String formatting {:d} vs %d on floating point number

Question

I realise that this question could be construed as similar to others, so before I start, here is a list of some possible "duplicates" before everyone starts pointing them out. None of these seem to really answer my question properly.

• Python string formatting: % vs. .format
• "%s" % format vs "{0}".format() vs "?" format

My question specifically pertains to the use of the string.format() method for displaying integer numbers.

Running the following code using % string formatting in the interpreter running python 2.7

    >>> print "%d" %(1.2345)  
    1

Whereas using the string.format() method results in the following

    >>> print "{:d}".format(1.2345)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: Unknown format code 'd' for object type 'float'

I was expecting the same behavior in both; for the interpreter to actually convert my floating point number to an integer prior to displaying. I realise that I could just use the int function to convert the floating point number to integer format, but I was looking for the same functionality you get with the %d formatting method. Is there any string.format() method that would do this for me?

Answer 1

The two implementations are quite separate, and some warts in the % implementation were ironed out. Using %d for floats may mask problems in your code, where you thought you had integers but got floating point values instead. Imagine a value of 1.999999 and only seeing 1 instead of 2 as %d truncates the value.

As such, the float.__format__() hook method called by str.format() to do the actual conversion work does not support the d format and throws an exception instead.

You can use the {:.0f} format to explicitly display (rounded) floating point values with no decimal numbers:

>>> '{:.0f}'.format(1.234)
'1'
>>> '{:.0f}'.format(1.534)
'2'

or use int() before formatting to explicitly truncate your floating point number.

As a side note, if all you are doing is formatting a number as a string (and not interpolating into a larger string), use the format() function:

>>> format(1.234, '.0f')
'1'

This communicates your intent better and is a little faster to boot.