initializing interactive mode

[Gribouillis]

You could start the interpreter with the -i option

Output:
λ echo 'x = 1009' > spam.py
λ 
λ python -i spam.py
>>> x
1009
>>> 

Second way, using an environment variable

Output:
λ PYTHONSTARTUP=spam.py python
Python 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x
1009
>>> 

Note that you cannot combine the two ways because with the PYTHONSTARTUP is not read if you start python with -i

Output:
λ echo 'y = 9000' > eggs.py
λ PYTHONSTARTUP=spam.py python -i eggs.py
>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
>>> y
9000
>>> 

Here is a third untested way, if you don't want to define PYTHONSTARTUP (I never define permanently python-related environment variables). In file usercustomize.py write this

# in usercustomize.py
import sys
if hasattr(sys, 'ps1'): # we are in interactive mode
    with open('spam.py') as ifh:
        exec(ifh.read(), vars(sys.modules['__main__']))