Моделирование стека

import re
 
F = open ( "input.txt" )
Err = False
stack = []
stack1 = []
 
def error():
    F = open ( "output.txt", "w" )
    F.write ( "ERROR" )
    F.close()
 
while Err != True:
    s = F.readline().split()
    if not s:
        error()
        break
    for k in s:
        stack.append( k )
        stack1.append( k )
        if len(stack) >= 2:
            a1 = stack.pop()
            a2 = stack.pop()
            stack.append( k )
            if a1 == "-" and a2 == "-":
                Err = True
F.close()
 
if Err == False:
    a = stack1.pop()
    b = stack1[0]
 
if Err == True:
    F = open ( "output.txt", "w" )
    F.write ( "EMPTY" )
    F.close()
else:
    a1 = re.sub(r'[+-]',"", a)
    b1 = re.sub(r'[+-]',"", b)
    F = open ( "output.txt", "w" )
    F.write ( b1 + " " + a1 )
    F.close()

with open("input.txt", "rt") as file_handle:
    in_put = file_handle.read()
 
stack, result = [], None
for line in in_put.splitlines():
    command, *args = line.split()
    if command == "+":
        stack.append(*args)
    elif command == "-":
        if len(stack) == 0:
            result = "ERROR"
            break
        stack.pop()
else:
    result = " ".join(map(str, stack)) if stack else "EMPTY"
 
with open("output.txt", "wt") as file_handle:
    file_handle.write(result)

stack=[]
flgError=False
with open("input.txt","rt") as fi:
    for stri in fi.readlines():
        if len(stri)==0:
            break        
        code=stri[0]
        if code=="+":
            num=int(stri[1:])
            stack.append(num)
        elif len(stack)==0:
            flgError=True
            break
        else:
            stack.pop()
with open("output.txt","wt") as fo:
    if flgError:
        print("Error",file=fo)
    elif len(stack)==0:
        print("Empty",file=fo)
    else:
        print(*stack,sep=' ',file=fo)

import sys
 
def solution():
    stack = []
    for command in iter(input, ""):
        if command.startswith("+"):
            stack.append(command.lstrip("+"))
        elif command == "-":
            stack.pop()
        else:
            print("ERROR")
            return
    print(*stack if stack else ("EMPTY",))
 
with open("input.txt") as f, open("output.txt", "w") as out:
    sys.stdin, sys.stdout = f, out
    solution()

for stri in fi: