Estimating the Size of Binary Data encoded as a B64 String in Python

Question

So I have some Binary data in python (a jpg Image, in this case) that I am retreiving from an API as a B64 encoded string. Is there an easy way to estimate the size of this (image) file on the disk?

<Image>/9j/4AAQSkZJRgABAgEASABIAAD/4RriRXhpZgAATU0AKgAAAAgABwESAAMAAAABAAEAAAEaAAUA
AAABAAAAYgEbAAUAAAABAAAAagEoAAMAAAABAAIAAAExAAIAAAAeAAAAcgEyAAIAAAAUAAAAkIdp
AAQAAAABAAAApAAAANAACvyAAAAnEAAK/IAAACcQQWRvYmUgUGhvdG9z ....... </Image>

Dogbert · Accepted Answer · 2011-07-22 17:38:47Z

18

You can multiply the length of the string by 3/4 to get the size in bytes.

(len(str) * 3) / 4

edited Jul 22, 2011 at 17:38

answered Jul 22, 2011 at 17:16

Dogbert

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2 Comments

varunsrin Over a year ago

isn't Base64 larger than the actual size in bytes? I always thought that size of B64 > Size in Bytes, whereas in this case it would be the opposite

varunsrin Over a year ago

As a follow up question, is the size of the Base64 string itself == the length of the string? I know that its true for 8bit strings, which are the default string encoding in python

Victor · Accepted Answer · 2015-07-02 08:35:14Z

1

I'm using this:

def size(b64string):
    return (len(b64string) * 3) / 4 - b64string.count('=', -2)

We remove the length of the padding, which is either no, one or two characters =, as explained here.

Probably not optimal. I don't how efficient str.count(char) is. On the other hand, it is only performed on a string of length 2.

answered Jul 2, 2015 at 8:35

Victor

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