core.pandas: dump the timezones in check_dateish

my/core/pandas.py

@@ -4,7 +4,8 @@ Various pandas helpers and convenience functions
 # todo not sure if belongs to 'core'. It's certainly 'more' core than actual modules, but still not essential
 # NOTE: this file is meant to be importable without Pandas installed
 from datetime import datetime
-from typing import Optional, TYPE_CHECKING, Any
+from pprint import pformat
+from typing import Optional, TYPE_CHECKING, Any, Iterable
 from . import warnings
 
 
@@ -19,18 +20,24 @@ else:
     DataFrameT = Any
 
 
-def check_dateish(s) -> Optional[str]:
+def check_dateish(s) -> Iterable[str]:
     import pandas as pd # type: ignore
     ctype = s.dtype
     if str(ctype).startswith('datetime64'):
-        return None
+        return
     s = s.dropna()
     if len(s) == 0:
-        return None
+        return
     all_timestamps = s.apply(lambda x: isinstance(x, (pd.Timestamp, datetime))).all()
-    if all_timestamps:
-        return 'All values are timestamp-like, but dtype is not datetime. Most likely, you have mixed timezones'
-    return None
+    if not all_timestamps:
+        return # not sure why it would happen, but ok
+    tzs = s.map(lambda x: x.tzinfo).drop_duplicates()
+    examples = s[tzs.index]
+    # todo not so sure this warning is that useful... except for stuff without tz
+    yield f'''
+    All values are timestamp-like, but dtype is not datetime. Most likely, you have mixed timezones:
+    {pformat(list(zip(examples, tzs)))}
+    '''.strip()
 
 
 from typing import Any, Callable, TypeVar
@@ -43,9 +50,8 @@ def check_dataframe(f: FuncT) -> FuncT:
         df = f(*args, **kwargs)
         # todo make super defensive?
         for col, data in df.reset_index().iteritems():
-            res = check_dateish(data)
-            if res is not None:
-                warnings.low(f"{f.__module__}:{f.__name__}, column '{col}': {res}")
+            for w in check_dateish(data):
+                warnings.low(f"{f.__module__}:{f.__name__}, column '{col}': {w}")
         return df
     # https://github.com/python/mypy/issues/1927
     return wrapper # type: ignore[return-value]