docs/backend.md

Customization

If you have explored the options and PySRRegressor reference, and still haven't figured out how to specify a constraint or objective required for your problem, you might consider editing the backend. The backend of PySR is written as a pure Julia package under the name SymbolicRegression.jl. This package is accessed with PyJulia, which allows us to transfer objects back and forth between the Python and Julia runtimes.

PySR gives you access to everything in SymbolicRegression.jl, but there are some specific use-cases which require modifications to the backend itself. Generally you can do this as follows:

1. Clone a copy of the backend:

git clone https://github.com/MilesCranmer/SymbolicRegression.jl

2. Edit the source code in src/ to your requirements:
   • The documentation for the backend is given here.
   • Throughout the package, you will often see template functions which typically use a symbol T (such as in the string where {T<:Real}). Here, T is simply the datatype of the input data and stored constants, such as Float32 or Float64. Writing functions in this way lets us write functions generic to types, while still having access to the specific type specified at compilation time.
   • Expressions are stored as binary trees, using the Node{T} type, described here.
   • Parts of the code which are typically edited by users include:
     • src/LossFunctions.jl, particularly the function eval_loss. This function assigns a loss to a given expression, using eval_tree_array to evaluate it, and loss to compute the loss with respect to the dataset.
     • src/CheckConstraints.jl, particularly the function check_constraints. This function checks whether a given expression satisfies constraints, such as having a complexity lower than maxsize, and whether it contains any forbidden nestings of functions.
       • Note that all expressions, even intermediate expressions, must comply with constraints. Therefore, make sure that evolution can still reach your desired expression (with one mutation at a time), before setting a hard constraint. In other cases you might want to instead put in the loss function.
     • src/Options.jl, as well as the struct definition in src/OptionsStruct.jl. This file specifies all the options used in the search: an instance of Options is typically available throughout every function in SymbolicRegression.jl. If you add new functionality to the backend, and wish to make it parameterizable (including from PySR), you should specify it in the options.
     • For reference, the main loop itself is found in the EquationSearch function inside src/SymbolicRegression.jl.
3. Specify the directory of SymbolicRegression.jl to PySR by setting julia_project in the PySRRegressor object, and run .fit when you're ready. That's it! No compilation or build steps required.
   • Note that it will automatically update your project by default; to turn this off, set update=False.

If you get comfortable enough with the backend, you might consider using the Julia package directly: the API is given on the SymbolicRegression.jl documentation.

If you make a change that you think could be useful to other users, don't hesitate to open a pull request on either the PySR or SymbolicRegression.jl repositories! Contributions are very appreciated.