Chris Wilcox <pyt...@crwilcox.com> added the comment:
Double curly braces do not indicate to not process the inner content. They
indicate to include a literal curly brace. That said, I think there may be
something not quite correct.
I came up with an example based on the example in the format specifiers section
of the PEP.
>From the PEP.
```
>>> width = 10
>>> precision = 4
>>> value = decimal.Decimal('12.34567')
>>> f'result: {value:{width}.{precision}}'
'result: 12.35'
```
The template in this instance is "10.4"
If we leave the sample the same, but don't wrap width or precision in single
curly braces,
```
>>> f'result: {value:width.precision}'
```
I would expect the template "width.precision".
Further, I would expect
```
>>> f'result: {value:{{width}}.{{precision}}}'
```
to have a template of "{width}.{precision}". This is not the case.
Here is some code that should demonstrate this.
```
class Decimal:
def __init__(self, value):
pass
def __format__(self, template):
return template
width = 10
precision = 4
value = Decimal('12.34567')
print("Expect Template to be '10.4' (TRUE)")
print(f'result0: {value:{width}.{precision}}')
print("Expect Template to be 'width.precision' (TRUE)")
print(f'result1: {value:width.precision}')
print("Expect Template to be '{width}.{precision}' (FALSE)")
print(f'result2: {value:{{width}}.{{precision}}}') # ACTUAL: {10}.{4}
```
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<https://bugs.python.org/issue39601>
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