chore: warn about switching to writing RNTuples by default (#1488)

* Added warning about behavior change

* Show warning only once

---------

Co-authored-by: Ianna Osborne <[email protected]>

Author: ariostas

pyproject.toml

@@ -124,7 +124,8 @@ addopts = ["-ra", "--showlocals", "--strict-markers", "--strict-config"]
 filterwarnings = [
   "error",
   "default:module 'sre_.*' is deprecated:DeprecationWarning",
-  "ignore:unclosed transport <asyncio.sslproto._SSLProtocolTransport"  # https://github.com/aio-libs/aiohttp/issues/1115
+  "ignore:unclosed transport <asyncio.sslproto._SSLProtocolTransport",  # https://github.com/aio-libs/aiohttp/issues/1115
+  "ignore: .*will default to writing RNTuples.*:FutureWarning"
 ]
 log_cli_level = "info"
 markers = [

src/uproot/writing/identify.py

@@ -17,6 +17,7 @@
 """
 from __future__ import annotations
 
+import warnings
 from collections.abc import Mapping
 
 import numpy
@@ -26,6 +27,9 @@
 import uproot.pyroot
 import uproot.writing._cascadetree
 
+# To keep track of whether we've warned about switching to writing RNTuple by default
+_warned_rntuple_by_default = False
+
 
 def add_to_directory(obj, name, directory, streamers):
     """
@@ -148,6 +152,16 @@ def add_to_directory(obj, name, directory, streamers):
             is_ttree = True
 
     if is_ttree:
+        global _warned_rntuple_by_default  # noqa: PLW0603
+        if not _warned_rntuple_by_default:
+            warnings.warn(
+                "Starting in version 5.7.0, Uproot will default to writing RNTuples instead of TTrees. "
+                "You will need to use `mktree` to create to explicitly create a TTree. "
+                "Please update your code accordingly.",
+                FutureWarning,
+                stacklevel=4,
+            )
+            _warned_rntuple_by_default = True
         tree = directory.mktree(name, metadata)
         tree.extend(data)