bug: Successfully imported module XXX but can't find it in the component registry.This is unexpected and most likely a bug.

[vmarchese]

Describe the bug
I have the following situation:

• a service using pipeline deserialization from yaml and executing the pipelines
• a custom components package

The custom components package is in the format:

comp/
    __init__.py
    A/
       __init__.py
       B/
            __init__.py
            b.py

in which the comp/A/B/b.py file is (in pseudo code):

from haystack import component

@component
class BComponent:

   def __init__(self, some_param: str):
      self.some_param = some_param

  @component.output_types(some_return=Any)
  def run(some_input: Any):
       # do some stuff...
       return {"some_input": ...}

The comp/A/B/__init__.py file is

from .b import BComponent

__all__ = ["BComponent"]

If the pipeline is written in yaml as:

components:
   b:
     init_parameters:... 
    type: comp.A.B.BComponent

I have the following error:

  File "[omitted]/.venv/lib/python3.12/site-packages/haystack/core/pipeline/base.py", line 167, in from_dict
    raise PipelineError(
haystack.core.errors.PipelineError: Successfully imported module comp.A.B but can't find it in the component registry.This is unexpected and most likely a bug.

If I write the fully qualified name:

components:
   b:
     init_parameters:... 
    type: comp.A.B.b.BComponent

it works perfectly.

Error message

  File "[omitted]/.venv/lib/python3.12/site-packages/haystack/core/pipeline/base.py", line 167, in from_dict
    raise PipelineError(
haystack.core.errors.PipelineError: Successfully imported module comp.A.B but can't find it in the component registry.This is unexpected and most likely a bug.

Expected behavior
The from_dict method of the Base Pipeline to support external exported symbols

To Reproduce

1. Write a package structure as in the previous example
2. create a new project
3. import the package
4. write the yaml with the exported name
5. write the following code:

from haystack import Pipeline


ptext = """
components:
   b:
     init_parameters:... 
    type: comp.A.B.BComponent
"""

p = Pipeline.loads(ptext)

6. execute the code

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System:

• OS: MacOS M4 Pro (doesn't matter... even in x86 docker images is the same)
• Haystack version (commit or version number): 2.8.0