Width and Precision using *

Question

I'm learning Python from a book and came across this example:

>>> '%f, %.2f, %.*f % (1/3.0, 1/3.0, 4, 1/3.0)
# Result: '0.333333, 0.33, 0.3333'

Don't quite understand what's happening here, especially the '4' in between.

Answer 1

There are two syntax errors in the line you posted. 1.3.0 isn't a valid number, and the string isn't closed.

This is a valid version of said string format.

'%f, %2.f, %.*f' % (1/3.0, 1/3.0, 4, 1/3.0)

and outputs:

'0.333333, 0.33, 0.3333'

I couldn't find documentation on %.*f in the official docs. However, it appears that it's parsing the 4 to be how many decimal places you want to do the next argument at.

For example:

'%.*f' % (5, 1/3.0)

returns

'0.33333'

and

'%.*f' % (6, 1/3.0)

returns

'0.333333'

It seems to be a way to offer variable length precision, so you could allow your users to specify it.

Answer 2

I think you meant something like this:

>>> '%f, %2.f, %.*f' % (1/3.0, 1.3, 4, 1/3.0)
'0.333333,  1, 0.3333'

4 is a wild card value that is used in place of asterisk *. When expanded it would be equivalent to:

>>> '%f, %2.f, %.4f' % (1/3.0, 1.3, 1/3.0)