[RavSinghChandan]

Formatting NaN treats it like a number with digits:

format_currency(Decimal('NaN'), 'USD', locale='en_US')      # '$NaN.00'
format_currency(Decimal('NaN'), 'EUR', locale='de_DE')      # 'NaN,00 €'
format_decimal(Decimal('NaN'), '#,##0.00', locale='en_US')  # 'NaN.00'
format_scientific(Decimal('NaN'), locale='en_US')           # 'NaNE0'
format_decimal(Decimal('NaN'), locale='fi_FI')              # 'NaN', but CLDR has 'epäluku'

Infinity is already replaced by the locale's symbol in _quantize_value; NaN now is too (number_symbols[...]['nan'], e.g. 'epäluku' in Finnish, 'ليس رقمًا' in Arabic), so no fraction digits are appended to it. Scientific notation is only rendered for finite numbers, which also turns format_scientific(Decimal('Infinity')) from '∞E0' into '∞'.

With the fix: '$NaN', 'NaN €', 'NaN', 'NaN', 'epäluku'. Finite numbers are unchanged. Assertions added to test_nan_and_infinity

[akx]

How does this compare to #1416?

[RavSinghChandan]

Same core fix, the NaN early return in _quantize_value. #1416 came first, sorry for not spotting it. This one adds two things on top:

it uses the locale's NaN symbol from CLDR, like infinity already does (epäluku in fi_FI), which changes plain format_decimal(nan) output for those locales
it skips the exponent for NaN and infinity in format_scientific (NaNE0 → NaN, ∞E0 → ∞)
If you'd rather keep this small, I'm happy to close this in favour of #1416, or to open just the format_scientific part separately on top of it.

[RavSinghChandan]

Same core fix, the NaN early return in _quantize_value. #1416 came first, sorry for not spotting it. This one adds two things on top:

• it uses the locale's NaN symbol from CLDR, like infinity already does (epäluku in fi_FI), which changes plain format_decimal(nan) output for those locales
• it skips the exponent for NaN and infinity in format_scientific (NaNE0 → NaN, ∞E0 → ∞)

If you'd rather keep this small, I'm happy to close this in favour of #1416, or to open just the format_scientific part separately on top of it.