Posts tagged python
The plea for stability in the SciPy ecosystem that I posted last week on this blog has generated a lot of feedback, both as comments and in a lengthy Twitter thread. For the benefit of people discovering it late, here is a summary of the main arguments and my reply to them.
Two NumPy-related news items appeared on my Twitter feed yesterday, just a few days after I had accidentally started a somewhat heated debate myself concerning the poor reproducibility of Python-based computer-aided research. The first was the announcement of a plan for dropping support for Python 2. The second was a pointer to a recent presentation by Nathaniel Smith entitled "Inside NumPy" and dealing mainly with the NumPy team's plans for the near future. Lots of material to think about... and comment on.
A few days ago, I noticed this tweet in my timeline:
I 'still' program in C. Why? Hint: it's not about performance. I wrote an essay to elaborate... appearing at Onward! https://t.co/pzxjfvUs5B— Stephen Kell (@stephenrkell) September 5, 2017
That sounded like a good read for the weekend, which it was. The main argument the author makes is that C remains unsurpassed as a system integration language, because it permits interfacing with "alien" code, i.e. code written independently and perhaps even in different languages, down to assembly. In fact, C is one of the few programming languages that lets you deal with whatever data at the byte level. Most more "modern" languages prohibit such interfacing in the name of safety - the only memory you can access is memory allocated through your safe language's runtime system. As a consequence, you are stuck in the closed universe of your language.
A few years ago, I decided to adopt the practices of reproducible research as far as possible within the technical and social constraints I have to live with. So how reproducible is my published code over time?
We all know that software deployment in a research environment can be a pain, but knowing this as a fact is not quite the same as experiencing it in reality. Over the last days, I spent way more time that I would have imagined on what sounds like a simple task: installing a scientific application written in Python on a Linux machine for use by a group of students in a training session. Here is an outline of the difficulties, in the hope that it will (1) help others who face similar problems and (2) contributes a little bit to improving the situation.
Yesterday I participated (as a visitor) in the kickoff meeting for OpenDreamKit, where one recurrent topic of discussion was notebooks, both Jupyter and Sage, including the question if they could be brought together. This reminded me of a recent blog post by Kirill Pomogajko entitled "Why I don't like Jupyter". And it reminded me of my own long-term project of integrating Jupyter with my ActivePapers system for reproducible research. That's three reasons for writing down my thoughts about notebooks and their role(s) in computational research, so here we go.
One key observation is in Gaël Varoquaux's comment on Kirill's blog post: using Jupyter for doing science creates a lock-in, because all collaborators on a project must agree on using Jupyter. There is no other tool that can be used productively for working with notebooks. It's a case of "wordization": digital content is taken hostage by a tool that defines a storage format for its own convenience without much consideration for other tools, be they competing or complementary. Wordization not only restricts the users' freedom to work with their data, but also creates headaches for the future. A data format defined by a tool can easily become unusable as the tool evolves and introduces incompatibilities, or of course if it disappears. In the case of Jupyter, its developers have always provided upgrade paths for notebooks between versions, but at some time this is bound to create trouble. Bugs are a fact of life, and I don't expect that the version-2-compatibility-feature will get much testing in Jupyter version 23. To make it worse, a Jupyter notebook can depend on third-party code that implements embedded widgets. This is one of the reasons why I don't use Jupyter for my research, although I am a big fan of using it for teaching. The other reason is that I cannot usefully link a notebook to other relevant information, such as code and data dependencies. Jupyter doesn't provide any functionality for this, and they are hard to implement externally exactly because of wordization.
Wordization is often associated with evil intentions of market dominance, as they are regularly assumed for a company like Microsoft. But I believe that the fundamental cause is the obsession with tools over content that has driven the computing industry for many years. The tool aspects of a piece of software, such as its feature list and its user interface, are immediately visible. On the contrary, its data model attracts attention only by a few specialists, if at all. Users feel the consequences of bad (or absent) data model design through the symptoms of wordization, in particular lock-in, but rarely understand where it comes from. Interestingly, this problem was also mentioned yesterday at the OpenDreamKit meeting, by Michael Kohlhase who discussed the digital representation of mathematical knowledge and the difficulty of exchanging it between different software tools. I have written earlier about another aspect, the representation of scientific models in computational science, which illustrates the extreme case of tools having absorbed scientific content to the point that its users don't even realize that something is missing.
Back to notebooks. Let's forget about tools for the moment and consider the question of what a notebook actually is, as a digital document. I think that notebooks are trying to be two different things, and that many of the problems we have with them come from this ambiguity. One role of notebooks is the documentation of computational work as a narrative with direct access to the data. This is why people publish notebooks. The other role is as a protocol of interactive explorative work, i.e. the computational scientist's equivalent of a lab notebook. The two roles are not completely unrelated, but they still significatively different.
To see the difference, look at how experimental scientists worked in the good old days of pencil, paper, and the printing press. As experiments were done, all the relevant information (preparation, results, …) was written down, immediately, with a time stamp, in the lab notebook. Like a bank ledger, a lab notebook is an immutable protocol of what happened. You don't go back and change earlier entries, that would even be considered fraud. You just add information at the end. Of course, the resulting protocol is not a good way to communicate one's findings. Therefore they are distilled and written up in a separate narrative, which surrounds a description of the work and its most important results by a motivating introduction and summarizing conclusions. This is the classic scientific article.
Today's computational notebooks are trying to be both protocol and narrative, and pretend that there is a fluent transition between them. One unfortunate consequence is that computational protocols disappear as they are edited to become narratives. This could be alleviated by keeping notebooks under version control, but I have yet to see good versioning support in any notebook-type tool. But, fundamentally, today's notebook tools don't encourage keeping a protocol. They encourage frequent changes to the code and the results, keeping only the latest version. As editors for narratives, notebook tools are also far from ideal because they encourage interactive execution of small code snippets, making it easy to lose track of what was actually executed and in what order. In Jupyter, the only way to ensure a coherent narrative is to (1) restart the kernel and (2) re-execute all cells. There is not even a single menu entry for this operation. Actually, I wonder how many Jupyter users are aware that they must restart the kernel before re-executing all the cells if they want to ensure reproducibility.
With all that said, here is my current idea of what a notebook should look like at the bit level. A notebook data model should have two distinct entries, one for a protocol and one for a narrative. The protocol entry is a sequence of code cells and results, as they were executed since the start of the computation (for Jupyter, that means the last kernel restart). The narrative is a user-edited sequence of code cells, documentation cells, and results. The actual cell contents could well be shared between the two views: store each cell with a unique ID, and make the protocol and the narrative simple lists of IDs. The representation of code and documentation cells in such a data model is straightforward, though there's a huge potential for bikeshedding in defining the details. The representation of results is much more difficult if you want to support more than plain text output. In the long run, it will be inevitable to define clear data models for every type of display widget, which is a lot of work.
From the tool point of view, the current Jupyter interface could be complemented by a non-editable protocol view. I'd also like to see a single command (menu/keyboard) for the "clean slate" operation: save the current state as a snapshot (or commit it directly to version control), restart the kernel, and re-initialize the protocol to an empty list. But what really matters to me is the data model. Contrary to the current one implemented in Jupyter, the one outlined above could be integrated into workflow management and archivation tools, such as my own ActivePapers. We'd probably see an Emacs mode for working with it as well. Plus pretty-printing tools, analysis tools, etc. We'd see an ecosystem of tools working with notebooks. A Dream of Openness.
SciPy 2015 is over, meaning that many non-participants like myself are now busy catching up with what happened by watching the videos. Today's dose for me was Jake VanderPlas' keynote entitled "State of the Tools". It's about the history, current state, and potential future of what is now generally known as the Scientific Python ecosystem: the large number of libraries and tools written in or for Python that scientists from many disciplines use to get their day-to-day computational work done.
History is done, the present status is a fact, but the future is open to both speculation and planning, so that's what I find most interesting in Jake's keynote. What struck me is that everything he discussed was about paying back technical debt: refactoring the core libraries, fixing compatibility problems, removing technical obstacles to installation and use of various tools. In fact, 20 years after Python showed up in scientific computing, the ecoystem is in a state that is typical for software projects of that age: a bit of a mess. The future work outlined by Jake would help to make it less of a mess, and I hope that something like this will actually happen. The big question mark for me is how this can be funded, given that it is "only" maintenance work, producing nothing fundamentally new. Fortunately there are people much better than me at thinking about funding, for example everyone involved in the NumFOCUS foundation.
Jake's approach to outlining the future is basically "how can we fix known problems and introduce some obvious improvements" (but please do watch the video to get the full story!). What I'd like to present here is an alternate approach: imagine an ideal scientific computing environment in 2015, and try to approximate it by an evolution of the current SciPy ecosystem while retaining a sane level of backwards compatibility. Think of it as the equivalent of Python 3 at the level of the core of the scientific ecosystem.
One aspect that has changed quite a bit over 20 years is the interaction between Python and low-level code. Back then, Python had an excellent C interface, which also worked well for Fortran 77 code, and the ease of wrapping C and Fortran libraries was one of the major reasons for Python's success in scientific computing. We have seen a few generations of wrapper code generators, starting with SWIG, and the idea of a hybrid language called Pyrex that was the ancestor of today's Cython. LLVM has been a major game changer, because it permits low-level code to be generated and compiled on-the-fly, without explicitly generating wrappers and compiling code. While wrapping C/C++/Fortran libraries still remains important, the equally important task of writing low-level code for performance can be handled much better with such tools. Numba is perhaps the best-known LLVM-based code generator in the Python world, providing JIT compilation for a language that is very similar to a subset of Python. But Numba is also an example of the mindset that has led to the current mess: take the existing ecosystem as given, and add a piece to it that solves a specific problem.
So how would one approach the high-/low-level interface today, having gained experience with LLVM and PyPy? Some claim that the distinction doesn't make sense any more. The authors of the Julia language, for example, claim that it "avoids the two-language problem". However, as I have pointed out on this blog, Julia is fundamentally a performance-oriented low-level language, in spite of having two features, interactivity and automatic memory management, that are traditionally associated with high-level languages. By the way, I don't believe the idea of a both-high-and-low-level language is worth pursuing for scientific computing. The closest realization of that idea is Common Lisp, which is as high-level as Python, perhaps more so, and also as low-level as Julia, but at the cost of being a very complex language with a very steep learning curve, especially for mastering the low-level aspects. Having two clearly distinct language levels makes it possible to keep both of them manageable, and the separation line serves as a clear warning sign to scientists, who should not attempt to cross it without first acquiring some serious knowledge about software development.
The model to follow, in my opinion, is the one of Lush and Terra. They embed a low-level language into a high-level language in such a way that the low-level code is a data structure at the high level. You can use literals for this data structure and get the equivalent of Numba. But you can also write code generators that specialize low-level code for a given problem. Specialization allows both optimization and simplification, both of which are desirable. The low-level language would have arrays as a primitive data structure, and both NumPy and Pandas, or evolutions such as xray, would become shallow Python APIs to such low-level array functionality. I think this is much more powerful than today's Numba building on NumPy. Moreover, wrapper generators become simple plain Python code, making the construction of interfaces to complex libraries (think of h5py) much easier than it is today. Think of it as ctypes on steroids. For more examples of what one could do with such a system, look at metaprogramming in Julia, which is exactly the same idea.
Another aspect that Jake talks about in some detail is visualization. There again, two decades of code written by people scratching their own itches has led to a mess of different libraries with a lot of overlap and no clear distinctive features. For cleaning it up, I propose the same approach: what are the needs and the available technologies for scientific visualization in 2015? We clearly want to profit from all the Web-based technologies, both for portability (think of mobile platforms) and for integration with Jupyter notebooks. But we also need to be able to integrate visualization into GUI applications. From the API point of view, we need something simple for simple plots (Toyplot looks promising), but also more sophisticad APIs for high-volume data visualization. The main barrier to overcome, in my opinion, is the current dominance of Matplotlib, which isn't particularly good in any of the categories I have outlined. Personally, I don't believe that any evolution of Matplotlib can lead to something pleasant to use, but I'd of course be happy to be proven wrong.
Perhaps the nastiest problem that Jake addresses is packaging. He seems to believe that conda is the solution, but I don't quite agree with that. Unless I missed some recent evolutions, a Python package prepared for installation through conda can only be used easily with a Python distribution built on conda as well. And that means Anaconda, because it's the only one. Since Anaconda is not Open Source, there is no way one can build a Python installation from scratch using conda. Of course, Anaconda is perfectly fine for many users. But if you need something that Anaconda does not provide, you may not be able to add it yourself. On the Mac, for example, I cannot compile C extensions compatible with Anaconda, because Mac Anaconda is built for compatibility with ancient OSX versions that are not supported by a standard XCode installation. Presumably that can be fixed, but I suspect that would be a major headache. And then, how about platforms unsupported by Anaconda?
Unfortunately I will have to leave this at the rant level, because I have no better proposition to make. Packaging has always been a mess, and will likely remain a mess, because the underlying platforms on which Python builds are already a mess. Unfortunately, it's becoming more and more of a problem as scientific Python packages grow in size and features. It's gotten to the point where I am not motivated to figure out how to install the latest version of nMOLDYN on my Mac, although I am a co-author of that program. The previous version is good enough for my own needs, and much simpler to install though already a bit tricky. That's how you get to love the command line… in 2015.
The release of NumPy 1.9 a few days ago was a bit of a revelation for me. For the first time in the combined history of NumPy and its predecessor Numeric, a new release broke my own code so severely thatI don't see any obvious way to fix it, given the limited means I can dedicate to software maintenance. And that makes me wonder for which scientific uses today's Python ecosystem can still be recommended, since the lack of means for code maintenance is a chronic and endemic problem in science.
I'll start with a historical review, for which I am particularly well placed as one of the oldtimers in the community: I was a founding member of the Matrix-SIG, a small group of scientists who in 1995 set out to use the still young Python language for computational science, starting with the design and implementation of a module called Numeric. Back then Python was a minority language in a field dominated by Fortran. The number of users started to grow seriously from 2000, to the point of now being a well-recognized and respected community that spans all domains of scientific research and holds several
conferences per year across the globe. The combination of technological change and the needs of new users has caused regular changes in the code base, which has grown as significantly as the user base: the first releases were small packages written and maintained by a single person (Jim Hugunin, who later became famous for Jython and IronPython), whereas today's NumPy is a complex beast maintained by a team.
My oldest published Python packages, ScientificPython and MMTK, go back to 1997 and are still widely used. They underwent a single major code reorganization, from module collections to packages when Python 1.5 introduced the package system. Other than that, most of the changes to the code base were implementations of new features and the inevitable bug fixes. The two main dependencies of my code, NumPy and Python itself, did sometimes introduce incompatible changes (by design or as consequences of bug fixes) that required changes on my own code base, but they were surprisingly minor and never required more than about a day of work.
However, I now realize that I have simply been lucky. While Python and its standard library have indeed been very stable (not counting the transition to Python 3), NumPy has introduced incompatible changes with almost every new version over the last years. None of them ever touched functionalities that I was using, so I barely noticed them when looking at each new version's release notes. That changed with release 1.9, which removes the compatbility layer with the old Numeric package, on which all of my code relies because of its early origins.
Backwards-incompatible changes are of course nothing exceptional in the computing world. User needs change, new ideas permit improvements, but existing APIs often prevent a clean or efficient implementation of new features or fundamental code redesigns. This is particularly true for APIs that are not the result of careful design, but of organic growth, which is the case for almost all scientific software. As a result, there is always a tension between improving a piece of software and keeping it compatible with code that depends on it. Several strategies have emerged to deal with, depending on the priorities of each community. The point I want to make in this post is that NumPy has made a bad choice, for several reasons.
The NumPy attitude can be summarized as "introduce incompatible changes slowly but continuously". Every change goes through several stages. First, the intention of an upcoming changes is announced. Next, deprecation warnings are added in the code, which are printed when code relying on the soon-to-disappear feature is executed. Finally, the change becomes effective. Sometimes changes are made in several steps to ease the transition. A good example from the 1.9 release notes is this:
In NumPy 1.8, the diagonal and diag functions returned readonly copies, in NumPy 1.9 they return readonly views, and in 1.10 they
will return writeable views.
The idea behind this approach to change is that client code that depends on NumPy is expected to be adapted continuously. The early warnings and the slow but regular rythm of change help developers of client code to keep up with NumPy.
The main problem with this attitude is that it works only under the assumption that client code is actively maintained. In scientific computing, that's not a reasonable assumption to make. Anyone who has followed the discussions about the scientific software crisis and the lack of reproduciblity in computational science should be well aware of this point that is frequently made. Much if not most scientific code is written by individuals or small teams for a specific study and then modified only as much as strictly required. One step up on the maintenance ladder, there is scientific code that is published and maintained by computational scientists as a side activity, without any significant means attributed to software development, usually because the work is not sufficiently valued by funding agencies. This is the category that my own libraries belong to. Of course the most visible software packages are those that are actively maintained by a sufficiently strong community, but I doubt they are representative for computational science as a whole.
A secondary problem with the "slow continuous change" philosophy is that client code becomes hard to read and understand. If you get a Python script, say as a reviewer for a submitted article, and see "import numpy", you don't know which version of numpy the authors had in mind. If that script calls array.diag() and modifies the return value, does it expect to modify a copy or a view? The result is very different, but there is no way to tell. It is possible, even quite probable, that the code would execute fine with both NumPy 1.8 and the upcoming NumPy 1.10, but yield different results.
Given the importance of NumPy in the scientific Python ecosystem - the majority of scientific libraries and applications depends on it -, I consider its lack of stability alarming. I would much prefer the NumPy developers to adopt the attitude to change taken by the Python language itself: accumulate ideas for incompatible changes, and apply them in a new version that is clearly labelled and announced as incompatible. Everyone in the Python community knows that there are important differences between Python 2 and Python 3. There's a good chance that a scientist publishing a Python script will clearly say if it's for Python 2 or Python 3, but even if not, the answer is often evident from looking at the code, because at least some of the many differences will be visible.
As for my initial question for which scientific uses today's Python ecosystem can still be recommended, I hesitate to provide an answer. Today's scientific Python ecosystem is not stable enough for use in small-scale science, in my opinion, although it remains an excellent choice for big communities that can somehow find the resources to maintain their code. What makes me hesitate to recommend not using Python is that there is no better alternative. The only widely used scientific programming language that can be considered stable, but anyone who has used Python is unlikely to be willing to switch to an environment with tedious edit-compile-run cycles.
One possible solution would be a long-time-support version of the core libraries of the Python ecosystem, maintained without any functional change by a separate development team. But that development team has be created and funded. Any volunteers?
A while ago I was chatting with two users of my Molecular Modelling Toolkit (MMTK), a library for molecular simulations written in Python. One of them asked me what I would do differently if I were to write MMTK today. That's an interesting question, but not the kind of question I can answer in a sentence or two, so I promised to write a blog post about this. Here it is.
First, a bit of history. The first version of MMTK was released about 16 years ago. I don't have the exact data, but the first message on the MMTK mailing list, announcing MMTK release 1.0b2, is dated 29 May 1997. Back then Python 1.4 was the state of the art and Numerical Python was a young project that was just beginning to stabilize. MMTK was one of the first domain-specific scientific libraries written in Python, at a time when the scientific Python community was very small and its members were mostly considered cranks by their peers. MMTK was designed from the start as a Python library, with relatively small bits of C code for the time-critical stuff (mainly energy evaluation and MD integration), with NumPy arrays at the Python-C interface. This has since become one of the two main approaches to using Python in scientific computing, the other one being wrapper code around libraries written in C/C++ or Fortran.
So what would I do differently if I were to start writing MMTK today? Many things, for different reasons. Lets first get the obvious stuff out of the way: the Python ecosystem has evolved significantly since 1997, and of course I would use Python 3, and Cython instead of C for the time-critical parts. I would also adopt many of the conventions that the community has developed but which weren't around in 1997. I might even be tempted to use bleeding-edge tools like Numba, although with hesitation: Numba is not only a moving target at this time, but also requires dependencies (I am thinking mostly of LLVM) which are big and non-trivial to install. One lesson I have learned in 16 years of scientific Python is that dependencies can cause more trouble than they are worth. It's nice in theory to re-use existing tested code, but it also makes installation and deployment more cumbersome.
So far for changes in the Python ecosystem. What has changed as well, though at a slower pace, is the role of computation in science and in particular in molecular simulation. Back in 1997, there were a few molecular simulation ecosystems that operated almost in isolation. The big players were the CHARMM, AMBER, and GROMOS/GROMACS communities. Each of them had their own software, their own file formats, and their own force fields. Members of these communities would of course talk about science to each other, but not share any software or data. Developing new computational methods required a serious investment into one of these ecosystems. That was in fact my main motivation for developing MMTK: I figured that I would be more efficient (not to mention more satisfied) writing a new system from scratch using modern development tools than trying to get familiar with crufty Fortran code. But I adopted basically the same approach with MMTK: I created a new ecosystem without much regard to sharing code or data with the rest of the world. As an illustration, MMTK defines its own trajectory format which I still consider superior to what the rest of the world is doing, but which is undeniably hard to use without MMTK, given that the definition of a universe is stored as an executable Python expression. MMTK also encourages storing data as Python pickle files, which are even harder to deal with for other programs.
Today we are seeing a change in attitude in computational science that I am sure will soon reach the molecular simulation community as well. People are starting to realize that computational results have serious reliability problems. The most publicized case in the structural biology community was the retraction of a few important published protein structures following the discovery of a bug in the data processing software that lead to completely wrong final structures. This and similar events point to the urgent need for better validation of computational results. One aspect of validation is re-running the same computation with different tools. Another aspect is publishing both software and raw data, enabling other scientists to inspect them and check their validity. Technology for sharing scientific code and data exists today (have a look at Github, Bitbucket, and figshare, for example). But in molecular simulation, there are still important practical barriers to such validation attempts, in particular the use of program-specific and badly documented file formats. While MMTK's file formats are documented, they are still program-specific and thus incompatible with the requirements of the future.
The sentence that I would like to write now is "If I were to rewrite MMTK today, I would use the exchange data formats accepted by the molecular simulation community". But those formats don't exist yet, although there are a few initiatives to develop them. My own contribution to this effort is the Mosaic data model and data formats - if you are interested in this subject, please have a look at it and send me your feedback. Mosaic will of course find its way into future versions of MMTK.
Finally, there are things I would do differently because the experience with MMTK has shown that a few initial design decisions were not the best ones. Number one is the absence of stable atom numbers. In MMTK, each atom and molecule is represented by a unique Python object, and there are ways to refer uniquely to everything by using Python expressions. But there is no such thing as a unique order of atoms that would assign a number to each one. Atoms do have numbers by which the low-level C code refers to them, but these numbers can be different every time you run a Python script. My original design goal was to discourage the use of numbers to refer to atoms, because this is an important source of mistakes if the simulated system undergoes changes. But every other molecular simulation program out there uses numbers to refer to atoms, so people are used to them. For interoperability with other programs, atom numbers are fundamental. There are ways to handle such situations, of course, but it's a constant source of headaches.
The other design aspect that I would change if I were to rewrite MMTK today is the hierarchy of chemical objects. MMTK has Atoms, Groups, Molecules, and Complexes, plus specializations such as AminoAcidResidue (a special Group), PeptideChain (a special Molecule), and Protein (a special Complex). While all of these correspond to some chemical reality, the system is more complex than required for molecular simulation, leading in some situations to code that is bloated by irrelevant special cases. Today I'd go for just Atoms and Groups, with special features of specific kinds of groups indicated by attributes rather than specific classes.
Tags: computational science, computer-aided research, emacs, mmtk, mobile computing, programming, proteins, python, rants, reproducible research, science, scientific computing, scientific software, social networks, software, source code repositories, sustainable software
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