Taskwarrior hook script that adds a "blocks:" pseudo-attribute for adding/modifying tasks.

on-modify.blocks_attr.py

#!/usr/bin/env python
#
# Adds the ability to add / modify tasks using a "blocks:" attribute,
# the opposite of "depends:".
#
# This script acts as an on-modify, on-add and on-launch hook at the same time.
#
### SETUP
# Save this file as
#   ~/.task/hooks/on-modify.blocks_attr.py
# change to that directory:
#   $ cd ~/.task/hooks
# make the script executable:
#   $ chmod +x on-modify.blocks_attr.py
# then create symlinks to that file with the other hook names:
#   $ ln -s on-modify.blocks_attr.py on-add.blocks_attr.py
#   $ ln -s on-modify.blocks_attr.py on-launch.blocks_attr.py
#
# Now tell Taskwarrior about the "blocks" attribute:
#   $ task config uda.blocks.type string
#   $ task config uda.blocks.label Blocks
#
# And we're ready to go!
#
# bf@id:~$ t add a
# Created task 1.
#
# bf@id:~$ t add b
# Created task 2.
#
# bf@id:~$ t add c blocks:1
# Created task 3.
#
# bf@id:~$ t 2 mod blocks:1
# Modifying task 2 'b'.
# Modified 1 task.
#
# bf@id:~$ t
# [t next]
#
# ID Age Deps Description Urg 
#  2 18s      b              8
#  3 8s       c              8
#  1 29s 2 3  a             -5
#

import json
import os
import subprocess
import sys

# Adjust this if "task" is not regular Taskwarrior.
TASK = 'task'
# Adjust path if needed. It is a temporary file managed by the hook.
SHIMFILE = "%s/.task/hooks/blocks_shim.txt" % os.getenv('HOME')

def on_launch():
    try:
        with open(SHIMFILE, 'r') as f:
            shim = f.readlines()
    except IOError:
        sys.exit(0)
    while shim:
        line = shim.pop(0).rstrip()
        if not line:
            continue
        if subprocess.call(line.split()) != 0:
            # Error handling by escalating to the user :)
            print("%s ERROR: First line in %s is failing to run!" % (sys.argv[0], SHIMFILE))
            print("Feel free to delete the file, or edit its contents to resolve the problem.")
            shim = [line + '\n'] + shim
            break
    if shim:
        with open(SHIMFILE, 'w') as f:
            f.write(''.join(shim))
        sys.exit(1)
    else:
        os.remove(SHIMFILE)

def handle_blocks_attribute(new):
    if 'blocks' in new:
        with open(SHIMFILE, "a") as f:
            # This needs error handling. If a later hook aborts, the UUID is invalid.
            f.write("%s rc.hooks=off rc.confirmation=no rc.bulk=10000 rc.verbose=nothing %s mod dep:%s\n" % (TASK, new['blocks'], new['uuid']))
        del new['blocks']
    print(json.dumps(new))

old = sys.stdin.readline()
new = sys.stdin.readline()

if not old:
    on_launch()
elif not new:
    handle_blocks_attribute(json.loads(old))
else:
    handle_blocks_attribute(json.loads(new))

task undo doesn't work for me. I don't have my task data.location at default but I changed the line where it is asked to do so. Basic functionality works (meaning I can add tasks with blocks: attribute) successfully.

The error it gives:
Hook Error: Expected 1 JSON task(s), found 0, in hook script: on-modify.blocks_attr.py

Weird to me is that it does not fail at removing the dependency but the blocking task. Here's the sequence:
create task a
create task b
create task c with blocks:1 and it works
task undo and it deletes the dependency of a on c successfully
task undo again but it fails to remove task c

Heres all the output:

The last modification was made 2022-03-11

                                          Prior Values                          Current Values                      
description                               a                                     a                                   
entry                                     2022-03-11                            2022-03-11
modified                                  2022-03-11                            2022-03-11                          
status                                    pending                               pending
uuid                                      fc90d4a0-04c5-4c25-a22c-9dc68f77be99  fc90d4a0-04c5-4c25-a22c-9dc68f77be99
dep_66b26bbf-5406-4656-8159-0aca20db9da8                                        x                                   
depends                                                                         66b26bbf-5406-4656-8159-0aca20db9da8

The undo command is not reversible.  Are you sure you want to revert to the previous state? (yes/no) yes
Modified task reverted.
[username@hostname ~/.local/share/task/hooks]$ tu

The last modification was made 2022-03-11

             Prior Values  Current Values                      
description                c                                   
entry                      2022-03-11                          
modified                   2022-03-11                          
status                     pending                             
uuid                       66b26bbf-5406-4656-8159-0aca20db9da8

The undo command is not reversible.  Are you sure you want to revert to the previous state? (yes/no) yes
Hook Error: Expected 1 JSON task(s), found 0, in hook script: on-modify.blocks_attr.py

Great script!

I've made one change in my fork: I changed the declaration of SHIMFILE to use the hook file's own location to determine the directory that the shim file should reside in. Please feel free to look at my fork (https://gist.github.com/rjray/f95d0919447afa956a1753d7ccec87b6)... I've tested the hook in my own taskwarrior repo and it works fine.