add format precision for string

Author: dvermd

Lib/test/test_ast.py

@@ -939,7 +939,7 @@ def test_iter_fields(self):
         self.assertEqual(d.pop('func').id, 'foo')
         self.assertEqual(d, {'keywords': [], 'args': []})
 
-    # TODO: RUSTPYTHON; redundant kind for Contant node
+    # TODO: RUSTPYTHON; redundant kind for Constant node
     @unittest.expectedFailure
     def test_iter_child_nodes(self):
         node = ast.parse("spam(23, 42, eggs='leek')", mode='eval')

Lib/test/test_fstring.py

@@ -1021,8 +1021,6 @@ def test_leading_trailing_spaces(self):
         self.assertEqual(f'expr={ {x: y for x, y in [(1, 2), ]} }',
                          'expr={1: 2}')
 
-    # TODO: RUSTPYTHON string should be cut at 3 chars
-    @unittest.expectedFailure
     def test_not_equal(self):
         # There's a special test for this because there's a special
         #  case in the f-string parser to look for != as not ending an

vm/src/format.rs

@@ -515,7 +515,14 @@ impl FormatSpec {
 
     pub(crate) fn format_string(&self, s: &str) -> Result<String, &'static str> {
         match self.format_type {
-            Some(FormatType::String) | None => self.format_sign_and_align(s, "", FormatAlign::Left),
+            Some(FormatType::String) | None => self
+                .format_sign_and_align(s, "", FormatAlign::Left)
+                .map(|mut value| {
+                    if let Some(precision) = self.precision {
+                        value.truncate(precision);
+                    }
+                    value
+                }),
             _ => Err("Unknown format code for object of type 'str'"),
         }
     }
@@ -944,7 +951,7 @@ impl<'a> FromTemplate<'a> for FormatString {
         let mut cur_text: &str = text;
         let mut parts: Vec<FormatPart> = Vec::new();
         while !cur_text.is_empty() {
-            // Try to parse both literals and bracketed format parts util we
+            // Try to parse both literals and bracketed format parts until we
             // run out of text
             cur_text = FormatString::parse_literal(cur_text)
                 .or_else(|_| FormatString::parse_spec(cur_text))