How to format percentage point differences in Python?

Question

I have to present the difference between two percentages using Python. For example, the difference between 40% and 41.234% is 1.234 percentage points, or 1.234 pp.

If I wanted to format the increase as a percentage, I would use the format specifier {:.0%} :

increase = 0.01234

"{:+.3%} ".format(increase)
>>> "+1.234 %"

What I'm looking for

A format specifier that replaces the % with pp , like this:

"{some_specifier} ".format(increase)
>>> "+1.234 pp"

What I'm not looking for

I know that I could use f-strings like this

f"{increase * 100:+.3f} pp"
>>> "+1.234 pp"

but I'm specifically looking for a format specifier that I can use with .format() .

Edit

I realised that a little bit of background may be useful here. In my implementation, the increase can represent a normal float, integer, percentage or percentage point. In order to get the right representation, I have use a variable-format mapping that looks something like:

format_mapping = {
    "var_x": "{:,.0f}",    # for large numbers
    "var_y": "{:+.3f}",    # for small numbers
    "var_z": "{:.4%}",     # for percentages
}

The variable names represent the column names of a pandas dataframe my_data , so that I can then apply the formatting to those columns, for the sake of reporting

for var in my_data.columns:
    fmt = format_mapping[var]
    my_data[var] = pd.Series(fmt.format(value) for value in my_data[var])

Answer 1

There are maybe things you can do with inline pipes and subclassing a Format class, but in this case I think it might be easier to just write functions to handle it:

def pct(num, digits=0):
    return '{:+.{}} pp'.format(num * 100, digits)

...

f"{pct(increase, 4)}"
# '+1.234 pp'

you can then put all those functions in their own file, if you want, and invoke them by just calling them in f-strings.

Answer 2

You could use the following:

increase = 0.01234

print("{:+.4} {}".format(increase*100, "pp"))

Result:

+1.234 pp

Answer 3

You can declare custom formatter and replace "%" with "pp" by overriding Formatter.format_field() method:

from string import Formatter    

class MyFormatter(Formatter):
    def format_field(self, value, spec):
        if spec.endswith("p"):
            return super(MyFormatter, self).format_field(value, spec[:-1] + "%")[:-1] + "pp"
        else:
            return super(MyFormatter, self).format_field(value, spec)

Usage:

print(MyFormatter().format("%: {0:+.3%}; pp: {0:+.3p}", 0.01234))

PS In python 3 you can use super() without arguments.