MF4Writer reports "asammdf package was not found" when asammdf is imported before can #2122

Describe the bug

If asammdf is imported before can, and canopen is also installed, can.io.mf4 decides that
asammdf is missing. Every MF4Writer created later in that process raises:

NotImplementedError: The asammdf package was not found. Install python-can with the optional
dependency [mf4] to use the MF4Writer.

asammdf is installed. The message hides the real cause, which is a circular import.

To Reproduce

The environment needs python-can, asammdf (which pulls in canmatrix) and canopen.

import asammdf
from can.io import MF4Writer

MF4Writer("out.mf4")  # NotImplementedError: The asammdf package was not found...

Importing can first avoids the error:

import can
import asammdf
from can.io import MF4Writer

MF4Writer("out.mf4").stop()  # works

Cause

import asammdf pulls in can.io.mf4 before asammdf has finished initialising:

asammdf/blocks/types.py:5         from canmatrix import CanMatrix
canmatrix/formats/__init__.py:24  importlib.import_module("canmatrix.formats." + module)  # every format
canmatrix/formats/eds.py:4        import canopen.objectdictionary.eds
canopen/network.py:8              import can
can/__init__.py:100               from .io import (...)
can/io/logger.py:36               from .mf4 import MF4Writer
can/io/mf4.py                     from asammdf import Signal, Source   # asammdf partially initialised

The from asammdf import Signal, Source raises ImportError because asammdf is only partly
initialised. The optional-dependency guard catches it:

try:
    import asammdf
    ...
except ImportError:
    asammdf = None

This guard treats "asammdf is not installed" and "asammdf failed to import" as the same thing.
asammdf = None is then stored as a module global, so the wrong answer lasts for the whole
process, even after asammdf finishes importing.

Suggested Fix

Either of these would avoid the problem:

1. Import asammdf lazily inside MF4Writer.__init__ / MF4Reader.__init__ instead of at module
   import. The cycle can no longer form, and import can gets cheaper too.
2. Narrow the guard so that only a genuinely missing package counts as missing

Expected behavior

asammdf and can can be imported regardless of order.

Additional context

• python-can 4.6.1. The same guard is in can/io/mf4.py on main.
• asammdf 8.7.2, canmatrix 1.2, canopen 2.4.1
• Python 3.13.11, Linux