How can I modify Bitbake variable using code in the local.conf?

I would like to dynamically modify some configuration variables regarding some others. To do so, I added a Python function (I don't mind using a shell function if that is the way to go) in my local.conf file:

# the variable I want to modify
PERSISTENT_BINDS += "${@add_persistent_binds(d)}"

def add_persistent_binds(d):
    bb.plain("Path: edit persistent bind list")
    binds = []
    for entry in oe.data.typed_value("ENTRIES", d):
        if not entry:
            continue
        ta, tb = line.split(None, 1)
        binds.append("/data/sys/" + ta + " /" + tb + "\n")
    return " ".join(binds)

Unfortunately Bitbake gives an error when it parses the function's definition line:

bb.parse.ParseError: ParseError at /home/user/workspace/yocto/build/conf/local.conf:173: unparsed line: 'def add_persistent_binds(d):'

For instance, the following content set in ENTRIES is expected to produce the following content to be added to PERSISTENT_BINDS:

ENTRIES ??= ""
ENTRIES[type] = "list"
ENTRIES[separator] = "\n"

ENTRIES = "\
    foo bar\n\
    dead beef\n\
"

PERSISTENT_BINDS = " \
    ...
    /data/sys/foo /bar\n\
    /data/sys/dead /beef\n\
"

NOTE I cannot just add this "as is" to the PERSISTENT_BINDS because the variable ENTRIES is used at other locations for other purposes.

Solution

I don't know what you are trying to do exactly, but here is a solution on how to define your new custom Python method:

If you want to add your custom Python library in Yocto, you can use the new feature addpylib : Link

Example:

meta-custom
└── lib
    └── custom
        └── __init__.py

Now, in layer.conf of the meta-custom you add:

addpylib ${LAYERDIR}/lib custom

Now, in meta-custom/lib/custom/__init__.py add:

from pkgutil import extend_path
__path__ = extend_path(__path__, __name__)

import bb 
import oe 

def add_persistent_binds(d):
    bb.plain("Path: edit persistent bind list")
    binds = []
    for entry in oe.data.typed_value("ENTRIES", d):
        if not entry:
            continue
        ta, tb = entry.split(None, 1)
        binds.append("/data/sys/" + ta + " /" + tb + "\n")
    return " ".join(binds)

In local.conf now:

ENTRIES ??= ""
ENTRIES[type] = "list"
ENTRIES[separator] = "\n"

ENTRIES = "\
    foo bar\n\
    dead beef\n\
"

# Call the custom function:
PERSISTENT_BINDS += "${@custom.add_persistent_binds(d)}"

To test it, run:

bitbake-getvar --value PERSISTENT_BINDS

You should get:

   /data/sys/foo /bar
   /data/sys/dead /beef

**** EDIT ****

The addpylib solution works only for versions >=4.2

The solution that comes to mind is to create a class and inherit it globally so local.conf can see the custom function:

meta-custom/classes/custom.bbclass :

def add_persistent_binds(d):
    bb.plain("Path: edit persistent bind list")
    binds = []
    for entry in oe.data.typed_value("ENTRIES", d):
        if not entry:
            continue
        ta, tb = entry.split(None, 1)
        binds.append("/data/sys/" + ta + " /" + tb + "\n")
    return " ".join(binds)

Then, in your local.conf just add:

INHERIT += "custom"

PERSISTENT_BINDS += "${@add_persistent_binds(d)}"
ENTRIES ??= ""
ENTRIES[type] = "list"
ENTRIES[separator] = "\n"

ENTRIES = "\
    foo bar\n\
    dead beef\n\
"

And it should work.

NOTE, remember that INHERIT inherits classes for all recipes, so you can use add_persistent_binds method from any recipe.