How to format a floating number to maximum fixed width in Python

I simply took the answer by @Harvey251 but split into test part and the part we need in production.

Usage would be:

# save the code at the end as formatfloat.py and then
import formatfloat

# do this first
width = 8
ff8 = formatfloat.FormatFloat(width)

# now use ff8 whenever you need
print(ff8(12345678901234))

And here is the solution. Save the code as formatfloat.py and import it to use FlotFormat class. As I said below, loop part of calculation better be moved to init part of the FormatFlot class.

import unittest

class FormatFloat:
    def __init__(self, width = 8):
        self.width = width
        self.maxnum = int('9'*(width - 1))  # 9999999
        self.minnum = -int('9'*(width - 2)) # -999999

    def __call__(self, x):

        # for small numbers
        # if -999,999 < given < 9,999,999:
        if x > self.minnum and x < self.maxnum:

            # o = f'{x:7}'
            o = f'{x:{self.width - 1}}'

            # converting int to float without adding zero
            if '.' not in o:
                o += '.'

            # float longer than 8 will need rounding to fit width
            elif len(o) > self.width:
                # output = str(round(x, 7 - str(x).index(".")))
                o = str(round(x, self.width-1 - str(x).index('.')))

        else:

            # for exponents
            # added a loop for super large numbers or negative as "-" is another char
            # Added max(max_char, 5) to account for max length of less 
            #     than 5, was having too much fun
            # TODO can i come up with a threshold value for these up front, 
            #     so that i dont have to do this calc for every value??
            for n in range(max(self.width, 5) - 5, 0, -1):
                fill = f'.{n}e'
                o = f'{x:{fill}}'.replace('+0', '+')

                # if all good stop looping
                if len(o) == self.width:
                    break
            else:
                raise ValueError(f"Number is too large to fit in {self.width} characters", x)
        return o


class TestFormatFloat(unittest.TestCase):
    def test_all(self):
        test = ( 
            ("1234567.", 1234567), 
            ("-123456.", -123456), 
            ("1.23e+13", 12345678901234), 
            ("123.4567", 123.4567), 
            ("123.4568", 123.45678), 
            ("1.234568", 1.2345678), 
            ("0.123457", 0.12345678), 
            ("   1234.", 1234), 
            ("1.235e+7", 12345678), 
            ("-1.23e+6", -1234567),
            )

        width = 8
        ff8 = FormatFloat(width)

        for expected, given in test:
            output = ff8(given)
            self.assertEqual(len(output), width, msg=output)
            self.assertEqual(output, expected, msg=given)

if __name__ == '__main__':
    unittest.main()

I've made a slight addition to yosukesabai's contribution to account for the rare case where rounding will make the width of the string 7 characters instead of 8!

class FormatFloat:
def __init__(self, width = 8):
    self.width = width
    self.maxnum = int('9'*(width - 1))  # 9999999
    self.minnum = -int('9'*(width - 2)) # -999999

def __call__(self, x):

    # for small numbers
    # if -999,999 < given < 9,999,999:
    if x > self.minnum and x < self.maxnum:

        # o = f'{x:7}'
        o = f'{x:{self.width - 1}}'

        # converting int to float without adding zero
        if '.' not in o:
            o += '.'

        # float longer than 8 will need rounding to fit width
        elif len(o) > self.width:
            # output = str(round(x, 7 - str(x).index(".")))
            o = str(round(x, self.width - 1 - str(x).index('.')))
            if len(o) < self.width:
                o+=(self.width-len(o))*'0'

    else:

        # for exponents
        # added a loop for super large numbers or negative as "-" is another char
        # Added max(max_char, 5) to account for max length of less 
        #     than 5, was having too much fun
        # TODO can i come up with a threshold value for these up front, 
        #     so that i dont have to do this calc for every value??
        for n in range(max(self.width, 5) - 5, 0, -1):
            fill = f'.{n}e'
            o = f'{x:{fill}}'.replace('+0', '+')

            # if all good stop looping
            if len(o) == self.width:
                break
        else:
            raise ValueError(f"Number is too large to fit in {self.width} characters", x)
    return o

You've clearly gotten very close, but I think your final solution is going to involve writing a custom formatter. For example, I don't believe the mini-formatting language can control the width of the exponent like you want.

(BTW, in your first example you don't have a "+" after the "e" but in the others you do. Making it clear which one you want might help other answerers.)

If I were writing this formatting function, the first thing I would do is write a thorough set of tests for it. Either doctest or unittest would be suitable.

Then you work on your formatting function until all those tests pass.

You could do something like, admittedly it's a little late and I spent way too long on it, but I came upon this when trying to figure out something similar.

import unittest


class TestStringMethods(unittest.TestCase):

    def test_all(self):
        test = (
            ("1234567.", 1234567),
            ("-123456.", -123456),
            ("1.23e+13", 12345678901234),
            ("123.4567", 123.4567),
            ("123.4568", 123.45678),
            ("1.234568", 1.2345678),
            ("0.123457", 0.12345678),
            ("   1234.", 1234),
            ("1.235e+7", 12345678),
            ("-1.23e+6", -1234567),
        )

        max_char = 8
        max_number = int("9" * (max_char - 1))  # 9,999,999
        min_number = -int("9" * (max_char - 2))  # -999,999
        for expected, given in test:
            # for small numbers
            # if -999,999 < given < 9,999,999:
            if min_number < given < max_number:

                # output = f"{given:7}"
                output = f"{given:{max_char - 1}}"

                # converting ints to floats without adding zero
                if '.' not in output:
                    output += '.'

                # floats longer than 8 will need rounding to fit max length
                elif len(output) > max_char:
                    # output = str(round(given, 7 - str(given).index(".")))
                    output = str(round(given, max_char - 1 - str(given).index(".")))

            else:
                # for exponents
                # added a loop for super large numbers or negative as "-" is another char
                # Added max(max_char, 5) to account for max length of less than 5, was having too much fun
                for n in range(max(max_char, 5) - 5, 0, -1):
                    fill = f".{n}e"
                    output = f"{given:{fill}}".replace('+0', '+')
                    # if all good stop looping
                    if len(output) == max_char:
                        break
                else:
                    raise ValueError(f"Number is too large to fit in {max_char} characters", given)

            self.assertEqual(len(output), max_char, msg=output)
            self.assertEqual(output, expected, msg=given)


if __name__ == '__main__':
    unittest.main()