This is a past issue of the mathspp insider 🐍🚀 newsletter. Subscribe to the mathspp insider 🐍🚀 to get weekly Python deep dives like this one on your inbox!
f-strings have been around ever since Python 3.6 and I cannot live without them.
If you still use the string method format to do formatting – or even %-style formatting! – then you can read up on the basics of f-strings in this article.
If you already know f-strings, let me share 3 tips with you that will make your life easier.
One of my favourite features of f-strings is that they can self-debug.
If your format specifier ends with an equals sign =, Python will format the string with the result value but also with the expression you typed!
Compare the two:
>>> value = 73
>>> f"{value}"
'73'
>>> f"{value = }"
'value = 73'The spaces around the equals sign = are relevant; if you don't include them, the formatted string also won't include them.
It also works with more “complex” expressions:
>>> x, y, z = 2, 10, 100
>>> f"{(y + z) ** x = }"
'(y + z) ** x = 12100'!rWhen using f-strings for any form of debugging, it might be useful to include !r in your expression to be formatted so that Python uses repr (and not str) to format the string.
(You can read about the differences between repr and str – or between __repr__ and __str__ in this article.)
For example, when formatting strings that actually look like numbers, you can only distinguish them if you use !r:
>>> num = 73
>>> s = "73"
>>> f"{num}, {s}"
'73, 73'
>>> f"{num!r}, {s!r}"
"73, '73'"Bonus tip: if you use the self-debugging features, Python will use repr instead of str:
>>> f"{num = }, {s = }"
"num = 73, s = '73'" # <-- '' around the s value.Sometimes, you'll want to use a format specifier that is, itself, a variable.
One way in which I use this is when I have a list of variables to align to the left and I don't know, ahead of time, how wide my column needs to be.
I compute the maximum width, I add a couple of spaces to make it neat, and then I use the nested braces {} to use the variable as the formatter:
>>> names = ["Harry", "Anne", "George"]
>>> width = 2 + max(map(len, names))
>>> for name in names:
... print(f"| {name:>{width}} |")
...
| Harry |
| Anne |
| George |I'm here to help, so here's a bonus tip:
To learn more about f-strings, a friend created the page
It's sort of a quick reference to the most common and useful features of f-strings.
This is a past issue of the mathspp insider 🐍🚀 newsletter. Subscribe to the mathspp insider 🐍🚀 to get weekly Python deep dives like this one on your inbox: