you can use this code:

binary= raw_input("Binary: ")
d= int(binary, 2)
print d

To convert from binary to decimal, just do:

int(binary_string, 2)  # The 2 is the base argument from which we convert.

Demo:

>>> int('0b10110', 2)
22

Note- There are many issues with the code you are using to convert decimal to binary. If you are insistent on not using a built-in function for that purpose, you may be interested by this post:

Convert an integer to binary without using the built-in bin function

Although personally if I were to want to avoid the bin() function, I would do something like:

"{0:#b}".format(an_integer)

Demo:

>>> "{0:#b}".format(22)
'0b10110'

This is much more Pythonic than your current code.

This algorithm could pretty easily be converted to a one-liner. Let's walk through the logic:

1. You want to work over each element of a string. You can just iterate over the string, rather than keeping track of an index.
2. You also need the index of that element, so you can use enumerate to get both.
3. You want to apply a mathematical operation if and only if the integer is not 0. This can be done using int(val), which converts the value to the number it represents.
4. You want to add all the values at the end. This can be done with sum.

To get error checking, you can convert the string to a set, and make sure the set is equal to a set with only 0 and 1 in it.

Also, you probably want the input handling in a while loop to avoid the recursion, which will fill up your memory, and then have a separate function to handle the actual conversion.

Also, I would only run main if the code is run as a script.

And keep in mind python has a built-in way to do binary to decimal conversion: int(binstr, 2), which can be used to test your implementation at the very least.

So here is how I would implement it:

def bin2int(binstr):
    if set(binstr) != {'0', '1'}:
        raise ValueError('String is not binary: "%s"' % binstr)

    return sum(int(x)*2**i for i, x in enumerate(a[::-1]))


def main():
    while True:
        binary = input("Enter a line of binary, enter 'q' to quit: ")

        if binary.lower() == 'q':
            break

        try:
            print(bin2int(binary))
        except ValueError as err:
            print(err)


if __name__ == "__main__":
    main()

You can use int and set the base to 2 (for binary):

>>> binary = raw_input('enter a number: ')
enter a number: 11001
>>> int(binary, 2)
25
>>>

However, if you cannot use int like that, then you could always do this:

binary = raw_input('enter a number: ')
decimal = 0
for digit in binary:
    decimal = decimal*2 + int(digit)
print decimal

Below is a demonstration:

>>> binary = raw_input('enter a number: ')
enter a number: 11001
>>> decimal = 0
>>> for digit in binary:
...     decimal = decimal*2 + int(digit)
...
>>> print decimal
25
>>>