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])

Have you considered just doing `"{:+.3%} ".format(increase).replace('%', 'pp')` instead of trying to make the f-string output 'pp' itself? — Green Cloak Guy, Aug 03 '20 at 13:36
That's a nice idea. I have a situation, though, where I have different variable types (percentages, floats, integers) that can be used as input for `increase`. I currently use a dictionary mapping between variable-types and format-specifiers to get the format I'm looking for. The format-specifier can then just be used without actually changing the `"{format_specifier}.format(value)"` syntax — KenHBS, Aug 03 '20 at 13:41

score 2 · Answer 1 · answered Aug 03 '20 at 13:52

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.

David Duran · Answer 2 · 2020-08-03T13:38:41.427

1

You could use the following:

increase = 0.01234

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

Result:

+1.234 pp

edited Aug 03 '20 at 13:38

answered Aug 03 '20 at 13:36

David Duran

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This isn't what OP asked for. They want the same output, but with `0.01234` as the input instead of `1.234` – Green Cloak Guy Aug 03 '20 at 13:37
You conveniently changed the value of `increase` – Nicolas Gervais Aug 03 '20 at 13:38
@GreenCloakGuy I edited the answer to have the same input. – David Duran Aug 03 '20 at 13:39

Olvin Roght · Accepted Answer · 2020-08-03T14:09:12.937

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))

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

How to format percentage point differences in Python?

What I'm looking for

What I'm not looking for

Edit

3 Answers3