Bug report

The f-string allows different lengths and zero padding and adding _ separators (every 3 digits for decimal and every 4 digits for binary and hexadecimal). The length of the generated string with different lengths is inconsistent.

See compiler explorer outputs: https://godbolt.org/z/dv7br5EzG

x = 1
for l in [4, 5, 6, 7, 8]:
    print(f'Length: {l}')
    print(f'  bin: {x:0{l}_b}')
    print(f'  hex: {x:0{l}_x}')
    print(f'  dec: {x:0{l}_}')

Output:

Length: 4
  bin: 0001
  hex: 0001
  dec: 0_001
Length: 5
  bin: 0_0001
  hex: 0_0001
  dec: 0_001
Length: 6
  bin: 0_0001
  hex: 0_0001
  dec: 00_001
Length: 7
  bin: 00_0001
  hex: 00_0001
  dec: 000_001
Length: 8
  bin: 000_0001
  hex: 000_0001
  dec: 0_000_001

Your environment

• CPython versions tested on: 3.9 and 3.11
• Operating system and architecture: Debian and compiler explorer

I would expect that the string is first padded (with zeros) to the specified length and then separator characters are inserted as much as needed. Correct me, if I am wrong.