tags: python string fstring

Python fstrings allow you to format strings by interpolating expressions.

```python
x = 257
y = 1.554
f"{x} {x:05d} {x:08x} {x:o}"
f"{y:e} {y:.2f} {0.5:.2%}"
z = 24242224224
f"{z:,.2f}"
# Alignment
f"{1:<8d} {1:<8d} {1:<8d}"
#vs
f"{1:>8d} {1:>8d} {1:>8d}"
```
## Padding
You can specify left, centre or right alignment using `<`, `>` or `^`, and the padding symbol before this.
You can use a variable as the padding symbol provided it is a single-character string.
```python
>>> a = 1
>>> b = 3.14
>>> c = "hello"
>>> width = 30
>>> f"{c:#^{width}}"
'############hello#############'
>>> f"{c:_<{width}}"
'hello_________________________'
>>> f"{c:*>{width}}"
'*************************hello'
>>> d="_*_"
>>> f"{c:{d}>{width}}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier
>>> d="*"
>>> f"{c:{d}>{width}}"
'*************************hello'
>>> f"{45:#>17.3f}"
'###########45.000'
```
# fstring formatters
You define a `__format__(self,fmtstring)` method in a class, and that is called by an fstring,
with whatever follows the `:` being passed as the argument `fmtstring`. Silly example:
```py
import sys
class A:
  n = 0
  @classmethod
  def inc(cls):
    cls.n += 1
    return cls.n
  def __format__(self,fmt):
    print(f"format: {fmt}",file=sys.stderr)
    return str(self.inc())
  
a = A()
print(f"{a:flibble} {a:hexvision}")
print(f"{a:flibble} {a:hexvision}")
print(f"{a:flibble} {a:hexvision}")
print(f"{a:flibble} {a:hexvision}")
```