Nested f-strings

Question

Thanks to David Beazley's tweet, I've recently found out that the new Python 3.6 f-strings can also be nested:

>>> price = 478.23 >>> f"{f'${price:0.2f}':*>20s}" '*************$478.23' 

Or:

>>> x = 42 >>> f'''-{f"""*{f"+{f'.{x}.'}+"}*"""}-''' '-*+.42.+*-' 

While I am surprised that this is possible, I am missing on how practical is that, when would nesting f-strings be useful? What use cases can this cover?

Note: The PEP itself does not mention nesting f-strings, but there is a specific test case.

Answer 1

I don't think formatted string literals allowing nesting (by nesting, I take it to mean f'{f".."}') is a result of careful consideration of possible use cases, I'm more convinced it's just allowed in order for them to conform with their specification.

The specification states that they support full Python expressions* inside brackets. It's also stated that a formatted string literal is really just an expression that is evaluated at run-time (See here, and here). As a result, it only makes sense to allow a formatted string literal as the expression inside another formatted string literal, forbidding it would negate the full support for Python expressions.

The fact that you can't find use cases mentioned in the docs (and only find test cases in the test suite) is because this is probably a nice (side) effect of the implementation and not it's motivating use-case.

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_{Actually, with two exceptions: An empty expression is not allowed, and a lambda expression must be surrounded by explicit parentheses.}

Answer 2

I guess this is to pass formatting parameters in the same line and thus simplify f-strings usage.

For example:

>>> import decimal >>> width = 10 >>> precision = 4 >>> value = decimal.Decimal("12.34567") >>> f"result: {value:{width}.{precision}}" 'result:      12.35' 

Of course, it allows programmers to write absolutely unreadable code, but that's not the purpose :)