from typing import Union

import astropy.units as u
from astropy.units import Quantity


@u.quantity_input
def afun(length_or_str: Union[Quantity["length"], str]):
    print(f"Worked with {length_or_str}")

afun(1 * u.m)  # Works
afun("1m")  # Fails

Worked with 1.0 m
Traceback (most recent call last):
  File "/home/vandal/repos/astro/astropy/astropy/units/decorators.py", line 69, in _validate_arg_value
    if arg.unit.is_equivalent(allowed_unit, equivalencies=equivalencies):
       ^^^^^^^^
AttributeError: 'str' object has no attribute 'unit'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/vandal/repos/astro/astropy/quantity_input_minimal.py", line 12, in <module>
    afun("1m")  # Fails
    ^^^^^^^^^^
  File "/home/vandal/repos/astro/astropy/astropy/units/decorators.py", line 298, in wrapper
    _validate_arg_value(
  File "/home/vandal/repos/astro/astropy/astropy/units/decorators.py", line 78, in _validate_arg_value
    raise TypeError(
TypeError: Argument 'length_or_str' to function 'afun' has no 'unit' attribute. You should pass in an astropy Quantity instead.

import astropy
try:
    astropy.system_info()
except AttributeError:
    import platform; print(platform.platform())
    import sys; print("Python", sys.version)
    import astropy; print("astropy", astropy.__version__)
    import numpy; print("Numpy", numpy.__version__)
    import erfa; print("pyerfa", erfa.__version__)
    try:
        import scipy
        print("Scipy", scipy.__version__)
    except ImportError:
        print("Scipy not installed")
    try:
        import matplotlib
        print("Matplotlib", matplotlib.__version__)
    except ImportError:
        print("Matplotlib not installed")