrant cloud of setfs and defstructs at the emaciated unfortunates below, scraping out their meager code in inflexible notation, and sneer superciliously. It's a good feeling. -- iterate manual, A.1 Introduction do..while is also the example used here https://www.tcl-lang.org/man/tcl9.0/TclCmd/uplevel.html https://www.tcl-lang.org/man/tcl9.0/TclCmd/uplevel.html
zitterbewegung 2y ago
This is a cool article. If you want to understand how to contribute to python there is a guide at https://devguide.python.org https://devguide.python.org
- tmgehrmann 2y ago
  Be aware, however, that the inner circle will use you, take your contributions, and, if you develop own opinions, publicly ban and defame you without any possibility of setting any record straight. pyhton-dev is a corporate shark tank where only personalities and employer matter (good code or ideas are optional).
  - pas 2y ago
    That's a pretty strong claim, (and since most inner circles are hard to get into, I even assume it's not without any basis in reality), yet could you please provide some exhibits to support it?
    - 0x1242149 2y ago
      The inner circle emerged after GvR resigned. It largely consists of people who haven't contributed that much to Python3 (sometimes nothing at all in terms of code). The members occupy different chairs in the PSF, Steering Council and the all-powerful CoC troika. They rotate, sometimes skip one election and then come back. Their latest achievement is the banning of Tim Peters and others: https://news.ycombinator.com/item?id=41234180 https://news.ycombinator.com/item?id=41234180 https://news.ycombinator.com/item?id=41385546 https://news.ycombinator.com/item?id=41385546 Tim Peters is just the tip of the iceberg. Many bans are private, intimidation is private. Steering council members can insult, bully and mock others without any CoC consequences for themselves and keep getting elected. That is how you know that they are in the inner circle.
      - pas 2y ago
        thanks for the details and links! how they are getting elected? who can vote?
cpburns2009 2y ago
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
- genter 2y ago
  So if "else None" is omitted, if bar is false, then does spam == None or is it unmodified? The former is what I think you want, but that would be very confusing.
  - 333c 2y ago
    Yeah, it would be confusing. In Ruby, this would leave spam unmodified.
    - 8n4vidtmkvmk 2y ago
      That's even more confusing. There's an =, I expect an assignment to happen. Maybe we just leave this one alone.
      - trehalose 2y ago
        Perhaps using a parenthesized assignment expression with the walrus operator would be unambiguous: (spam := eggs) if bar I think that seems reasonable? It would act just the same as it already does with an explicit `else None`, if I'm not mistaken. I don't find it beautiful though.
      - 333c 2y ago
        In Ruby, the `if [condition]` modifier at the end of a line is used for more cases than just assignment. For example, `return if s.blank?` and `raise "invalid" if input.length > 100`. In Ruby, this pattern makes it clear that the statement is only executed if the condition is met. I'm not advocating for this feature to be added to Python, just explaining why it's not confusing in Ruby.
bbb651 2y ago
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`.
- steveklabnik 2y ago
  Option is already a lang item, so that would t be an issue. I don’t know what the actual underlying reason is, or if it was ever even proposed.
  - bbb651 2y ago
    I meant more than it already is, I referenced this: https://stackoverflow.com/a/43339003 https://stackoverflow.com/a/43339003. I do think it would be neat but it's niche enough to not be worth the non-obvious behavior of boxing with `Option`, e.g. you might forget an `else { ... }` and get complex type errors.
zahlman 2y ago
>Condensed version of this cool blog post. The effort is very much appreciated.
dalke 2y ago
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/2008/03/09/python4ply_tutorial_3.html 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/ 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/GardenSnake 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.
m463 2y ago
I never knew this syntax worked in python 2 and 3 1 if True else 2

161 points by leontrolski 2y ago

Y_Y 2y ago

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...

BiteCode_dev 2y ago
Python can actually do this using "# coding:", albeit less elegantly than lisp. I would say it's a good thing, I don't want to see a hundred of half baked, badly tested and vaguely document DSL with no decent tooling support.
- develatio 2y ago
  Can you provide some more info / links regarding the “# coding:” feature? I wasn’t able to find anything.
  - deleted 2y ago
    [deleted]
  - klibertp 2y ago
    If you place `# coding: utf-8` on the first line of a Python script, it'll try to interpret the raw bytes contained later in the file by passing them through a relevant codec[1]. Since the codec receives the raw source and transforms it before any interpretation happens, you can (here's the point I'm starting to guess) supply a codec that parses the code, transforms the AST, and dumps it back to the source for execution. It would be similar to how "parse transforms" work in Erlang, though there you're handed AST and not bytes. [1] https://docs.python.org/3/library/codecs.html https://docs.python.org/3/library/codecs.html
    - zahlman 2y ago
      >how "parse transforms" work in Erlang, though there you're handed AST and not bytes. Oh, I didn't know about this. Was thinking about doing something similar in my own language, so I'll have to check that out.
  - JackC 2y ago
    The idea is you can register a custom file encoding, analogous to built-in file encodings like utf-8, and use it to transform the source file before python loads it. An example is pyxl, kind of like jsx, where you put `# coding: pyxl` at the top and then it will transform bare `<html>` in the python file into calls to a template builder: https://github.com/gvanrossum/pyxl3 https://github.com/gvanrossum/pyxl3 Incidentally this turns out to be super hard to search for without asking an LLM, since "python coding" is so overloaded, and using the feature this way is intentionally undocumented because it's not really what it's for, and not something I think most python users really want to encourage. So, forbidden python knowledge!
    - kevindamm 2y ago
      It's specified in PEP 263 [0] which pulls up easier on search if you quote it and include the typical "special comment symbol" -*- around it. [0]: https://peps.python.org/pep-0263/ https://peps.python.org/pep-0263/
  - BiteCode_dev 2y ago
    Codecs (https://docs.python.org/3/library/codecs.html https://docs.python.org/3/library/codecs.html) can change the content of the file on the fly. You can abuse that to create new syntax, although it is evidently not the original intent. Let's say you hate significative spaces, here is a (very fragile) PoC for your pain: https://0bin.net/paste/42AQCTIC#dLEscW0rWQbE70cdnVCCiY72VuJwnSoLfJ0FlEGl82m https://0bin.net/paste/42AQCTIC#dLEscW0rWQbE70cdnVCCiY72VuJw... Import that into a *.pth file in your venv, and you can then do: # coding: braces_indent def main() { print("Hello, World!") if True { print("This is indented using braces.") } } You also can use import hooks (python ideas does that), bytecode manipulations (pytest does that) or use the ctypes module (forbiddenfruit does that). Still, I'm very happy it stays limited to super specific niches. Big power, big responsibilities, and all that.
- pas 2y ago
  That's probably an argument for a language with good DSL support. When this comes up I usually link to the work of Alan Kay and others (the very mystical sounding STEPS project at VPRI) """ The big breakthrough is making it easy to create new DSLs for any situation. Every area of the OS has its own language (and you can just add more if you feel the need) so that the whole OS including networking and GUI is very compact, understandable, and hackable. This particular project focused on compactness, just to prove that it is quantitatively more expressive. """ comment by sp332 https://news.ycombinator.com/item?id=11687952 https://news.ycombinator.com/item?id=11687952 final report from 2016 https://news.ycombinator.com/item?id=11686325 https://news.ycombinator.com/item?id=11686325
knighthack 2y ago
Try Nim's macros.
klibertp 2y ago
You're Lisp-baiting, aren't you? ;) I'd add Elixir next to Nim (already mentioned); also Rust. Recently, also Scala. The reason we don't have such metaprogramming available everywhere is mostly because you have to subscribe to a particular ideology to allow it. If you think programmers are generally intelligent and responsible, you put macros and metaclasses in your language. If, on the other hand, you think most programmers are dumb code monkeys (with a few exceptions, maybe) your language becomes like early Java or early Go.
- keybored 2y ago
  That dichotomy is interesting considering Guy Steele’s Growing a Language talk and his Scheme background. But maybe just mentioning Scheme is misleading here...
  - klibertp 2y ago
    Thanks for bringing it up. I had a good read. I agree entirely with Steele! It's ironic, however, that it's written in the context of Java - he mentions just three features that would make Java growable: generics, operator overloading, and value types. We all know how it went: generics were added sometime later, but there's still no operator overloading, and value types came to the language 20 years after the talk :)
- Y_Y 2y ago
  (Isn't lisp-baiting the raison d'être of hn?) Since you mention it, Python does have a fairly elaborate metaclass system, but it seems like it's only really used for implementing the language and rarely if ever wielded by "users". I guess that's a reflection of the language ideology you're talking about. Also for what it's worth, I know myself to be a dumb code monkey, but being in the CRUD gutter doesn't preclude me from looking at the metasyntactic stars.
radarsat1 2y ago
Why would I want to extend the language syntax as part of normal execution?

nickpresta 2y ago

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

sph 2y ago
Did OP basically summarise another blog post and publish it on their own blog, even though the original is linked? Not a good look, also because it's "narrated" in the first person, as if they were the one to implement this change.
- Wowfunhappy 2y ago
  ...it's not, although I have to say, I really appreciated the succinctness of OP's article versus the original.

nemoniac 2y ago

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-adding-a-new-statement-to-python/ https://eli.thegreenplace.net/2010/06/30/python-internals-ad...

pansa2 2y ago
That blog post seems a lot more involved - it adds code to the bytecode compiler as well as to the parser. I suspect that’s mostly because the `until` statement is more complex - but another factor seems to be Python’s change in parsing technology from LL(1) to PEG.
asplake 2y ago
I sometimes think it would be fun to do “do: ... where:/while/until”, wherein the “where” part does local bindings.
- tpoacher 2y ago
  there is a nice "staticscope" package in python, which acts as a context manager, and does pretty much what you refer to. pretty neat. https://pypi.org/project/scoping/ https://pypi.org/project/scoping/
deleted 2y ago
[deleted]

pkolchanov 2y ago

Great post! I have worked on a similar series of CPython customization articles that you might find interesting: https://github.com/pkolchanov/CustomizingPython https://github.com/pkolchanov/CustomizingPython

tgv 2y ago

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

dig1 2y ago

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))

BoingBoomTschak 2y ago
Yeah, this is the kind of material that shows that any attempt to prevent/hamper meta-programming or punt it to preprocessors is at best misguided. It also feeds the smug Lisp weenie within me. Above all the wonders of Lisp's pantheon stand its metalinguistic tools; by their grace have Lisp's acolytes been liberated from the rigid asceticism of lesser faiths. Thanks to Macro and kin, the jolly, complacent Lisp hacker can gaze through a fragrant cloud of setfs and defstructs at the emaciated unfortunates below, scraping out their meager code in inflexible notation, and sneer superciliously. It's a good feeling. -- iterate manual, A.1 Introduction do..while is also the example used here https://www.tcl-lang.org/man/tcl9.0/TclCmd/uplevel.html https://www.tcl-lang.org/man/tcl9.0/TclCmd/uplevel.html

zitterbewegung 2y ago

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

tmgehrmann 2y ago
Be aware, however, that the inner circle will use you, take your contributions, and, if you develop own opinions, publicly ban and defame you without any possibility of setting any record straight. pyhton-dev is a corporate shark tank where only personalities and employer matter (good code or ideas are optional).
- pas 2y ago
  That's a pretty strong claim, (and since most inner circles are hard to get into, I even assume it's not without any basis in reality), yet could you please provide some exhibits to support it?
  - 0x1242149 2y ago
    The inner circle emerged after GvR resigned. It largely consists of people who haven't contributed that much to Python3 (sometimes nothing at all in terms of code). The members occupy different chairs in the PSF, Steering Council and the all-powerful CoC troika. They rotate, sometimes skip one election and then come back. Their latest achievement is the banning of Tim Peters and others: https://news.ycombinator.com/item?id=41234180 https://news.ycombinator.com/item?id=41234180 https://news.ycombinator.com/item?id=41385546 https://news.ycombinator.com/item?id=41385546 Tim Peters is just the tip of the iceberg. Many bans are private, intimidation is private. Steering council members can insult, bully and mock others without any CoC consequences for themselves and keep getting elected. That is how you know that they are in the inner circle.
    - pas 2y ago
      thanks for the details and links! how they are getting elected? who can vote?

cpburns2009 2y ago

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

genter 2y ago
So if "else None" is omitted, if bar is false, then does spam == None or is it unmodified? The former is what I think you want, but that would be very confusing.
- 333c 2y ago
  Yeah, it would be confusing. In Ruby, this would leave spam unmodified.
  - 8n4vidtmkvmk 2y ago
    That's even more confusing. There's an =, I expect an assignment to happen. Maybe we just leave this one alone.
    - trehalose 2y ago
      Perhaps using a parenthesized assignment expression with the walrus operator would be unambiguous: (spam := eggs) if bar I think that seems reasonable? It would act just the same as it already does with an explicit `else None`, if I'm not mistaken. I don't find it beautiful though.
    - 333c 2y ago
      In Ruby, the `if [condition]` modifier at the end of a line is used for more cases than just assignment. For example, `return if s.blank?` and `raise "invalid" if input.length > 100`. In Ruby, this pattern makes it clear that the statement is only executed if the condition is met. I'm not advocating for this feature to be added to Python, just explaining why it's not confusing in Ruby.

bbb651 2y ago

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`.

steveklabnik 2y ago
Option is already a lang item, so that would t be an issue. I don’t know what the actual underlying reason is, or if it was ever even proposed.
- bbb651 2y ago
  I meant more than it already is, I referenced this: https://stackoverflow.com/a/43339003 https://stackoverflow.com/a/43339003. I do think it would be neat but it's niche enough to not be worth the non-obvious behavior of boxing with `Option`, e.g. you might forget an `else { ... }` and get complex type errors.

zahlman 2y ago

>Condensed version of this cool blog post. The effort is very much appreciated.

dalke 2y ago

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/2008/03/09/python4ply_tutorial_3.html 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/ 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/GardenSnake 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.

m463 2y ago

I never knew this syntax worked in python 2 and 3 1 if True else 2