Formatting raw string in Python when using exact match regex

Question

The fr prefix can combine f and r flag.
But when it comes to the regex's exact match, it seems that it can't format the raw string well:

import re

RE1 = r'123'
RE2 = re.compile(fr'@{3} {RE1}')

Then, the RE2.pattern will become '@3 123',

but what I want is '@{3} 123'.

Answer 1

You have to escape the braces surrounding the 3 like this, otherwise they will be interpreted as string interpolation:

import re

RE1 = r'123'
RE2 = re.compile(fr'@{{3}} {RE1}')
print(RE2)

This produces:

@{3} 123

Ref: https://www.python.org/dev/peps/pep-0498/

Note that the correct way to have a literal brace appear in the resulting string value is to double the brace:

>>> f'{{ {4*10} }}'
'{ 40 }'
>>> f'{{{4*10}}}'
'{40}'