Have function reject bad argument when argument is called by another function?

[jobroinc]

Thank you I will take a look.

I will clarify what I meant if this does not solve the issue.

---

x = {"F": "Fluorine", "Cl": "Chlorine"}
y = {"O": "Oxygen"}
 
def hal(a):
  for halogens in x:
    if halogens == a:
      print(x[a])
  return(x[a])  
   
 
def acc(b):
  for acceptors in y:
    if acceptors == b:
      print(y[b])
  return y[b]
  
def is_halogen_bond(halogen, acceptor ): 
  hal(a)
  acc(b)
 
is_halogen_bond("F", "O")

On the code that I have I want is

 is_halogen_bond("F", "O") 

to print Fluorine and Oxygen, which it does.

I get a "KeyError" every time that "halogen" or "acceptor" is put inside of the arguments. Perhaps my failed code would be a better way to illustrate what I am speaking of . . .

Error:
Traceback (most recent call last):
  File "halogen.py", line 24, in <module>
    is_halogen_bond("B", "O")
  File "halogen.py", line 21, in is_halogen_bond
    hal(halogen)
  File "halogen.py", line 8, in hal
    return(x[a])
KeyError: 'B'

To resolve this issue I attempted a while loop and all loops under the sun and I failed. What I want my code to do from this point is then print something like this if the halogen argument produces a key error:

print("Error, not a halogen.")

and then prompt the user to enter a halogen that is in the list x. I want the same to apply for acceptors with the list y.

I know loops are involved in here but I am struggling to create one correctly.

Thanks for your time!

---

Sorry, I made a mistake...

I meant " I get a "KeyError" every time a "halogen" or "acceptor" is put inside of the arguments that is not listed in x or y. Perhaps my failed code would be a better way to illustrate what I am speaking of . . ."