f-string (UP032)

lint.select = ["UP032"]

Repeated arguments with side effects

A str.format argument is evaluated once, but an f-string re-evaluates it on every interpolation. So when an argument is used more than once and can have a side effect, the call is left unconverted to avoid running that side effect twice. This used to cover only call expressions; it now covers any side-effecting expression, such as a subscript or a walrus binding. Even a builtin call like list() counts, since the f-string would construct a new object on each interpolation.

def foo(): ...


d = {}

"{x} {x}".format(x=foo())
"{x} {x}".format(x=d["k"])
"{x} {x}".format(x=(y := 1))
"{x.append} {x.append}".format(x=list())

A single use is still converted

When the argument is interpolated once, the side effect runs once either way, so the fix is safe.

def foo(): ...


"{x}".format(x=foo())  # snapshot: f-string

error[UP032]: Use f-string instead of `format` call
 --> src/mdtest_snippet.py:4:1
  |
4 | "{x}".format(x=foo())  # snapshot: f-string
  | ^^^^^^^^^^^^^^^^^^^^^
  |
help: Convert to f-string
1 | def foo(): ...
2 |
3 |
  - "{x}".format(x=foo())  # snapshot: f-string
4 + f"{foo()}"  # snapshot: f-string