書式指定子の入れ子

プログラミング言語Pythonの str.format や f-string*1 において、書式指定子部のみ１段階の入れ子が許容される。下記コードではいずれも文字列 Hello!!!!! が得られる。

msg = 'Hello'
'{:!<10}'.format(msg)
f'{msg:!<10}'

f, a, w = '!', '<', 10
'{:{}{}{}}'.format(msg, f, a, w)
f'{msg:{f}{a}{w}}'

# 各typeフィールドを明示
f'{msg:{f:s}{a:s}{w:d}s}'

Top-level format specifiers may include nested replacement fields. These nested fields may include their own conversion fields and format specifiers, but may not include more deeply-nested replacement fields. The format specifier mini-language is the same as that used by the string .format() method.

Lexical analysis, Formatted string literals

*1:formatted string literal