patterns.md

Programming patterns

The code makes use of the simpy discrete event simulation library. This means that functions representing processes are implemented as generators, so that the library can simulate timeouts and asynchronous communication (typically faster than real time).

We use the convention of putting "(process)" in the doc comment of these functions. They are also annotated with a ProcessEffect return type. They either must use the yield construct, or return the result of calling another "(process)" function (not both).

Objects that implement processes typically hold the simpy.Environment in an instance variable self.env.

To wait for another process f() before continuing, use yield from f(). (If it is the last thing to do in a function with no other yield statements, return f() can be used as an optimization.)

A "(process)" function that does nothing should return skip(), using simtfl.util.skip.

Type annotations

The codebase is written to use static type annotations and to pass the pyanalyze static analyzer.

Each source file should have from __future__ import annotations at the top. This (usually) allows types defined in the same file to be referenced before their definition, without needing the workaround of writing such types as string literals. The preferred style is for this line to be immediately followed by other imports that are needed for type annotations.

The default annotation for argument and return types is Any. This works well for interoperating with libraries that don't use static typing, but please don't rely on it for code in this project. It is better to add explicit Any annotations in the few cases where that is needed. This means that functions and methods that do not return a value should be annotated with -> None.

pyanalyze has some limitations and is not claimed to be fully sound, but it does a pretty good job in practice; the result feels pretty much like a statically typed variant of Python. Importing the code it checks allows it to be more compatible with some Python idioms. The following workarounds for its limitations may be needed:

• It is sometimes unable to see that a None value cannot occur in a particular context. In that case, adding an assertion that the value is not None may help.

• In plain Python it is common to have classes that share an interface and are used polymorphically, but have no superclass in common. It might be necessary to create an abstract base class to define the interface.

• There is no easy way to precisely check uses of *args or **kwargs.

• If two files have mutually dependent types, they may end up circularly importing each other, which Python does not support. This is more likely for types than for implementation code. There are several possible workarounds:

  • merge the files;
  • move part of one file that is causing the circularity into the other;
  • create an abstract base class for the type that is being used circularly (with methods that raise NotImplementedError), and use that as the type instead of the concrete subclass.

• Adding type annotations might require exposing internal classes that would otherwise be intentionally hidden. Since this hiding is normally only possible by convention (e.g. using underscore-prefixed names) in any case, that does not really cause any problem. Please just refrain from directly using the constructors of such classes.

As is often the case for static typing in other languages, it typically works best to use more general types for arguments and more specific types for results.

Flake8

We also use flake8 to encourage a consistent coding style. However, if you disagree with a particular error or warning produced by flake8 and have a good justification for why it should not be enforced, please just add it to the ignore list in .flake8, and document the justification there.