Introduction

Py-gnumake is a GNU make plugin that allows you to extend a makefile with Python. Python code run through Py-gnumake has access to variables, make functions, and etc., just like functions built with conventional makefile syntax.

Rationale

First and foremost, this is a tech demo using the fairly new plugin feature for GNU make, introduced in version 4.0. But does it have any real applications?

Let’s start with a real example: this actual makefile from the Android NDK. Peruse that file for a moment and bask in the glory of a topological sort written in pure make syntax.

The great thing about a makefile is its simplicity, and even though (or perhaps because) it is simple it scales remarkably well, even to huge builds. Where it doesn’t scale is when builds begin to require more complicated rules than .c –> .o –> .exe. It’s not that make can’t do it, but rather that complicated flows become difficult to express in make syntax. This becomes especially acute when makefiles are used to build projects with multiple interconnected sub-components.

So what exactly does it do?

Py-gnumake adds the following functions to make:

  • python-eval – Expands to the result of the Python expression.

  • python-exec – Executes arbitrary Python code. Expands to the value of any output printed to stdout (much like $(shell ...)).

  • python-file – Like python-exec but runs code from a script, optionally passing arguments to sys.argv.

Python code run using these functions are all run within the same process as make. They all share globals and imports, and can interact with the makefile in the same way.

What can the Python code do from within the makefile?

You have full access to the Python interpreter. You can import and use any installed module. To make it easier to interact with the makefile, the gnumake module provides the following functions:

  • export() – A decorator that will cause a Python function to be exposed to the makefile as a new function. So running the following code using one of the above make functions will cause a new function $(newer <file1>,<file2>) to be created, which expands to the name of the file with the more recent modification time:

    from gnumake import export
import os

@export
def newer(file1, file2):
    if os.path.getmtime(file1) > os.path.getmtime(file2):
        return file1
    else:
        return file2

  • evaluate() – Executes the given make code, as if $(eval ...) had been used. This can be used to set variable values, raise errors, etc.

  • expand() – Returns the result of expanding the given expression, so gnumake.expand("$(lastword $(SOME_LIST))") could be used to get the final word in the variable $(SOME_LIST)

  • variables or vars – A dictionary-like object of type Variables that can be used in place of expand() and evaluate() to access variables.