Add use-str-func check

refurb/checks/readability/use_str_func.py

@@ -0,0 +1,68 @@
+from dataclasses import dataclass
+
+from mypy.nodes import CallExpr, ListExpr, MemberExpr, NameExpr, StrExpr
+
+from refurb.checks.common import stringify
+from refurb.error import Error
+from refurb.checks.string.use_fstring_fmt import CONVERSIONS as FURB_119_FUNCS
+
+
+@dataclass
+class ErrorInfo(Error):
+    """
+    If you want to stringify a single value without concatenating anything, use
+    the `str()` function instead.
+
+    Bad:
+
+    ```
+    nums = [123, 456]
+    num = f"{num[0]}")
+    ```
+
+    Good:
+
+    ```
+    nums = [123, 456]
+    num = str(num[0])
+    ```
+    """
+
+    name = "use-str-func"
+    code = 183
+    categories = ("readability",)
+
+
+ignore = set[int]()
+
+
+def check(node: CallExpr, errors: list[Error]) -> None:
+    if id(node) in ignore:
+        return
+
+    match node:
+        case CallExpr(
+            callee=MemberExpr(
+                expr=StrExpr(value=""),
+                name="join",
+            ),
+            args=[ListExpr(items=items)],
+        ):
+            ignore.update(id(item) for item in items)
+
+        case CallExpr(
+            callee=MemberExpr(
+                expr=StrExpr(value="{:{}}"),
+                name="format",
+            ),
+            args=[arg, StrExpr(value="")],
+        ):
+            match arg:
+                case CallExpr(callee=NameExpr(fullname=fn)) if fn in FURB_119_FUNCS:
+                    return
+
+            x = stringify(arg)
+
+            msg = f'Replace `f"{{{x}}}"` with `str({x})`'
+
+            errors.append(ErrorInfo.from_node(node, msg))

refurb/visitor/visitor.py

@@ -45,14 +45,14 @@ def __init__(self, checks: defaultdict[type[Node], list[Check]], settings: Setti
                 setattr(self, name, func.__get__(self))
 
     def visit_call_expr(self, o: CallExpr) -> None:
+        for check in self.checks[CallExpr]:
+            self.run_check(o, check)
+
         for arg in o.args:
             self.accept(arg)
 
         self.accept(o.callee)
 
-        for check in self.checks[CallExpr]:
-            self.run_check(o, check)
-
     def run_check(self, node: Node, check: Check) -> None:
         # Hack: use the type annotations to check if the function takes 2 or
         # 3 arguments. There is an extra field for return types, hence why we

test/data/err_119.py

@@ -19,7 +19,7 @@
 
 # these will not
 
-f"{123}"
+f"{123}"  # noqa: FURB183
 
 f"{0b1010:b}"
 

test/data/err_183.py

@@ -0,0 +1,12 @@
+x = " "
+
+# these should match
+
+f"{x}"
+f"{123}"
+
+
+# these should not
+
+f"hello{x}world"
+f"{x} {x}"

test/data/err_183.txt

@@ -0,0 +1,2 @@
+test/data/err_183.py:5:1 [FURB183]: Replace `f"{x}"` with `str(x)`
+test/data/err_183.py:6:1 [FURB183]: Replace `f"{123}"` with `str(123)`