Converting hexadecimal to decimal in python

There are a lot of issues in your code. Let's go through it :

hexidecimal= "7ac8965f" #hexadecimal value

decimal=[hexidecimal[i:i+1] for i in range(0,len(hexidecimal), 1)]
# >> decimal : ["7","a","c","8","9","6","5","f"]

for i in range(0,len(decimal)):
# first path : i = 0

        # First Error : 'in' is a array-wide search.
        # you want to use :'if decimal[i] == 'a' '
        if 'a' in decimal: # a is in decimal (second pos) so decimal[0] is modified !
            decimal[i]='10'
            # >> decimal : ["10","a","c","8","9","6","5","f"]

        if 'b' in decimal:
            decimal[i]='11'
        if 'c' in decimal:
            decimal[i]='12'
        if 'd' in decimal:
            decimal[i]='13'
        if 'e' in decimal:
            decimal[i]='14'
        if 'f' in decimal: # f is in decimal (last pos) so decimal[0] is modified !
            decimal[i]='15'
            # >> decimal : ["15","a","c","8","9","6","5","f"]

        #Second Error : anticipated return
        #Assuming the indentation is correct, the function exit here, on the 
        #first run of the function
        return decimal

Now a solution :

#dict for conversion (there are more elegant ways to do it, 
#                                                        but this is good enough)
conversion_table = { '0' : 0 ,
                     '1' : 1 ,
                     '2' : 2 ,
                     '3' : 3 ,
                      .......
                     'a' : 10 ,
                     'b' : 11 ,
                     ......
                     'f' : 15
                    }

hexidecimal= "7ac8965f" #hexadecimal value

hexa_list=[ digit for digit in hexidecimal]
# same thing as before, just more "elegant"

decimal = [ conversion_table[hex_digit] for hex_digit in hexa_list]
# convert everything in base10

# reduce the list
return reduce( lambda x,y : x*16 + y, decimal )

I think it would be helpful for you, to read this idiomatic python tutorial: http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html

def hex2dec(hexadecimal):
    conversion_table = {'A': 10, 'B': 11, 'C': 12, 'D': 13, 'E': 14, 'F': 15}

    hex_list = list(hexadecimal)

    for index, number in enumerate(hex_list):
        number = number.upper()   

        if number in conversion_table:
            hex_list[index] = conversion_table[number]

    int_list = [int(number) for number in hex_list]

    return reduce(lambda x, y: x*16+y, int_list)


print hex2dec('7Ac8965f')    # 2059966047

I'm just having a bit of fun here, but to rewrite your function...

def toDecimal(hexadecimal):
    decimal = int(hexadecimal, 16)
    return decimal

toDecimal('0xdeadbeef')

I'm presuming you're re-inventing the wheel just to see how it's done? :)

There is a much simpler way to do this:

Something large:

>>> s = "6f48f8248e828ce82f82"
>>> int(s, 16)
525528725744949635067778L

Something smaller:

>>> int('BF', 16)
191

Adding a 0x in front of the number does not change the end result.

As a function:

def hex2dec(s):
    """return the integer value of a hexadecimal string s"""
    return int(s, 16)