Q:

What does ":" do in the format methods in python?

print("{:2d} file: {:>25}".format(idx + 1, filename_list[idx]) )

This code is from a forloop trying to loop through file names.
The  idx variable is the iterator and the filename_list is the list of filenames.
The list is made up of file names like this ie name or names seperated by underscore, number and the file extension
Boston_Chiwawa_0478.jpg

My question is, what does :2d and :>25 do in this code?

A:

Using your example...
{:2d} formats idx + 1 to 2 characters using additional leading padding if required.
{:>25} formats filename_list[idx] to the right hand side with a fixed width of 25 characters. If there is less than 25 characters in the file name, leading spaces will be shown.
Also worth noting, in Python versions 3.6 and above, it can be re-written with a f-string as follows:
print(f"{idx + 1:2d} file: {filename_list[idx]:>25}")
To the left of the colon inside the curly braces is the expression and to the right side of the colon is the formatting string that it applies to it.

