Nested f-strings

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.

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.

---

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

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 :)

I've actually just come across something similar (i think) and thought i'd share.

My specific case is a big dirty sql statement where i need to conditionally have some very different values but some fstrings are the same (and also used in other places).

Here is quick example of what i mean. The cols i'm selecting are same regardless (and also used in other queries elsewhere) but the table name depends on the group and is not such i could just do it in a loop.

Having to include mycols=mycols in str2 each time felt a little dirty when i have multiple such params.

I was not sure this would work but was happy it did. As to how pythonic its is i'm not really sure tbh.

mycols='col_a,col_b'

str1 = "select {mycols} from {mytable} where group='{mygroup}'".format(mycols=mycols,mytable='{mytable}',mygroup='{mygroup}')

group = 'group_b'

if group == 'group_a':
	str2 = str1.format(mytable='tbl1',mygroup=group)
elif group == 'group_b':
	str2 = str1.format(mytable='a_very_different_table_name',mygroup=group)

print(str2)

Any basic use case is where you need a string to completely describe the object you want to put inside the f-string braces {}. For example, you need strings to index dictionaries.

So, I ended up using it in an ML project with code like:

scores = dict()
scores[f'{task}_accuracy'] = 100. * n_valid / n_total
print(f'{task}_accuracy: {scores[f"{task}_accuracy"]}')

Working on a pet project I got sidetracked by writing my own DB library. One thing I discovered was this:

>>> x = dict(a = 1, b = 2, d = 3)
>>> z = f"""
    UPDATE TABLE 
        bar 
    SET 
        {", ".join([ f'{k} = ?'     for k in x.keys() ])} """.strip()
>>> z
'UPDATE TABLE 
    bar 
SET 
    a = ?, b = ?, d = ?  '

I was also surprised by this and honestly I am not sure I would ever do something like this in production code BUT I have also said I wouldn't do a lot of other things in production code.

I found nesting to be useful when doing ternaries. Your opinion will vary on readability, but I found this one-liner very useful.

logger.info(f"No program name in subgroups file. Using {f'{prg_num} {prg_orig_date}' if not prg_name else prg_name}")

As such, my tests for nesting would be:

• Is the value reused? (Variable for expression re-use)
• Is the expression clear? (Not exceeding complexity)

A simple example of when it's useful, together with an example of implementation: sometimes the formatting is also a variable.

num = 3.1415
fmt = ".2f"
print(f"number is {num:{fmt}}")

The following nested f-string one-liner does a great job in constructing a command argument string

cmd_args = f"""{' '.join([f'--{key} {value}' for key, value in kwargs.items()])}"""

where the input {'a': 10, 'b': 20, 'c': 30, ....}

gets elegantly converted to --a 10 --b 20 --c 30 ... `

I use it for formatting currencies. Given values like:

a=1.23
b=45.67

to format them with a leading $ and with the decimals aligned. e.g.

 $1.23
$45.67

formatting with a single f-string f"${value:5.2f}" you can get:

$ 1.23
$45.67

which is fine sometimes but not always. Nested f-strings f"${f'${value:.2f}':>6}" give you the exact format:

 $1.23
$45.67

In F-string the open-paren & close-paren are reserved key characters. To use f-string to build json string you have to escape the parentheses characters. in your case only the outer parens.

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

Nested f-strings vs. evaluated expressions in format specifiers

This question is about use-cases that would motivate using an f-string inside of some evaluated expression of an "outer" f-string.

This is different from the feature that allows evaluated expressions to appear within the format specifier of an f-string. This latter feature is extremely useful and somewhat relevant to this question since (1) it involves nested curly braces so it might be why people are looking at this post and (2) nested f-strings are allowed within the format specifier just as they are within other curly-expressions of an f-string.

F-string nesting can help with one-liners

Although certainly not the motivation for allowing nested f-strings, nesting may be helpful in obscure cases where you need or want a "one-liner" (e.g. lambda expressions, comprehensions, python -c command from the terminal). For example:

print('\n'.join([f"length of {x/3:g}{'.'*(11 - len(f'{x/3:g}'))}{len(f'{x/3:g}')}" for x in range(10)]))

If you do not need a one-liner, any syntactic nesting can be replaced by defining a variable previously and then using the variable name in the evaluated expression of the f-string (and in many if not most cases, the non-nested version would likely be more readable and easier to maintain; however it does require coming up with variable names):

for x in range(10):
    to_show = f"{x/3:g}"
    string_length = len(to_show)
    padding = '.' * (11 - string_length)
    print(f"length of {to_show}{padding}{string_length}")

Nested evaluated expressions (i.e. in the format specifier) are useful

In contrast to true f-string nesting, the related feature allowing evaluated expressions within the "format specifier" of an f-string can be extremely useful (as others have pointed out) for several reasons including:

1. formatting can be shared across multiple f-strings or evaluated expressions
2. formatting can include computed quantities that can vary from run to run

Here is an example that uses a nested evaluated expression, but not a nested f-string:

import random

results = [[i, *[random.random()] * 3] for i in range(10)]
format = "2.2f"

print("category,precision,recall,f1")
for cat, precision, recall, f1 in results:
    print(f"{cat},{precision:{format}},{recall:{format}},{f1:{format}}")

However, even this use of nesting can be replaced with more flexible (and maybe cleaner) code that does not require syntactic nesting:

import random

results = [[i, *[random.random()] * 3] for i in range(10)]
def format(x):
    return f"{x:2.2f}"

print("category,precision,recall,f1")
for cat, precision, recall, f1 in results:
    print(f"{cat},{format(precision)},{format(recall)},{format(f1)}")

If some fancy formatting is needed, such nesting could be, perhaps, useful.

for n in range(10, 1000, 100):
	print(f"{f'n = {n:<3}':<15}| {f'|{n:>5}**2 = {n**2:<7_}'} |")

You could use it for dynamicism. For instance, say you have a variable set to the name of some function:

func = 'my_func'

Then you could write:

f"{f'{func}'()}" 

which would be equivalent to:

'{}'.format(locals()[func]()) 

or, equivalently:

'{}'.format(my_func())