Scientific Computing

VTK test data can be useful to test ParaView data flows and code as a reference of known good data. The VTK file I/O documentation itself recommends using the example data files to test one’s own project software. A straightforward way to obtain the data is by “building” a single ExternalData target, where virtually all of the time is spent downloading.

git clone --recursive https://gitlab.kitware.com/vtk/vtk.git --depth 1

cmake -Bbuild -DVTK_BUILD_TESTING=ON

cmake --build build -t VTKData

Numerous data folders are created under build/ExternalData/Testing/Data. For example, VTKHDF data in HDF5 file is the file mandelbrot-vti.hdf

pyproject.toml specifies Python package prerequisites and typically all Python metadata. This helps security by allowing extremely fast recursive machine-parsing of prerequisites without installing packages first. Generally, specify Python package prerequisites in pyproject.toml as much as possible.

Python packages should minimize the size of their directed dependency graph for best package longevity with minimum maintenance effort. However, the most effective use of programmer/scientist/engineer time generally comes from reusing code wherever appropriate. How do we evaluate quality of prereqs? Modern Python code includes these factors:

• use pyproject.toml to specify all package metadata
• continuous integration
• code coverage tests
• PEP8 compliance
• Static type hinting

Long term archiving of Python software requires direct and indirect dependencies. This is commonly done by pip freeze, but provides no direct sense of module hierarchy. The techniques described below provide a detailed, zoomable hierarchical view of Python module dependencies.

Python dependency analysis where packages use setup.py to specify package prerequisites generally require modules to be installed to determine their dependencies. That is, setup.py is recursively executed for each module to determine what modules are needed overall. This is bad for automated security analysis, which is slowed greatly by needing to install packages to determine prereqs. Modern Python packages solve this problem by specifying most package configuration in pyproject.toml.

Currently, pipdeptree is the most practical solution to generate plots of Python directed dependency graphs. This method assumes:

• self-test has adequate coverage to be meaningful for most users
• packages only used as convenience methods for some users are under [project.optional-dependencies] in pyproject.toml
• strictly necessary modules are specified
• minimum Python version is specified
• CI-only requirements are specified

The process below is targeted for packages used in “development mode” that is, not installed into site-packages, except for a link back to the code directory.

Install prereqs:

pip install virtualenv

In the Python package directory, create a new Python virtual environment, since pipdeptree depends on having only the analyzed package and its dependencies installed.

virtualenv testdep
. testdep/bin/activate

pip install pipdeptree[graphviz]

Install the package to examine (and whatever dependencies it automatically installs)

pip install -e .

Make a hierarchical dependency graph

pipdeptree

This should be a very short tree (unless testing with a big package). Try it with a simple package, seeing if the dependency list is expected.

Now create the directed dependency graph for the package. Install GraphViz by

• Linux: apt install graphviz
• macOS: brew install graphviz
• Windows

and then:

pipdeptree --graph-output svg > dep.svg

View the SVG in web browser or image viewer software such as IrfanView.

Wrap up the previous discussion and scripts in this Bash script pydeptree.sh for a one-click Python dependency graph.

#!/usr/bin/env bash

set -o errexit

[[ ! -z $1 ]] && cd $1

virtualenv testdep     # it's OK if it already exists

. testdep/bin/activate

pip install pipdeptree[graphviz]

pip install -e .[tests]

pipdeptree --graph-output svg > dep.svg

. deactivate

eog dep.svg &  # image viewing program

Notes

• python dependency graph analysis, with code and data
• Why xarray?

To make Modulegraph useful, the output must be post-processed, as almost all of the output is system stdlib modules. Modulegraph is an established, maintained tool for creating a .dot dependency graph. It lists extremely verbose output. It’s necessary to post-process .dot output with pydot to make use of modulegraph output. What if we instead preemptively excluded from a list of known stdlib modules, removing say 98% of modulegraph output from the start?

pip install modulegraph

Examine a file’s requirements, creating a .dot graph.

python -mmodulegraph file.py -q -g > graph.dot

dot -Tsvg graph.dot > graph.svg

Modulegraph command line options

Snakefood is another dependency graph checker.

• Matlab dependency graph
• CMake dependency graph

Regardless of the macOS Terminal shell, the key bindings are generally distinct versus Linux terminals. It is possible to use keybind commands in ~/.zshrc to make macOS Terminal key shortcuts work like Linux terminal emulators. Learning the macOS Terminal key shortcut defaults can be useful when at another person’s laptop.

Exit status by convention has integer zero to represent “OK” no error status. Non-zero status is generally considered a failure. Coding languages such as Fortran also have built-in syntax to manage program exit code returned to the operating system.

C++ defines EXIT_SUCCESS and EXIT_FAILURE macros in header cstdlib. C defines EXIT_SUCCESS and EXIT_FAILURE macros in header stdlib.h.

Because typical headers already included often #include <cstdlib> or #include <stdlib.h>, developers may not realize these exit status macros need to be included somewhere. As compilers transition to providing stdlib via C++20 modules and generally cleanup excessive includes from built-in headers, code may suddenly complain about missing exit status macros at build time.

We feel it’s a good practice to use exit status macros as a findable and readable indication that program flow is ending and returning to system. A best practice is to include the appropriate header in any code file where the exit status macros are used.

#include <stdlib.h>

int main(void){
  return EXIT_SUCCESS;
}

#include <cstdlib>

int main(){
  return EXIT_SUCCESS;
}

The conda clean command allows cleaning up cached downloads and unused packages. Over time, Conda’s cache can grow over several gigabytes, even more than 10 GB of disk storage. To clean the unused files, use a command like:

conda clean --all --verbose

Many Windows users of Zoom may have downloaded the 32-bit Zoom client. We have observed that manually reinstalling the Zoom 64-bit Windows client can significantly help avoid choppiness of Zoom audio/video under high CPU usage. In most cases, it’s good to use the 64-bit version of a program for a 64-bit CPU and operating system. For Zoom as with many other programs, if the 32-bit version is installed, the updater doesn’t go to 64-bit version. The user typically has to manually once reinstall the 64-bit version of the program and from there on the updates stay at 64-bit program. Latency-sensitive 32-bit Windows programs may not work as well as 64-bit programs on 64-bit Windows due to WOW64 emulation for 32-bit programs on 64-bit Windows.

CMake presets can have specific build preset option for particular build tools. This CMakePresets.json shows example build presets for Ninja. The preset name “ninja” in this example is arbitrary. As usual, the presets help avoid copying script parameters and the possibility of typos in duplicated script code, whether for CI or developers themselves.

For example, to have ninja “explain” why a target is dirty:

cmake --build --preset explain

{
  "version": 2,
  "configurePresets": [
  {
    "name": "ninja",
    "generator": "Ninja",
    "binaryDir": "${sourceDir}/build"
  }
  ],
  "buildPresets":[
    {
      "name": "explain",
      "configurePreset": "ninja",
      "nativeToolOptions": ["-d", "explain"]
    },
    {
      "name": "keep",
      "configurePreset": "ninja",
      "nativeToolOptions": ["-d", "keeprsp", "-d", "keepdepfile"]
    },
    {
      "name": "stats",
      "configurePreset": "ninja",
      "nativeToolOptions": ["-d", "stats"]
    }
  ]
}

On Windows, compiler option syntax is generally MSVC-like or GCC-like. On Windows, the compiler option families are:

• MSVC-like: Intel oneAPI (IntelLLVM), Clang-CL, Visual Studio
• GCC-like: GCC, Clang, non-Windows OS.

Meson and CMake can each detect compiler option family. In general, the “else” branch would have further nested “if” to handle compiler options with distinct syntax.

CMake uses the MSVC variable to detect compiler option family. More specific compiler option selection is often handled with an if-else tree for each compiler ID and / or check_compiler_flag().

project(Hello LANGUAGES C)

if(MSVC)
  message(STATUS "${CMAKE_C_COMPILER_ID} is MSVC-like")
else()
  message(STATUS "${CMAKE_C_COMPILER_ID} is GCC-like")
endif()

Meson uses the .get_argument_syntax() property to detect compiler option family. Getting more specific compiler options can be done with an if-else tree for each compiler ID and / or the .has_argument() method.

project('hello', 'c')

cc = meson.get_compiler('c')

if cc.get_argument_syntax() == 'msvc'
  message(cc.get_id() + ' is MSVC-like')
else
  message(cc.get_id() + ' is GCC-like')
endif

A 5-way shell detect script can detect bash, csh, dash, ksh, or zsh. However, often we simply want to detect if the shell is Bash or something else. For example, in a Bash-dependent script we may thereby stop and recommend the user use Bash to avoid needing to test across numerous shells. There are a number of pitfalls in detecting a shell type so this method is not completely robust either, but has worked. The key to this technique is that while different shells have distinct “if-else” syntax, most shells have “case-esac”.

When relocating / moving / renaming a CMake script within a project, it may be desired to leave a placeholder at the original script location until users know to use the new script path in the project. Many CMake scripts call other scripts in the project. In general, CMake scripts should use absolute paths, except for install() parameters. That is, CMake scripts should generally start file paths with CMake built-in absolute paths like CMAKE_CURRENT_LIST_DIR and CMAKE_CURRENT_FUNCTION_LIST_DIR. If the script directory is changed, the script references to paths within the project will of course also need to be changed.

Legacy users may take a while to start calling the CMake script at the new location. Note that making a softlink will NOT work in general because the path references would be from the placeholder script location, which in general is distinct from the new script location. To bridge this UX gap, create a trivial one-line script at the original script path like:

include(${CMAKE_CURRENT_LIST_DIR}/../path/to/new-script.cmake)