Python custom formatting

Thursday 14 April 2022

Python f-strings use a formatting mini-language, the same as the older .format() function. After the colon comes short specifications for how to format the value:

>>> word = "Hello"
>>> f"{word:/^20}"
>>> amt = 12345678
>> f"{amt:20,}"
'          12,345,678'

Datetimes can use strftime syntax:

>>> f"{now:%Y-%m on day %d}"
'2022-04 on day 14'

The reason datetime uses different formatting specs than strings is because datetime defines its own __format__ method. Any object can define its own formatting mini-language. F-strings and .format() will use the __format__ method on an object, and pass it the formatting directives being used:

>>> class Confused:
...     def __format__(self, fmt):
...         return f"What is {fmt}?"
>>> c = Confused()
>>> f"{c:xyz12}"
'What is xyz12?'

Of course, __format__ can be used for more useful formatting than Confused is doing...

Geographic latitude and longitude are conventionally presented in a few different formats: degrees; or degrees and minutes; or degrees, minutes and seconds. Then the numbers can have varying number of decimal places, and sometimes the units are represented by symbols.

Here’s an implementation of those possibilities in __format__. The format string starts with “d”, “dm”, or “dms” to indicate the basic format. The number of decimal places can be specified with “.N”. Finally, symbols can be added, either plain or fancy, by adding a quote or minute symbol:

import dataclasses, re

class LatLong:
    lat: float
    long: float

    def __format__(self, fmt):
        dms, nfmt, opts = re.fullmatch(r"(dm?s?)([.\d]*)([′']?)", fmt).groups()
        formatted = []
        for num in [, self.long]:
            parts = []
            for ms in dms[1:]:
                num = abs((num - int(num)) * 60)
            parts.append(format(num, nfmt + "f"))
            syms = None
            if "'" in opts:
                syms = "°'\""
            elif "′" in opts:
                syms = "°′″"
            if opts:
                parts = [p + s for p, s in zip(parts, syms)]
            formatted.append(" ".join(parts))
        joined = ", ".join(formatted)
        return joined
>>> where = LatLong(42.359764937, -71.092068768)
>>> print(f"Location: {where:d'}")
Location: 42.359765°, -71.092069°
>>> print(f"Location: {where:d.4}")
Location: 42.3598, -71.0921
>>> print(f"Location: {where:dm'}")
Location: 42° 21.585896', -71° 5.524126'
>>> print(f"Location: {where:dms.4'}")
Location: 42° 21' 35.1538", -71° 5' 31.4476"
>>> print(f"Location: {where:dms.4}")
Location: 42 21 35.1538, -71 5 31.4476
>>> print(f"Location: {where:dms.6′}")
Location: 42° 21 35.153773, -71° 5 31.447565
>>> print("There: {:dms.6′}".format(where))
There: 42° 21 35.153773, -71° 5 31.447565
>>> print(format(where, "dms.6′"))
42° 21 35.153773, -71° 5 31.447565

This implementation doesn’t handle errors properly, but shows the basic idea. Also, lat/long are often shown with N/S E/W instead of positive and negative values. That’s left as an exercise for the reader.


Add a comment:

Ignore this:
Leave this empty:
Name is required. Either email or web are required. Email won't be displayed and I won't spam you. Your web site won't be indexed by search engines.
Don't put anything here:
Leave this empty:
Comment text is Markdown.