How does this formatting code work?

Question

I always have to know why, rather than just how, so here I go:

How does this work:

'{0:01.2f}'.format(5.555)  #returns '5.55'

'{0:01.1f}'.format(5.555)  #returns '5.5'

'{0:1.2f}'.format(5.555)   #returns '5.55' again

'{0:1.1f}'.format(5.555)   #returns '5.5' again

Why does this not add zero padding by returning '05.5' instead of just '5.5' when I have the extra zero. It just doesn't seem to work.

Also, why is the string 0:1.1f instead of 0:0.1f, etc. Is it just a convention to use the number 1 before the decimal instead of a zero, or is that the syntax?

Answer 1

Its because the 0 character enables zero-padding, but you have a width of 1 set. Set the width to 2 (like '{0:02.1f}' and you will see a leading 0.

Edit - actually, I'm not sure if 2 will suffice, because I don't know how it behaves with more digits after the decimal point. So to be safe, make it something like 5 and see what it does then.

Edit 2 - ok, I just tried it. The width corresponds to the TOTAL width, including all digits and the decimal point. So you'll get this:

>>> '{0:05.2f}'.format(5.555)
'05.55'

Note that the total length of the string is 5 now, with a leading 0 because zero-padding is enabled, and that it truncates to 2 digits after the decimal point because of .2

Answer 2

You should specify a width taking into account all the characters, including the decimals and the dot. If you want 05.55 that's 5 characters, so:

>>>'{0:05.2f}'.format(5.555)
'05.55'