Tuesday, March 19, 2019

Don't Solve My Problem.

Two and a half examples of "Don't solve the problem I described. Provide the implementation I dream about."

Can't Use Enums for Constants

I was asked to see this because sometimes there's just too much abstraction https://stackoverflow.com/questions/2668355/how-much-abstraction-is-too-much

The accepted answer links to some useful design principles. Read the answer. It's useful.

The question objects to abstract superclasses without much (or any) actual implementation.  I've seen folks toy around with frameworks where there are classes that introduce a name, but little else. So I understand the complaint. I once tried to use Python classes as surrogates for Java interfaces. It was a bad idea. And irrelevant to solving the underlying problem.

The problem that lead to the "yet another abstraction" complaint, however, was not related to a design with empty layers of framework abstractions. It was not related to classes used to define an interface-like feature in Python. It wasn't related to *anything* in the Stack Overflow question or answer.

The "yet another abstraction" complaint was based on Python not having constants. Seriously. How did we get here? Right. They don't want a solution. They want to complain.

I lift this situation up to folks who are trapped in conversations where things devolve into bizarro-world like "Yet Another Abstraction is bad" when we're not talking about abstractions. The solution is simple, but, it's not what they wanted so, it's labeled as bad in some way.

The solution is bad because it's unexpected. Consequently peripheral, tangential, weird-ass nonsense will show up in trying to avoid an unexpected solution.

Can't Assign Numbers

There's an API to load some data.  They have 100's of clients happily loading data. In some cases, the clients must assign numbers in addition to names; it's a disambiguation thing. Most of the time, the name is good. In a few cases, (name, number) is a two-part key because they have multiple instances with the same name.

We're good here. The data structure's key can be (name, number) and the default number is zero. Works for almost everyone.

Almost everyone. Exception they have one client who cannot count or enumerate their data.

Really.

The client can't even pre-process the data to add numbers because reasons.

The stated reason is "the data originates off-line and the numbers might be inconsistent." The key needs a number. It doesn't need to consistent. The point is asking the client to own the identity -- a name and a number.

The solution seemed easy. Assign a number. If your data comes from a spread-sheet, use the row number. The =row() function works. Use that.  If your data doesn't come from a spread-sheet write a tiny utility to laminate a number into the data. This doesn't seem hard. And then the client owns the object identity.

Nope.

Can't do it. The web service will have to assign the number for them.

It's not a difficult feature to add. It's a complicated, stateful default value. This will turn into trouble tickets in the future when the numbers are unacceptable because they change with each load or something even more obscure than that.

Can't Fork a Repo

This isn't recent, and I may not have the details right. But.

The team had evolved an approach where they had several different pieces of software spread among multiple branches in a single Git repository.

This was weird. And they were -- of course -- about to start having CI/CD problems as they moved away from manual builds into a world of git commit hooks and relatively fixed CI/CD pipelines.

And they were really unhappy. They liked having multiple branches in a single repo. The idea of forking this into separate repos was unacceptable. Unworkable. Breaks everything. (Breaks everything they had. Everything they needed to replace. Or so they claimed.)

They had some vision of having the CI/CD jobs all reworked to move beyond the common dev/master world into their uniquely odd world of lots of parallel branches, each it's own private "master". But all in one repo.

They seemed to have locked into a strange world view, and weren't happy discarding it. The circular discussions of how multiple repos would break something something in their something was more examples of tangential, irrelevant discussion to cloak empty whining.

tl;dr

I think there are people who don't really want a "solution." They want something else.

There are people who have a vision: How Things Should Be (HTSB™,) They seem to be utterly unwilling to consider something that is not literally their (narrow) vision of HTSB.

It's very much as "Don't confuse me with facts, my mind is made up" situation. It's exasperating to me because of the irrelevant side-channel discussions they use to avoid confronting (or even stating) the actual problem.

Tuesday, March 12, 2019

Python's multi-threading and the GIL

Got this in an email.
"Python's multi-threading module seems not efficient because of the global interpreter lock?" 
Yep.
Is the trick is to use "Thread-Local Data"?
Nope.

It Gets Worse

Interestingly, there was no further ask. The questioner had decided on thread-local data because the questioner had decided to focus on threads. And they were done making choices at that point.

Sigh.

No question on "What was recommended?" or "What's a common solution?" or "What is Dask?" Nothing other than "confirm my assumptions."

This is swirling around a bunch of emails on trying to determine the maximum number of concurrent threads or processes based on the number of cores or CPU's or something.

Maximum.

I'll repeat that for those who skim.

They think there's a maximum number of concurrent threads or processes.

If you have some computation which (1) makes zero OS requests and (2) is never interrupted, I can imagine you'd like to have all of the cores fully committed to executing that theoretical stream of instructions. You might even be able to split that theoretical workload up based on the number of cores.

Practically, however, that stream of uninterrupted computing rarely exists.

Maybe. Maybe you've got some basin-hopping or gradient-following or random forest ML algorithm which is going to do a burst of computation on an in-memory data structure. In that (rare) case, Dask is still ideal for exploiting all of the cores on your processor.

The upper-bound idea bugs me a lot.

  • Any OS request leads to a context switch. Any context switch leads to waiting. Any waiting means you can have more threads than you have cores. 
  • AFAIK, any memory write outside the local cache will lead to a stall in the pipeline. Another thread can (and should) leap in to the core's processing stream. The only way you can create the "all-computing" sequence of instructions bounded by the number of cores is to *also* be sure the entire thing fits in cache. Hahahaha.

What's the maximum number of threads or processes? It depends on the wait times. It depends on memory writes. It depends on the size of the data structure, the size of cache, and the size of the instruction stream.

Because it depends on a lot of things, it's rather difficult to predict. And that makes it rather difficult to determine a maximum.

Replying about the uselessness of trying to establish a maximum, of course, does nothing. AFAIK, they're still assiduously trying to use os.cpu_count() and os.sched_getaffinity() to put an upper bound on the size of a thread pool.

Acting as if Dask doesn't exist.

Solution

Use Dask.

Or

Use a multiprocessing pool.

These are simple things. They don't require a lot of hand-wringing over the GIL and Thread Local Data. They're built. They work. They're simple and effective solutions.

Side-bar Nonsense

From "a really smart guy. He got his PhD in quantum mechanics and he got major money to actually go build … . He initially worked for ... and now he is working for .... So, when he says something or asks a question, I listen very carefully."
The laudatory blah-blah-blah doesn't really change the argument. It can be omitted. It is an "Appeal to Authority" fallacy, and the Highest Paid Person's Opinion (HIPPO) organizational pattern. Spare me.

Indeed. Asking for my confirmation of using Thread-Local Data to avoid the GIL is -- effectively -- yet another Appeal to Authority. Don't ask me if you have a good idea. An appeal to me as an authority is exactly as bad as appeal to some other authority to convince me you've found a corner case that no one has ever seen before.

Worse is to ask me and then blah-blah-blah Steve Lott says blah-blah-blah. Please don't.

I can be (and often am) wrong.

Write your code. Measure your code's performance. Tell me your results. Explore *all* the alternatives while you're at it.

Tuesday, March 5, 2019

Python exceptions considered an anti-pattern

https://sobolevn.me/2019/02/python-exceptions-considered-an-antipattern

While eloquent and thorough, I remain unconvinced that this is a significant improvement over try/except.

It's common enough in some functional languages to have strong support and a long, successful history.

I think it replaces one problem with another. It's not a "solution". It's an alternative. Instead of exceptions being raised, they're returned. Cool.

Tuesday, February 12, 2019

On the uselessness of Enum -- wait, what?

Had a question about an enumerated set of constant values.

"Where do I put these constants?" they asked. It was clear what they wanted. This is another variation on their personal quest which can be called "I want Python to have CONST or Final." It's kind of tedious when a person asks -- repeatedly -- for a feature that's not present in the form they want it.

"Use Enum," I said.

"Nah," they replied. "It's Yet Another Abstraction."

Wait, what?

This is what I learned from rest of their nonsensical response: There's an absolute upper bound on abstractions, and Enum is one abstraction too many. Go ahead count them. This is too many.

Or.

They simply rejected the entire idea of learning something new. They wanted CONST or Final or some such. And until I provide it, Python is garbage because it doesn't have constants. (They're the kind of person that needs to see CONST minutes_per_hour = 60 in every program. When I ask why they don't also insist on seeing CONST one = 1 they seem shocked I would be so flippant.)

YAA. Seriously. Too many layers.

As if all of computing wasn't a stack of abstractions on top of stateful electronic circuits.

Tuesday, February 5, 2019

Python Enhancement Proposal -- Floating an Idea

Consider the following code

def max(m: int, n: int) -> int:
    if m >= n:
        return m
    elif n >= m:
        return n
    else:
        raise Exception(f"Design Error: {vars()}")

There's a question about else: clause and the exception raised there.
  • It's impossible. In this specific case, a little algebra can provide that it's impossible. In more complex cases, the algebra can be challenging. In some cases, external dependencies may make the algebra impossible.
  • It's needless in general. An else: would have been better than the elif n >= m:.  The problem with else: is that a poor design, or poor coordination with the external dependencies, can lead to undetectable errors.
Let's look at something a little more complex.

def ackermann(m: int, n: int) -> int:
    if m < 0 or n < 0:
        raise ValueError(f"{m} and {n} must be non-negative")
    if m == 0:
        return n + 1
    elif m > 0 and n == 0:
        return ackermann(m - 1, 1)
    elif m > 0 and n > 0:
        return ackermann(m - 1, ackermann(m, n - 1))
    else:
        raise Exception(f"Design Error: {vars()}")

It's somewhat less clear in this case that the else: is impossible. A little more algebra is required to create a necessary proof.

The core argument here is Edge Cases Are Inevitable. While we can try very assiduously to prevent them, they seem to be an emergent feature of complex software. There are two arguments that seem to indicate the inevitability of edge and corner cases:

  • Scale. For simple cases, with not too many branches and not too many variables, the algebra is manageable. As the branches and variables grow, the analysis becomes more difficult and more subject to error. 
  • Dependencies. For some cases, this kind of branching can be refactored into a polymorphic class hierarchy, and the decision-making superficially simplified. In other cases, there are multiple, disjoint states and multiple conditions related to those states, and the reasoning becomes more prone to errors.
The noble path is to use abstraction techniques to eliminate them. This is aspirational in some cases. While it's always the right thing to do, we need to check our work. And testing isn't always sufficient.

The noble path is subject to simple errors. While we can be very, very, very, very careful in our design, there will still be obscure cases which are very, very, very, very, very subtle. We can omit a condition from our analysis, and our unit tests, and all of our colleagues and everyone reviewing the pull request can be equally snowed by the complexity. 

We have two choices.
  1. Presume we are omniscient and act accordingly: use else: clauses as if we are incapable of error. Treat all complex if-elif chains as if they were trivial.
  2. Act more humbly and try to detect our failure to be omniscient.
If we acknowledge the possibility of a design error, what exception class should we use?
  • RuntimeError. In a sense, it's an error which didn't occur until we ran the application and some edge case cropped up. However. The error was *always* present. It was a design error, a failure to be truly omniscient and properly prove all of our if-elif branches were complete.
  • DesignError. We didn't think this would happen. But it did. And we need debugging information to see what exact confluence of variables caused the problem.
I submit that DesignError be added to the pantheon of Python exceptions. I'm wondering if I should make an attempt to write and submit a PEP on this. Thoughts?

Tuesday, January 29, 2019

Eager and Lazy Properties

See this


My answer was -- frankly -- vague. Twitter being what it is, I should have written the blog post first and linked to it.

The use case is this.

class X:
    def __init__(self, x):
        self._value = f(x)
    @property
    def value(self):
        return self._value

We've got a property where the returned value is already an instance variable.

I'm not a fan.

This reflects an eager computation strategy. f(x) was computed eagerly and the value made available via a property. One can justify the use of a property to make the value read-only, but... still nope.

There are a lot of alternatives that make more sense to me.

Option 1. We're All Adults Here.

Here's an approach I think is better.

class X:
    def __init__(self, x):
        self.value = f(x)

It's read-only because -- really -- if you change it, you break the class. So don't change it.

This is my favorite. Read-onlyness is sometimes described has a way protect utter idiots from breaking a library they don't seem to understand. Or. It's described as a way to prevent some Evil Genius Programmer (EGP) from doing something intentionally malicious and breaking things.

Bah.

It's Python. They have access to the source. Why mess around breaking things this way?

Option 2. Lazyiness

Here's an approach that hits at the essential feature.

class X:
    def __init__(self, x):
        self.x = x
    @property
    @lru_cache(None)
    def value(self):
        return f(self.x)

This seems to hit at the original intent without an explicit cached variable. Instead the caching is pushed off into another space. (I'm writing a chapter on decorators, so this may be a bit much.)

The idea, though, is to make properties lazy. If they're expensive, then the result should be cached.

There may be other choices, but I think lazy and eager cover the bases. I don't think eager is wrong, but I don't see the need for a property to hide the attribute created by an eager computation.


Things that start badly

Today's Example of Starting Badly: Building HTML.

The code has a super-simple email message with f"<html><body><p>stuff {data}</p></body></html>". It was jammed into an email object along with the text version. All very nice.

For a moment, I considered suggesting that f-string substitution wasn't a good long-term solution, since it doesn't cover anything more than the most trivial case.

Two things stopped me from complaining:
  • The case really was trivial.
  • It's administrative code: it sends naggy reminder emails periodically. Why over-engineer it?
What an idiot I was.

Today, the {data} has been replaced with a complex table instead of a summary. (Why? The user story evolved. And we needed to replace the summary with details.)

The engineer was pretty sure they could use htmlify(data) or data.htmlify() to transform the data into an HTML structure without seriously breaking the f-string nature of the app.

I should have commented "Don't build HTML that way, it's a bad way to start" on the previous release.

The f-string solution turns rapidly into complexities layered on complexities dusted over the top with sprinkles of NOPE.

This is a job for Jinja2 or Mako or something similar. 

There's a step function change in the app's perceived "complexity". Instead of a simple f-string, we now have to populate a template. It goes from one line of code to more than one (three seems typical.) And. The file-system loader for templates seems more appropriate rather than hard-coding the template in the body of the code. So there are now more files in the app with the HTML templates in them.

However. The Jinja {{variable|round(2)}} was an immediate victory. The use of {%for%} to build the HTML table was the goal, and that simplification was worth the price of entry. Now we're arguing over CSS nuances to get the columns to look "right."

Lessons learned.

Don't let the currently superficial trivial case slide past without at least a warning. Make the suggestion that functions like "get template" and "populate template" will be necessary even for trivial f-string or string.Template processing.

HTML isn't a first-class part of anything. It's external serialization.  Yes, it's for people, but it's only serialization. Serialization has to be separated from the other aspects of the data gathering, map-reduce summarization, and email distribution. There's a pipeline of steps there and the final app should reflect the complete separation of these concerns. Even if it is admin overhead.

Tuesday, January 22, 2019

Read this: 10 Reasons to Learn Python in 2019

See 10 Reasons to Learn Python in 2019.

I want to dwell on #4 for a moment.

The Python community actually has a Code of Conduct. We try to stick by it and conferences will have reporting mechanisms in place so we can educate folks who are being inconsiderate or disrespectful.

The consequence of this is a welcoming and intentionally helpful community. It's hard to emphasize the "intentionally helpful" enough. We don't have much patience for snark. And we're willing to call each other out on being unhelpful.

In my Day Job, we have an in-house Slack channel with well over 1,000 Python folks. The single most common class of questions is a variation on "My Corporate Firewall Setup Doesn't Let Me Use PIP." This is ubiquitous. And confusing. And frustrating for folks who are surprised there is a corporate firewall.

We have a number of pinned answers in Slack for this. And -- perhaps once a week -- someone will patiently repeated the pinned answers for someone who's truly and deeply in over their head trying to get pip to work. (We have an in-house PyPI, also, but it requires doing something in addition to typing `pip install whatever` at the command line, and that can require hand-holding.)

As Python2 winds to a close, and we uncover folks working with Python 2, we have to issue guidance. I've switched tone from "please consider rewriting your app/tool/framework" to "we strongly recommend you start using Python 3." In June, I'm plan to switch to "You have only six months to convert whatever you're working on."

We've had some sidebar conversations on making sure I'm being properly positive, supportive, considerate, and respectful of the folks who think Python2 might be useful.

The point of the Python community is to help each other. We're actively and intentionally trying to be helpful and inclusive.

Technical Sidebar -- Conda and Virtual Environments 

What about the trickle of people trying to make use of the built-in Python2 in Mac OS X or the Python2 that comes with Linux on a cloud-based server?

Some important coaching: Don't Use The OS Default Python.

This is kind of negative. It helps to state this positively. Always Use A Virtual Environment.

Because we have a *large* community of data scientists, this becomes: Always Use A Conda Environment.

And yes, there are some packages that also require a pip install. And yes, XKCD 1987 describes the consequence of the rapid growth of Python and the variety of ways it can be made to work. (While all the strands of spaghetti look like a negative, they reflect the variety of clever solutions, all of which work without any problems.)

Therefore. This.

1. conda create --name=working python=3.7 --file conda_install.txt
2. conda activate working
3. python3 -m pip install pip_install.txt

The absolute worst case is a project with two lists of requirements, one in a conda conda_install.txt and some extra stuff in a pip conda_install.txt. We're able to use `python3 -m pip install requirements.txt` for almost everything.

If you're just starting out, you can use miniconda to bootstrap everything else you might need.

Tuesday, January 15, 2019

Super-picky Writing Advice

There are patterns to bad writing. I'll give some examples based on a blog post I was sent. It's also based -- indirectly -- on some of proposals I saw for PyCon and PyDataDC.

For the conference calls for papers, I can ask a few questions of the author, but that's about it.

For the blog post, I suggested a bunch of changes.

They balked.

Why ask for advice and then refuse to do anything?  (We can conjecture they wanted a "good job" pat on the head. They didn't want to actually have me give them a list of errors to fix.)

One of the points of contention was "Not everyone has your depth of expertise."

Sigh.

The blog post was on Ubuntu admin: something I know approximately nothing about. Let me step away from being an expert, while sticking closely with being able to write. I work with editors who -- similarly -- can write without being deep technology experts. I'm trying to learn what and how they do it.

In this case, my editing was based on general patterns of weak writing.

  1. Contradictions
  2. Redundancy.
  3. Waffling.
  4. Special-Casing.

Any blog post on Ubuntu admin that starts with "This is not just another blog post..." has started off with a flat-out contradiction. It emphatically is another blog post. You can't rise above the background noise of blog posts by writing a blog post that claims it's not a blog post. Sheesh. Find a better hook.

Any blog post on Ubuntu admin that includes "This blog post assumes the reader is familiar with linux sys administration." As if -- somehow -- a reader interested in Ubuntu admin could be confused by the required skills. It's clearly redundant. Cut it.

(This led to an immense back-and-forth with repeated insistence that *somehow* someone once got confused and *something* bad happened to someone. Once. My response was adamant. "It's redundant. The title says Ubuntu. That covered it. More repetition is an insult to the reader.")

Anything that has "this may nor may not work depending on your filesystem" is flat-out confusing: it covers both bases. Does it work or does it not work? Which is it? Clearly, there's some kind of precondition -- "must be this file system" or "most not be this file system" -- buried under "may or may not work." It's not that I know anything about Ubuntu file systems. But I can spot waffling.

Indeed, when you look at it, this is a "hook" to make the blog post useful and interesting. Some advice doesn't work. This advice always works. Simple statements of fact are better than contradictions and waffling.

Finally, there was a cautionary note that replacing "/swapfile" with "/ swapfile" would brick your OS. Which. Was. Crazy. It's really difficult to arbitrarily introduce spaces into shell commands and still have proper syntax. Sometimes a shell command with rando spaces may have proper syntax and may work. Most commands won't work at all with a rando space added. Try some and see.

What's more important is only one random punctuation error was listed. The special-case nature of this was a tip-off that something was not right with this advice.

What about a random ">" in the command? Or a random "|"? Not covered. A single space was considered worthy of mention. The rest. Meh.

(If you want details, it's was `dd -of=/swapfile ...`. If asked, I'd guess `dd -of=/ swapfile` is a syntax error because `swapfile` isn't a valid operand to `dd`. Not an expert. I didn't check. AFAIK, the author didn't actually check, either.)

None of this editing was based on any vast expertise. It was simple editorial work looking for some common problems with hasty writing.

  1. Avoid clear contractions.
  2. Avoid redundancy.
  3. Don't waffle.
  4. Special cases are instances of a more general pattern. The pattern is more important than the case.

One of the contentious back-and-forth issues was "I'm not writing a book, it's only a blog post."

Wait. What? Blog posts are often more widely-read than books. Look at Stack Overflow. If one of my books had the kind of readership my Stack Overflow answers have, I'd be living on royalty payments alone.

A blog post requires the same depth of care as a book.

This applies to proposed talks at conferences, also. The proposal needs to be carefully edited to reflect the final presentation's care and quality.

Thank goodness, most of the 100's of proposals I've looked at rarely have the four obvious problems listed above.

The problems I see in conference proposals are minor.

  1. Incompleteness. A 45 minute talk boiled down to 4 bullet points doesn't give us any confidence. It's hard to imagine filling the whole time with useful content when looking at a four-sentence outline. Will it be rambling digressions? Or will we have disgruntled attendees who had hoped for more?
  2. Weirdly cute style. Things like "This is where I jokingly outline something something and the real fun begins." We assume everyone is witty and charming, you don't need to tell us. We assume all talks will be fun. Can we move on to the Python (or PyData) topics you'll cover?
  3. Sales Pitches. "[Speaker's name] is a respected industry expert who delivers exciting and transformative keynote addresses and will dynamically cover the state-of-the-art blah blah blah..." Please. What Python topic is this? We like to review the outlines without speaker information; we need to focus on the content. Subverting this by including the speaker's name in the description or outline is irritating.
I'm really pleased we see very few PyCon Code-of-Conduct problems in the calls for proposals. That is a delight.

Editing is hard work. I'm sorry to report that editing means making changes. If you ask for editorial advice, it helps to listen.

Tuesday, January 8, 2019

Tuesday, January 1, 2019

PyCon Call for Proposals

PyCon CFP closes in a day or so. See https://us.pycon.org/2019/speaking/ for details.

After looking at 100's of proposals there are several broad categories.

  • Some cool stuff I wrote (or helped write, or contributed to)
  • Some cool aspect of Python the language
  • Some cool thing happening in the community

These are -- of course -- refined into separate tracks by the program committee. But as a reader of outlines, there are a few BIG buckets I tend to throw things into.

There are some tangentially Python things.

  • Using k8s (or some other tool or framework) with Python. The focus is often the tool, not Python.
  • Doing some machine learning with Python. The focus is often on the ML application domain or the cool model that got produced. Again, Python sometimes feels secondary.
  • Optimizing a personal (or enterprise) workflow or CI/CD pipeline. This can be helpful. But it can sometimes skip past Python and focus on IDE or something not solidly Python.
One of the most challenging are the personal journeys. These are often interesting stories. We like to see how others have succeeded. But. (And this is important.) Some of these cool personal journeys are only tangentially Pythonic, which can be disappointing. Some people can make good things happen in spite of big obstacles.

A personal journey can happen with a lot of technologies. When the proposal doesn't seem to show how Python was a unique enabler of the journey, then it's a cool talk for SXSW or OSCON, but might not be ideal for PyCon.

It's an honor to see everyone's ideas streaming through my in-box.