Debug Taskgraph

There are several approaches you could take when you need to debug changes in Taskgraph.

Print Debugging

It’s possible to add print statements to transforms which will show up in the log. Just beware that because transforms tend to loop over every task, it can be difficult to tell which output was generated from the task(s) you’re trying to debug.

Here’s an example pattern you can use to limit debug output:

@transforms.add
def some_transform(config, tasks):
    for task in tasks:
        def debug(msg):
            # Note "name" is not a standard field, use whatever field
            # is appropriate here.
            if task["name"] == "my-task":
                print(msg, file=sys.stderr)

        # Will only be printed when "my-task" is being processed.
        debug("FOO")

pdb

It’s also possible to use the built-in Python debugger, pdb. Simply add calls to pdb.set_trace() and run the taskgraph binary as normal. Note that as with print debugging, you’ll want to ensure that breakpoints are only triggered on the tasks you are aiming to debug:

@transforms.add
def some_transform(config, tasks):
    import pdb
    for task in tasks:
        # Note "name" is not a standard field, use whatever field
        # is appropriate here.
        if task["name"] == "my-task":
            pdb.set_trace()

debugpy

Debugpy is the Python debugger that comes bundled with VSCode. But it can be used with other editors as well, such as Neovim via the nvim-dap-python plugin. The advantage of debugpy is that you can set breakpoints directly via your editor (so you don’t need to edit your source). You can trigger new runs from within your editor as well.

How to setup debugpy varies editor by editor and is out of scope for these docs. But typically there will be a way of defining a “launch config”. For taskgraph it should look similar to:

{
    "name": "Taskgraph Full",
    "type": "python",
    "request": "launch",
    "program": "/path/to/bin/taskgraph",
    "args": ["full"],
    "console": "integratedTerminal",
    "justMyCode": false
}

Make sure to adjust “program” to point to your taskgraph binary. Also be sure to tweak args as you see fit. See Useful Arguments for more information on available arguments.