I remember first having to make these sorts of functions whilst doing GCSE Computing many years ago :-)

Decimal Number to Binary:

def decimal_to_binary(decimal_num):  
    binary_num = ""  
    if decimal_num == 0:  
        return "0"  
    while decimal_num > 0:  
        remainder = decimal_num % 2  
        binary_num = str(remainder) + binary_num  
        decimal_num //= 2  
    return binary_num  

decimal_number = 100  
binary_number = decimal_to_binary(decimal_number)  
print(binary_number)  

Binary Number to Decimal:

def binary_to_decimal(binary_num):  
    decimal_num = 0  
    power = len(binary_num) - 1   
    for digit in binary_num:  
        decimal_num += int(digit) * (2 ** power)  
        power -= 1   
    return decimal_num  

binary_number = "101011"  
print(binary_to_decimal(binary_number))