File getting written to only after the program ends

Question

I am currently creating a programming language for beginners. My plan is to append all of the user's input to a python file out.py, and I will convert the then I plan to run that file and show the output.

The problem I am having is when I add the line to out.write(equation), it only writes to the file when the program ends. I also have the same problems on the first lines where I append the functions file. Essentially, the file only gets written to when the program wins.

Python file with code:

# Open the file that we will write all of the lines of code to
out = open("out.py","w")

# Open the file with all of the built in functions
functions = open("functions.py","r").read()

# Add built in functions to out so that the user can use them
out.write(functions)
out.write("\n")
operators = ['+', '-', '*', '/']
var_assign = '='
variables = {}

# Lexer
def lex(line):
    idx = 0
    line = line.replace(" ", "")
    num = ""
    tok = ""
    isNum = False
    tokens = []
    for char in line:
        if char.isdigit():
            isNum = True
            num+=char
        elif char in operators:
            if isNum:
                tokens.append(f"NUM:{num}")
                num=""
                isNum = False
            tokens.append(f"OP:{char}")
        elif char == "=":
            try:
                if line[:idx].isdigit():
                    return None
                else:
                    tokens.append(f"VAR_ASSIGN:{line[:idx]}={line[idx+1:]}")
            except:
                return ""
        elif char.isalpha() and "=" not in line:
            try:
                if '-' in line or '+' in line or '*' in line or '/' in line:
                    tokens.append(f"VAR_USE:{char}")
                else:
                    tokens.append(f"VAR_CALL:{line}")
            except:
                return ""
        idx+=1
        

    if isNum:
        tokens.append(f"NUM:{num}")
    return tokens

# Parser
def parse(tokens):
    equation = ""
    if not tokens:
        return "Syntax error"
    for tok in tokens:
        if "NUM" in tok:
            equation+=tok[4:]
        elif "OP" in tok:
            equation+=tok[3:]
        elif "VAR_ASSIGN" in tok:
            tok = tok[11:]
            variables[tok[:tok.index('=')]] = tok[tok.index('=')+1:]
            return variables[tok[:tok.index('=')]]
        elif "VAR_CALL" in tok:
            try:
                return variables[tok[9:]]
            except:
                return "Variable not defined"
        elif "VAR_USE" in tok:
            try:
                equation += variables[tok[8:]]
            except:
                return "Variable not defined"
    try:
        out.write(equation)
        out.write("\n")
        return eval(equation)
    except:
        return f"Invalid equation {equation}"

while True:
    data = input("KPP>")
    print(parse(lex(data)))

Output file with the input of 1+2, after I end the file

def show(arg):
    print(arg)
1+2