Create my own if() statement function in python

So first I want to say that I am creating a mini coding language, and It is meant for people just coming from Scratch to text based programming. Right now, it does basic command line functions like making a variable, and performing operations. I have written it all by myself in Python and it has a lexer and a parser. What I want to do is allow the user to make call an if() function inside of my language and then it will do the things inside the block. I am just confused on how I would define a function that does something that the user would want other then them making an lambda function, but since my language is for beginners, I don't want that. So esentially what I want to do is create an if function.

python code:

operators = ['+', '-', '*', '/']
var_assign = '='
variables = {}

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

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:
        return eval(equation)
    except:
        return f"Invalid equation {equation}"

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