You use the built-in int() function, and pass it the base of the input number, i.e. 2 for a binary number:

>>> int('11111111', 2)
255

Here is documentation for Python 2, and for Python 3.

It works fine when you remove all the typos and indentation problems:

def binStrToInt(binary_str):


    """The function binStrToInt() takes in one input, a string of ones and
    zeros (no spaces) called BINARY_STR.  It treats that input as a binary
    number (base 2) and converts it to a decimal integer (base 10). It
    returns an integer result."""

    length = len(binary_str)

    num = 0
    for i in range(length):
        num = num + int(binary_str[i])
        num = num * 2
    return num / 2

But honestly, how did you not see these? The interpreter tells you about most of them (all except the return indentation). And as commenters have mentioned, the built-in int() function will do the same thing if you pass it a string, and the number 2 to cause it to convert from a binary string.

EDIT Based on your comment below, you are somehow using a special quote, perhaps copied from a web-page or something. That won't work -- here is an example of something that worked and something that didn't in the interpreter:

>>> def binStrToInt(binary_str):
...     length = len(binary_str)
...     num = 0
...     for i in range(length):
...         num = num + int(binary_str[i])
...         num = num * 2
...     return num / 2
... 
>>> binStrToInt('101')
5
>>> binStrToInt(’101’)
  File "<stdin>", line 1
    binStrToInt(’101’)
                ^
SyntaxError: invalid syntax

In python you can only create it as a binary value (as a syntactic sugar), it will be converted into an integer immediately. Try it for yourself:

>>> 0b11001010
202

The same thing will happen with octal and hexadecimal values. So you can convert your binary string to an integer, with the int() function's base argument like:

>>> int('0b11001010', 2)
202

After the conversion you can do any operations on it -- just like with an integer, since it is an integer.

Of course you can convert it back at any time to a binary string, with the builtin bin() function:

>>> bin(202)
0b11001010

Python's string format method can take a format spec.

>>> "{0:b}".format(37)
'100101'

Format spec docs for Python 2

Format spec docs for Python 3