Convert single-argument %-style format calls (#3600)

crates/ruff/resources/test/fixtures/pyupgrade/UP031_0.py

@@ -73,3 +73,13 @@ print("%s \N{snowman}" % (a,))
 print("%(foo)s \N{snowman}" % {"foo": 1})
 
 print(("foo %s " "bar %s") % (x, y))
+
+# Single-value expressions
+print('Hello %s' % "World")
+print('Hello %s' % f"World")
+print('Hello %s (%s)' % bar)
+print('Hello %s (%s)' % bar.baz)
+print('Hello %s (%s)' % bar['bop'])
+print('Hello %(arg)s' % bar)
+print('Hello %(arg)s' % bar.baz)
+print('Hello %(arg)s' % bar['bop'])

crates/ruff/resources/test/fixtures/pyupgrade/UP031_1.py

@@ -1,6 +1,4 @@
 # OK
-"%s" % unknown_type
-
 b"%s" % (b"bytestring",)
 
 "%*s" % (5, "hi")
@@ -57,3 +55,9 @@ pytest.param('"%8s" % (None,)', id="unsafe width-string conversion"),
     """
     % (x,)
 )
+
+'Hello %s' % bar
+
+'Hello %s' % bar.baz
+
+'Hello %s' % bar['bop']

crates/ruff/src/rules/pyupgrade/rules/printf_string_formatting.rs

@@ -335,7 +335,9 @@ pub(crate) fn printf_string_formatting(
     }
 
     // Parse each string segment.
-    let mut format_strings = vec![];
+    let mut num_positional_arguments = 0;
+    let mut num_keyword_arguments = 0;
+    let mut format_strings = Vec::with_capacity(strings.len());
     for (start, end) in &strings {
         let string = checker.locator.slice(Range::new(*start, *end));
         let (Some(leader), Some(trailer)) = (leading_quote(string), trailing_quote(string)) else {
@@ -351,12 +353,52 @@ pub(crate) fn printf_string_formatting(
             return;
         }
 
+        // Count the number of positional and keyword arguments.
+        for (.., format_part) in format_string.iter() {
+            let CFormatPart::Spec(ref fmt) = format_part else {
+                continue;
+            };
+            if fmt.mapping_key.is_none() {
+                num_positional_arguments += 1;
+            } else {
+                num_keyword_arguments += 1;
+            }
+        }
+
+        // Convert the `%`-format string to a `.format` string.
         let format_string = percent_to_format(&format_string);
         format_strings.push(format!("{leader}{format_string}{trailer}"));
     }
 
     // Parse the parameters.
     let params_string = match right.node {
+        ExprKind::Constant { .. } | ExprKind::JoinedStr { .. } => {
+            format!("({})", checker.locator.slice(right))
+        }
+        ExprKind::Name { .. }
+        | ExprKind::Attribute { .. }
+        | ExprKind::Subscript { .. }
+        | ExprKind::Call { .. } => {
+            if num_keyword_arguments > 0 {
+                // If we have _any_ named fields, assume the right-hand side is a mapping.
+                format!("(**{})", checker.locator.slice(right))
+            } else if num_positional_arguments > 1 {
+                // If we have multiple fields, but no named fields, assume the right-hand side is a
+                // tuple.
+                format!("(*{})", checker.locator.slice(right))
+            } else {
+                // Otherwise, if we have a single field, we can't make any assumptions about the
+                // right-hand side. It _could_ be a tuple, but it could also be a single value,
+                // and we can't differentiate between them.
+                // For example:
+                // ```python
+                // x = (1,)
+                // print("%s" % x)
+                // print("{}".format(x))
+                // ```
+                return;
+            }
+        }
         ExprKind::Tuple { .. } => clean_params_tuple(checker, right),
         ExprKind::Dict { .. } => {
             if let Some(params_string) = clean_params_dictionary(checker, right) {

crates/ruff/src/rules/pyupgrade/snapshots/ruff__rules__pyupgrade__tests__UP031_0.py.snap

@@ -582,4 +582,164 @@ expression: diagnostics
       row: 75
       column: 35
   parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 78
+    column: 6
+  end_location:
+    row: 78
+    column: 26
+  fix:
+    content: "'Hello {}'.format(\"World\")"
+    location:
+      row: 78
+      column: 6
+    end_location:
+      row: 78
+      column: 26
+  parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 79
+    column: 6
+  end_location:
+    row: 79
+    column: 27
+  fix:
+    content: "'Hello {}'.format(f\"World\")"
+    location:
+      row: 79
+      column: 6
+    end_location:
+      row: 79
+      column: 27
+  parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 80
+    column: 6
+  end_location:
+    row: 80
+    column: 27
+  fix:
+    content: "'Hello {} ({})'.format(*bar)"
+    location:
+      row: 80
+      column: 6
+    end_location:
+      row: 80
+      column: 27
+  parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 81
+    column: 6
+  end_location:
+    row: 81
+    column: 31
+  fix:
+    content: "'Hello {} ({})'.format(*bar.baz)"
+    location:
+      row: 81
+      column: 6
+    end_location:
+      row: 81
+      column: 31
+  parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 82
+    column: 6
+  end_location:
+    row: 82
+    column: 34
+  fix:
+    content: "'Hello {} ({})'.format(*bar['bop'])"
+    location:
+      row: 82
+      column: 6
+    end_location:
+      row: 82
+      column: 34
+  parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 83
+    column: 6
+  end_location:
+    row: 83
+    column: 27
+  fix:
+    content: "'Hello {arg}'.format(**bar)"
+    location:
+      row: 83
+      column: 6
+    end_location:
+      row: 83
+      column: 27
+  parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 84
+    column: 6
+  end_location:
+    row: 84
+    column: 31
+  fix:
+    content: "'Hello {arg}'.format(**bar.baz)"
+    location:
+      row: 84
+      column: 6
+    end_location:
+      row: 84
+      column: 31
+  parent: ~
+- kind:
+    name: PrintfStringFormatting
+    body: Use format specifiers instead of percent format
+    suggestion: Replace with format specifiers
+    fixable: true
+  location:
+    row: 85
+    column: 6
+  end_location:
+    row: 85
+    column: 34
+  fix:
+    content: "'Hello {arg}'.format(**bar['bop'])"
+    location:
+      row: 85
+      column: 6
+    end_location:
+      row: 85
+      column: 34
+  parent: ~