格式化字符串文字语法

Question

In Python official document 2.4.3. Formatted string literals the following grammar is defined:

f_string          ::=  (literal_char | "{{" | "}}" | replacement_field)*

Can someone explain why there are two curly braces here?

Answer 1

This is explained immediately below the grammar definition:

The parts of the string outside curly braces are treated literally, except that any doubled curly braces '{{' or '}}' are replaced with the corresponding single curly brace.

This is so that you can "escape" the braces if you want literal braces to appear. As an example:

>>> print(f"braces look like {{this}}")
braces look like {this}