fix: Invert case_sensitive logic in StructType (apache#1147)

* fix: Invert  logic in StructType

* Add test for StructType.field_by_name

* Remove var I forgot about.

* Fix formatting post-lint

pyiceberg/types.py

@@ -377,13 +377,13 @@ def field(self, field_id: int) -> Optional[NestedField]:
 
     def field_by_name(self, name: str, case_sensitive: bool = True) -> Optional[NestedField]:
         if case_sensitive:
-            name_lower = name.lower()
             for field in self.fields:
-                if field.name.lower() == name_lower:
+                if field.name == name:
                     return field
         else:
+            name_lower = name.lower()
             for field in self.fields:
-                if field.name == name:
+                if field.name.lower() == name_lower:
                     return field
         return None
 

tests/test_types.py

@@ -149,6 +149,20 @@ def test_struct_type() -> None:
     assert type_var == pickle.loads(pickle.dumps(type_var))
 
 
+def test_struct_field_by_name() -> None:
+    lower_field = NestedField(1, "lower_case_field", IntegerType(), required=True)
+    upper_field = NestedField(2, "UPPER_CASE_FIELD", IntegerType(), required=True)
+    type_var = StructType(lower_field, upper_field)
+
+    assert type_var.field_by_name("lower_case_field", case_sensitive=False) == lower_field
+    assert type_var.field_by_name("upper_case_field", case_sensitive=False) == upper_field
+    assert type_var.field_by_name("nonexistent_field", case_sensitive=False) is None
+
+    assert type_var.field_by_name("lower_case_field", case_sensitive=True) == lower_field
+    assert type_var.field_by_name("upper_case_field", case_sensitive=True) is None
+    assert type_var.field_by_name("nonexistent_field", case_sensitive=True) is None
+
+
 def test_list_type() -> None:
     type_var = ListType(
         1,