将str附加到str

Question

I've been working on a simple Caesar Shift in python, but when I try to run it it says that:

File "Ceaser Shift.py", line 36, in main
ciphertext += shift(letter,shift)
TypeError: 'str' object is not callable

I've tried to figure out why it does this, and I can add to a string in the normal IDLE environment and haven't seen anything online relatable because I haven't redefined str anywhere in my script. Any help would be great!

My Code:

## Doesn't support changing shifts during computation, to do this either the database must be re-written or script restarted

import time, os, string

global selmemo
shiftmemo = {}

def shift(l,shift):
    if l not in shiftmemo:
        charset = list(string.ascii_lowercase)
        place = charset.index(l.lower())
        shiftplace = charset.index(shift.lower())

        shiftmemo[l] = charset[(place+shiftplace)%25]

    return shiftmemo[l]

def main():
    shift = None
    ciphertext = ""

    print("--- Welcome ---")
    print("--- Ceaser Shifter ---")
    print("Commands: shift, encrypt, clear, print, quit")
    choice = input(": ")

    while choice != "quit":
        if choice == "shift":
            shift = input("Please enter a shift letter: ")

        elif choice == "encrypt" and shift != None:
            uparse = input("Enter your plaintext: ")
            for letter in uparse:
                if letter.lower() in string.ascii_lowercase:
                    ciphertext += shift(letter,shift)
                else:
                    ciphertext += letter

        elif choice == "clear":
            shift = ""
            ciphertext = ""
            shiftmemo = {}

        elif choice == "print":
            print(ciphertext)

        else:
            pass

        choice = input(": ")

main()

Answer 1

The problem is that you defined your function shift and your string variable shift .

A quick fix is to rename your functions and variables so that there is no conflict.

Answer 2

The shift is just name. It is the value of the name that is recognized as a user-defined function by the interpreter. So you can use function like this by assign the value to another name:

>>> def func():
...     print('a')
... 
>>> f = func
>>> f()
a
>>> 

But also if you assign a new value to the name, it may not be a function again.

>>> func = None
>>> type(func)
<class 'NoneType'>
>>> func()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable
>>>