First draft of dtparse post
Bradlee Speice
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layout: post
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title: "What I Learned: Porting Dateutil Parser to Rust"
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description: ""
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category:
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tags: [dtparse, rust]
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---
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Hi. I'm Bradlee.
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I've mostly been a lurker in Rust for a while, making a couple small contributions here and there.
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So launching [dtparse](https://github.com/bspeice/dtparse) feels like nice step towards becoming a
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functioning member of society. But not too much, because then you know people start asking you to
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pay bills, and ain't nobody got time for that.
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But I built dtparse, and you can read about my thoughts on the process. Or don't. I won't tell you
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what to do with your life (but you should totally keep reading).
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# Slow down, what?
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OK, fine, I guess I should start with *why* someone would do this.
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[Dateutil](https://github.com/dateutil/dateutil) is a Python library for handling dates.
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While the standard library support for time in Python is kinda dope, there's a lot of pieces
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that go into making it useful beyond just the [datetime](https://docs.python.org/3.6/library/datetime.html)
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module.
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Specifically, `dateutil.parser` is code to take all the super-weird time formats people
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come up with and turn them into something actually useful. Just like [everything else](https://zachholman.com/talk/utc-is-enough-for-everyone-right)
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[involving](https://i.redd.it/syw7q6gc77f01.jpg) [computers](https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time)
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and [time](https://infiniteundo.com/post/25509354022/more-falsehoods-programmers-believe-about-time),
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it feels like it shouldn't be that difficult to do, until you try to do it,
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and you realize that people just suck and this is why can't we have nice things.
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But alas, we can still try and make contemporary art out of the rubble.
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What makes `dateutil.parser` great is that there's single super-important function: `parse(time_string)`.
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It takes in the time as a string, and gives you back a reasonable "look, this is the best
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anyone can possibly do to make sense of your input" value. It doesn't expect much of you.
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Which is great. And now it's in Rust.
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# Lost in Translation
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Having worked at Bank of America and seeing Java programmers try to be Python programmers,
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I'm admittedly hesitant to publish Python code that's pretending to be Rust.
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Interestingly, Rust code can actually do a great job of mimicking Python.
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It's certainly not idiomatic Rust, but [the Iterator pattern is the same](https://webcache.googleusercontent.com/search?q=cache:wkYMpktJtnUJ:https://jackstouffer.com/blog/porting_dateutil.html+&cd=3&hl=en&ct=clnk&gl=us).
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When transcribing code, **stay as close to the original library as possible**. I'm talking
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about using the same variable names, same access patterns, the whole shebang.
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It's way too easy to make a couple of typos, and all of a sudden
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your code blows up in new and exciting ways. Having a reference manual for verbatim
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what your code should be means that you don't spend that long debugging complicated logic,
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you're more looking for typos.
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Also, **don't use nice Rust things like enums**. While
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[one time it worked out OK for me](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L88-L94),
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I also managed to shoot myself in the foot a couple times because `dateutil` stores AM/PM as a boolean
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and I got mixed up on the enum trying to figure out which AM and PM were (side note: AM is false, PM is true).
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In general, writing nice code *should not be a first-pass priority* when you're just trying to recreate
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the same functionality.
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**Exceptions are a pain.** Make peace with it. Python code is just allowed to skip stack frames.
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So when a co-worker told me "Rust is getting try-catch syntax" I properly freaked out.
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Turns out [he's not quite right](https://github.com/rust-lang/rfcs/pull/243), and I'm OK with that.
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And while `dateutil` is pretty well-behaved about not skipping multiple stack frames,
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[130-line try-catch blocks](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L730-L865)
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take a while to verify.
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As another Python quirk, **be very careful about [long nested if-elif-else blocks](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L494-L568)**.
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I used to think that [Python's whitespace](https://www.xkcd.com/353/) was just there
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to get you to format your code correctly. I think that no longer. It's way too easy
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to close an extra block and have incredibly weird issues in the logic.
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**Rust macros are not free.** I originally had the
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[main test body](https://github.com/bspeice/dtparse/blob/b0e737f088eca8e83ab4244c6621a2797d247697/tests/compat.rs#L63-L217)
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wrapped up in a macro using [pyo3](https://github.com/PyO3/PyO3). It took two minutes to compile. After
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[moving things to a function](https://github.com/bspeice/dtparse/blob/e017018295c670e4b6c6ee1cfff00dbb233db47d/tests/compat.rs#L76-L205)
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compile times dropped down to ~5 seconds. Turns out 150 lines * 100 tests = a lot of redundant code.
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My new rule of thumb is that any macros longer than 10-15 lines are actually functions that need to be liberated, man.
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Finally, **I really miss list comprehensions and dictionary comprehensions.**
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As a quick comparison, see
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[this dateutil code](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L476)
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and [the implementation in Rust](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L619-L629).
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Ultimately, I hope that these can be added through macros, but I have a feeling that they'd actually
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need to be syntax extensions. Either way, they're expressive, save typing, and super-readable. Let's get more of that.
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# Using a young language
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Now, Rust is exciting and new, which means that there's opportunity to make a substantive impact.
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On more than one occasion I've had issues navigating the Rust ecosystem though.
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What I'll call the "canonical library" is still being built. In Python, if you need datetime parsing,
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you use `dateutil`. If you want [Decimal](https://docs.python.org/3.6/library/decimal.html) types,
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it's already in the standard library. It's probably
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[not strictly necessary in `dateutil`](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L1242),
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but I wanted to follow the principle of **stay as close to the original library as possible**
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and thus began my quest to find a decimal library in Rust. What I quickly found was summarized
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in a comment:
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> Writing a BigDecimal is easy. Writing a *good* BigDecimal is hard.
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>
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> [-cmr](https://github.com/rust-lang/rust/issues/8937#issuecomment-34582794)
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In practice, this means that there are at least [4](https://crates.io/crates/bigdecimal)
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[different](https://crates.io/crates/rust_decimal) [implementations](https://crates.io/crates/decimal)
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[available](https://crates.io/crates/decimate). And that's a lot of decisions to worry about
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when all I'm thinking about is "I just want a reasonable Decimal library" and I'm forced to dig through a
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[couple](https://github.com/rust-lang/rust/issues/8937#issuecomment-31661916)
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[different](https://github.com/rust-lang/rfcs/issues/334)
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[threads](https://github.com/rust-num/num/issues/8) to figure out if the library I'm look at is DOA stable.
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And even when the "canonical library" exists for something like timezones ([`pytz`](https://pythonhosted.org/pytz/) and
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more recently [`dateutil.tz`](https://dateutil.readthedocs.io/en/stable/tz.html) in Python), there's no guarantees
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that it will be well-maintained. [Chrono](https://github.com/chronotope/chrono) is currently the canonical datetime
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library in Rust, and just released version 0.4.3 like a week ago. Meanwhile, [chrono-tz](https://github.com/chronotope/chrono-tz)
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appears to be dead in the water even though [there are people happy to help maintain it](https://github.com/chronotope/chrono-tz/issues/19).
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I know relatively little about it, but it appears that most of the release process is automated; keeping
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that up to date should be a no-brainer.
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## Trial Maintenance Policy
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Specifically given "maintenance" being an [oft-discussed](https://www.reddit.com/r/rust/comments/48540g/thoughts_on_initiators_vs_maintainers/)
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issue, I'm going to try out the following policy to keep things moving on [dtparse]:
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1. Issues/PRs needing *maintainer* feedback will be updated at least weekly. I want to make sure nobody's blocking on me.
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2. To keep issues/PRs needing *contributor* feedback, I'm going to (kindly) ask the contributor to check in after two weeks,
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and close the issue without resolution if I hear nothing back after a month.
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The second point I think has the potential to be a bit controversial, so I'm happy to receive feedback on that.
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And if a contributor responds with "hey, still working on it, had a kid and I'm running on 30 seconds of sleep a night,"
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then first congratulations on sustaining human life, and second I don't mind keeping those going indefinitely.
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I just want to try and balance keeping things moving with giving people the necessary time.
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I should also note that I'm still getting some best practices in place - CONTRIBUTING and CONTRIBUTORS files
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need to be added, as well as issue/PR templates. In progress.
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# Roadmap and Conclusion
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So if I've now built a `dateutil`-compatible parser, we're done, right? Of course not! That's not
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nearly ambitious enough.
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Ultimately, I'd love to have a library that's capable of essentially everything the Linux `date`
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command can do (and not `date` on OSX, because seriously, it's the worst). I know Rust has a
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coreutils rewrite going on, and this would be potentially an interesting candidate since it
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doesn't bring in a lot of extra dependencies for the functionality it provides.
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[`humantime`](https://crates.io/crates/humantime) also is able to parse durations,
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so maybe we negotiate something to integrate it all together?
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All in all, I'm really hoping that nobody's already done this and I've spent a bit over a month
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on redundant code. So if it exists, tell me because I need to know, but be nice about it.
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And in the mean time, I'm looking forward to building more. Onwards.
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