Why do I need another pair of curly braces when using a variable in a format specifier in Python f-strings?

Question

I'm learning how Python f-strings handle formatting and came across this syntax:

a = 5.123
b = 2.456
width = 10
result = f"The result is {(a + b):<{width}.2f}end"
print(result)

This works as expected, however I don't understand why {width} needs its own curly braces within the format specification. Why can't I just use width directly just as "a" and "b"? Isn't width already inside the outer curly braces?

Answer 1

The braces are needed because the part starting with : is the format specification mini-language. That is parsed as its own f-string, where all is literal, except if put in braces.

That format specification language would bump into ambiguities if those braces were not required: that language gives meaning to certain letters, and it would not be clear whether such letter(s) would need to be taken literally or as an expression to be evaluated dynamically.

For instance, in Python 3.11+ we have the z option that can occur at the position where you specified the width:

z = 5
f"The result is {(a + b):<z.2f}end"

So what would this mean if braces were not needed? Would it be the z option or what the z variable represents?

There are many more examples that could be constructed to bring the same conclusion home: the braces are needed for anything that needs to be evaluated inside the format specification part.

Answer 2

The grammar for f-string is (emphasis mine)

f_string          ::=  (literal_char | "{{" | "}}" | replacement_field)*
replacement_field ::=  "{" f_expression ["="] ["!" conversion] [":" format_spec] "}"
f_expression      ::=  (conditional_expression | "*" or_expr)
                         ("," conditional_expression | "," "*" or_expr)* [","]
                       | yield_expression
conversion        ::=  "s" | "r" | "a"
format_spec       ::=  (literal_char | replacement_field)*
literal_char      ::=  <any code point except "{", "}" or NULL>

It says a replacement_field should have f_expression along with optional [":" format_spec]. format_spec says it can have a replacement_field.

According to the grammar it allows having replacement_field that has format_spec with a replacement_field. so, f"{0.777:{'.02f'}}" is a valid string.

replacement_field -> {0.777:{'.02f'}}
                            |      |
                            --------
                           format_spec with replacement_field

Another example,

f"{10.12345466768789:{0.100:{'.2f' if True else ''}}}"
# '10.12345467'

# replacement_field -> {10.12345466768789:{0.100:{'.2f' if True else ''}}}
# format_spec with replacement_field  -> {0.100:{'.2f' if True else ''}}
# the above format_spec has replacement_field with format_spec

# Breakdown
- {'.2f' if True else ''} -> .2f

- {0.100:{'.2f' if True else ''}}
  {0.100:.2f} -> 0.10

- f"{10.12345466768789:{0.100:{'.2f' if True else ''}}}"
  f"{10.12345466768789:0.10}" -> '10.12345467'

Let's say you want to left-pad with char x stored in a variable a.

a = 'x'
f"{10.022:a>10.2f}"
# 'aaaaa10.02'

# with {}
f"{10.022:{a}>10.2f}"
# 'xxxxx10.02'

# If the padding character is either { or } 
f"{10.022:{'{'}<10}"
# '10.022{{{{'

# Without {}
# f"{10.022:{<10}" # Error

f-strings allows to write conditional_expressions in f_expressions. Without {} it would be very hard to parse the format_spec.

pos = False
approx = True
f"{-0.00000 :{'z' if pos else ''}{'.2f' if approx else '.7f'}}"
# '-0.00'

f"{-0.00000 :{'z' if not pos else ''}{'.2f' if not approx else '.7f'}}"
# '0.0000000'

# Without {} it would be impossible to parse through this.
# f"{0.777:z if pos else ''.2f if approx else .7f}" # error