Add str.__format__

vm/src/format.rs

@@ -422,7 +422,7 @@ impl FormatSpec {
             }
         };
 
-        self.format_sign_and_align(magnitude_string, sign_str)
+        self.format_sign_and_align(&magnitude_string, sign_str)
     }
 
     pub fn format_int(&self, num: &BigInt) -> Result<String, &'static str> {
@@ -486,12 +486,19 @@ impl FormatSpec {
             },
         };
 
-        self.format_sign_and_align(magnitude_string, sign_str)
+        self.format_sign_and_align(&magnitude_string, sign_str)
+    }
+
+    pub fn format_string(&self, s: &str) -> Result<String, &'static str> {
+        match self.format_type {
+            Some(FormatType::String) | None => self.format_sign_and_align(s, ""),
+            _ => Err("Unknown format code for object of type 'str'"),
+        }
     }
 
     fn format_sign_and_align(
         &self,
-        magnitude_string: String,
+        magnitude_string: &str,
         sign_str: &str,
     ) -> Result<String, &'static str> {
         let align = self.align.unwrap_or(FormatAlign::Right);

vm/src/obj/objstr.rs

@@ -26,7 +26,7 @@ use crate::cformat::{
     CFormatPart, CFormatPreconversor, CFormatQuantity, CFormatSpec, CFormatString, CFormatType,
     CNumberType,
 };
-use crate::format::{FormatParseError, FormatPart, FormatPreconversor, FormatString};
+use crate::format::{FormatParseError, FormatPart, FormatPreconversor, FormatSpec, FormatString};
 use crate::function::{single_or_tuple_any, OptionalArg, PyFuncArgs};
 use crate::pyhash;
 use crate::pyobject::{
@@ -727,6 +727,16 @@ impl PyString {
         }
     }
 
+    #[pymethod(name = "__format__")]
+    fn format_str(&self, spec: PyStringRef, vm: &VirtualMachine) -> PyResult<String> {
+        match FormatSpec::parse(spec.as_str())
+            .and_then(|format_spec| format_spec.format_string(&self.value))
+        {
+            Ok(string) => Ok(string),
+            Err(err) => Err(vm.new_value_error(err.to_string())),
+        }
+    }
+
     /// Return a titlecased version of the string where words start with an
     /// uppercase character and the remaining characters are lowercase.
     #[pymethod]