F-strings do not properly pad when ANSI colour codes are included in the input

[Redstone118]

Bug report

Bug description:

#ANSI colour code for red foreground, '\033[31m', and '0'
example_string = "\033[31m0"
print(f"|{ example_string:<3}|")

Produces the following output:
|0|
Which is not padded as would be expected

Removing the colour code however

example_string = "0"
print(f"|{ example_string:<3}|")`

Produces the expected value
|0 |

This leads to misalignment when padding coloured content

CPython versions tested on:

3.13

Operating systems tested on:

No response

[gvanrossum-ms]

IMO it's unreasonable to expect Python to have specific knowledge of something like the ANSI color codes (or other escape sequences for moving the cursor etc.).

What you can do is having the color codes outside the data, e.g.

RED = '\033[31m'  # Or colorama.Fore.RED
example_string = "0"
print(f"|{RED}{example_string:<3}|")

[ericvsmith]

I agree it’s not reasonable that Python knows about color escape sequences, or how various devices interpret escape sequences. The same sequence could show up as different widths on different devices, for example. Guido provided a decent workaround. I’m going to close this.