Trying To Understand Some F-string Magic (formatting Mini Language In F-strings)

In a comment on this post, somebody dropped this line of code: print('\n'.join(f'{a:{a}<{a}}' for a in range(1,10))) 1 22 333 4444 55555 666666 7777777 88888888 999999999 And

Solution 1:

You can demistify the output if you substitute some things:

print("\n".join(f'{a:4<5}'for a inrange(1,10)))

and read up on String format mini language:

It left aligns the value of a in 5 spaces using 4 as filler:

14444
24444
34444
44444
54444
64444
74444
84444
94444

Playing around with code is a good way to get what it does ...

Solution 2:

It's quite simple if you visualize the iterations:

1# f'{1:1<1}', means start with 1, left align with 1 spaces filled with 122# f'{2:2<2}', means start with 2, left align with 2 spaces filled with 2333# f'{3:3<3}', means start with 3, left align with 3 spaces filled with 34444# f'{4:4<4}', means start with 4, left align with 4 spaces filled with 455555# f'{5:5<5}', means start with 5, left align with 5 spaces filled with 5666666# f'{6:6<6}', means start with 6, left align with 6 spaces filled with 67777777# f'{7:7<7}', means start with 7, left align with 7 spaces filled with 788888888# f'{8:8<8}', means start with 8, left align with 8 spaces filled with 8999999999# f'{9:9<9}', means start with 9, left align with 9 spaces filled with 9

You already know what the f-string f'{a:{a}<{a}' does - when given an {object} within the string it will replace with said object. In this case, a is range of 1 to 9.

Then all you need to understand is what {9:9<9} does. It's a string formatter as the documentation that the answer has provided:

'<' Forces the field to be left-aligned within the available space (this is the default for most objects).

The x<y portion signifies to left-align the text with a width of y spaces. For any unused space, pad it with character x. So you start with {9} as the first character, and for the rest of the 8 unused spaces, fill it with {9}. This is what {9:9<9} does.

Then you apply the same logic and see how each iteration came about.

More importantly, it should be noted that what feels like "magic" is often just a lack of understanding. Once you take the time to digest and understand the process, it becomes very disillusioning and you become enlightened.