Python - %n.nf format

Question

I'd like to understand the first argument %n for the format command %n.nf.

IE:

print("number is %1.3f" %(1.1000000000))

number is 1.100

Ok, I've understand the "3f" is the total of total of digits after decimal point.

but what about the fist argument?

another example:

print("number is %3.3f" %(1111.1000000000))

number is 1111.100

Argument say 3, but return 4 digits.

or

print("number is %10.3f" %(1111.1000000000))

number is   1111.100

Even saying 10, not fill with 0.

Answer 1

The first argument in %n.nf is the minimum field width. It doesn't appear to do anything in your first example (%1.3f) because the output is already wider than the minimum specified.

It left-pads with spaces by default, so your second example does work. Just add a leading 0 to get it to pad with zeroes instead of spaces.

>>> print("number is %010.3f" %(1111.1000000000))
number is 001111.100

See printf-style String Formatting for more details. (As mentioned in the comments, this is the old style of string formatting. Ctrl-F on that page to see newer methods.)