Convert an excel or spreadsheet column letter to its number in Pythonic fashion

Question

Is there a more pythonic way of converting excel-style columns to numbers (starting with 1)?

Working code up to two letters:

Answer 1

Use:

LETTERS = list(string.ascii_uppercase)
def column_number(column_id):
    return sum([(LETTERS.index(j)+1)*(26**i) for i,j in enumerate(column_id[::-1])])

There are several parts to this one-liner, so here's the explanation:

column_id[::-1]: reverses the string, e.g. converts 'AZ' to 'ZA', there's a good reason to do so, which we will see in a bit.

enumerate(): produces a iterable, e.g. (0, 'Z'), (1, 'A')

With some observation:

 A -> 1  = (26**0)*1              # ** is the exponential operator
 B -> 2  = (26**0)*2 
 Z -> 26 = (26**0)*26
AA -> 27 = (26**0)*1  + (26**1)*1
AB -> 28 = (26**0)*2  + (26**1)*1
AZ -> 52 = (26**0)*26 + (26**1)*1  # recall that we have (0, 'Z'), (1, 'A')

Reversing the column_id and enumerate() allows us to use the index as the exponent for 26. The rest is now trivial.

LETTERS.index(j): gives us the index of the letter in LETTERS

sum(): takes a list of numbers and returns the total.