The problem during debugging python within Emacs: NameError: name '__file__' is not defined

[Hongyi Zhao <hongyi.zhao@gmail.com>]

I try to debug the following simple python script within Emacs with
the help of elpy:

###
import os

def realdirname(file):
    return os.path.dirname(os.path.abspath(os.path.expanduser(file)))

script_realdirname = realdirname(__file__)
print(script_realdirname)
###

When I evaluate the buffer, I meet the following error:

----------------
Python 3.9.1 (default, Feb 10 2021, 15:30:33)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.23.1 -- An enhanced Interactive Python. Type '?' for help.
>>>
python.el: native completion setup loaded
>>> import os
...
...
... def realdirname(file):
...     return os.path.dirname(os.path.abspath(os.path.expanduser(file)))
...
...
... script_realdirname = realdirname(__file__)
... print(script_realdirname)

Traceback (most recent call last):
  File "/home/werner/Desktop/work/python/emacs-python-ide.py", line 8,
in <module>
    script_realdirname = realdirname(__file__)
NameError: name '__file__' is not defined

>>>
----------------

After some googling, I still couldn't find any effective solution. How
to deal with this problem? Any hints will be highly appreciated.

Regards
-- 
Assoc. Prof. Hongyi Zhao <hongyi.zhao@gmail.com>
Theory and Simulation of Materials
Hebei Vocational University of Technology and Engineering
No. 473, Quannan West Street, Xindu District, Xingtai, Hebei province