core: add @warn_if_empty decorator

my/core/__init__.py

@@ -1,4 +1,5 @@
 # this file only keeps the most common & critical types/utility functions
 from .common import PathIsh, Paths, Json
 from .common import get_files, LazyLogger
+from .common import warn_if_empty
 from .cfg import make_config

my/core/common.py

@@ -3,7 +3,7 @@ from pathlib import Path
 from datetime import datetime
 import functools
 import types
-from typing import Union, Callable, Dict, Iterable, TypeVar, Sequence, List, Optional, Any, cast, Tuple
+from typing import Union, Callable, Dict, Iterable, TypeVar, Sequence, List, Optional, Any, cast, Tuple, TYPE_CHECKING
 import warnings
 
 # some helper functions
@@ -161,7 +161,6 @@ def get_files(pp: Paths, glob: str=DEFAULT_GLOB, sort: bool=True) -> Tuple[Path,
 
 
 # TODO annotate it, perhaps use 'dependent' type (for @doublewrap stuff)
-from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from typing import Callable, TypeVar
     from typing_extensions import Protocol
@@ -269,3 +268,40 @@ import re
 def get_valid_filename(s: str) -> str:
     s = str(s).strip().replace(' ', '_')
     return re.sub(r'(?u)[^-\w.]', '', s)
+
+
+from typing import Generic, Sized, Callable
+
+
+def _warn_iterator(it):
+    emitted = False
+    for i in it:
+        yield i
+        emitted = True
+    if not emitted:
+        warnings.warn(f"Function hasn't emitted any data, make sure your config paths are correct")
+
+
+def _warn_iterable(it):
+    if isinstance(it, Sized):
+        sz = len(it)
+        if sz == 0:
+            warnings.warn(f"Function is returning empty container, make sure your config paths are correct")
+        return it
+    else:
+        return _warn_iterator(it)
+
+
+from functools import wraps
+X = TypeVar('X')
+
+class G(Generic[X], Iterable[X]):
+    pass
+
+CC = Callable[[], G]
+def warn_if_empty(f: CC) -> CC:
+    @wraps(f)
+    def wrapped(*args, **kwargs) -> G[X]:
+        res = f(*args, **kwargs)
+        return _warn_iterable(res)
+    return wrapped

tests/misc.py

@@ -48,3 +48,25 @@ def prepare(tmp_path: Path):
 
 # meh
 from my.core.error import test_sort_res_by
+
+
+from typing import Iterable, List
+import warnings
+from my.core import warn_if_empty
+def test_warn_if_empty():
+
+    @warn_if_empty
+    def nonempty() -> Iterable[str]:
+        yield 'a'
+        yield 'aba'
+
+    @warn_if_empty
+    def empty(arg: str) -> List[str]:
+        return []
+
+    with warnings.catch_warnings(record=True) as w:
+        assert list(nonempty()) == ['a', 'aba']
+        assert len(w) == 0
+
+        assert empty('whatever') == []
+        assert len(w) == 1