concatenation $($A$,$B$),$ concatenate string B to the end of string A; return A$||$B

Write in Python:

the selected function

a program to:

$--$ input$/$read input data from the keyboard,

$--$ call and execute the selected function, and

$--$ output result$/$output data on the screen of terminal.

The function may print$/$report only errors.

The function should not print the computed value.

The function should return the computed value.

The function$($s$)$ and program must be into two separate python files.

The function$($s$)$ and program must be designed from scratch.

It is not allowed to use standard and library functions.

A character is "a decimal digit", $"$an upper$-$case letter", or "a lower$-$case letter".

A string is a sequence of characters.