Is it bad practice in Python to define a function in the middle of operational code?

I learned from a similar question on imports that one should not mix "operational code" and import (they should be at the top).

What is the consensus around defining functions in the middle of "operational code"?

I have a case where a function is used in just one place (it has to be a function) and having its definition close to that piece of code would make sense (all would be grouped together). It visually breaks the flow of the code, though.

Any advice from PEP? (I did not find anything relevant, as opposed to import)

EDIT: the apparent trend in the answers is that it is not a nice thing to do, but unlike the importcase mentioned above there are no definitive directives (PEP for instance)

Solution

All code in a python script or module (which is basically the same thing - the difference being how it is used) is "operational code" one way or another. The clean way to structure a script's code is to have everything in functions - what you name "operational code" being in a "main()" function - and just have a call to that "main" function if the module is used as a script, ie:

# mymoduleorscript.py
import something
from somewhere import somethingelse

def somefunc(arg1, arg2):
   # ...

def otherfunc(arga, argb):
   # ...

def main(*args):
   # "operational" code here
   return status_code # 0 if ok, anything else if error

# only call main if used as a script
if __name__ == "__main__":
     import sys
     sys.exit(main(*sys.argv[1:]))

Note that this is not a "in my opinion" answer, but the officially blessed OneTrueWay of doing things.