QPY-84: Match json.dumps default separators

src/builtins/json.rs

@@ -38,14 +38,51 @@ fn json_dumps(args: Vec<Value>) -> Result<Value, Value> {
     }
 
     let json_value = value_to_json(&args[0])?;
-    let json_str = serde_json::to_string(&json_value).map_err(|e| {
+    let mut json_str = String::new();
+    write_json_default(&json_value, &mut json_str)?;
+
+    Ok(Value::String(json_str))
+}
+
+fn write_json_default(json: &serde_json::Value, output: &mut String) -> Result<(), Value> {
+    match json {
+        serde_json::Value::Array(items) => {
+            output.push('[');
+            for (index, item) in items.iter().enumerate() {
+                if index > 0 {
+                    output.push_str(", ");
+                }
+                write_json_default(item, output)?;
+            }
+            output.push(']');
+        }
+        serde_json::Value::Object(object) => {
+            output.push('{');
+            for (index, (key, value)) in object.iter().enumerate() {
+                if index > 0 {
+                    output.push_str(", ");
+                }
+                write_json_scalar(&serde_json::Value::String(key.clone()), output)?;
+                output.push_str(": ");
+                write_json_default(value, output)?;
+            }
+            output.push('}');
+        }
+        scalar => write_json_scalar(scalar, output)?,
+    }
+
+    Ok(())
+}
+
+fn write_json_scalar(json: &serde_json::Value, output: &mut String) -> Result<(), Value> {
+    let encoded = serde_json::to_string(json).map_err(|error| {
         Value::error(
             ExceptionType::RuntimeError,
-            format!("Failed to serialize: {}", e),
+            format!("Failed to serialize: {}", error),
         )
     })?;
-
-    Ok(Value::String(json_str))
+    output.push_str(&encoded);
+    Ok(())
 }
 
 fn json_to_value(json: &serde_json::Value) -> Result<Value, Value> {
@@ -118,3 +155,34 @@ fn value_to_json(value: &Value) -> Result<serde_json::Value, Value> {
         )),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn dumps_json(source: &str) -> String {
+        let json: serde_json::Value = serde_json::from_str(source).unwrap();
+        let value = json_to_value(&json).unwrap();
+        json_dumps(vec![value])
+            .unwrap()
+            .as_string()
+            .unwrap()
+            .to_owned()
+    }
+
+    #[test]
+    fn dumps_uses_python_default_separators() {
+        assert_eq!(dumps_json(r#"{"a":1}"#), r#"{"a": 1}"#);
+        assert_eq!(dumps_json("[1,2,3]"), "[1, 2, 3]");
+        assert_eq!(
+            dumps_json(r#"{"a":[1,2,{"b":3}]}"#),
+            r#"{"a": [1, 2, {"b": 3}]}"#
+        );
+    }
+
+    #[test]
+    fn medium_response_dump_has_cpython_length() {
+        let source = include_str!("../../benchmarks/talk_comparison/fixtures/medium_response.json");
+        assert_eq!(dumps_json(source).len(), 1815);
+    }
+}