adding indicative error message to union decoding

pyrallis/parsers/decoding.py

@@ -18,7 +18,8 @@
     is_enum,
     ParsingError,
     format_error,
-    has_generic_arg
+    has_generic_arg,
+    add_tab_to_new_lines
 )
 
 logger = getLogger(__name__)
@@ -195,19 +196,31 @@ def _decode_optional(val: Optional[Any]) -> Optional[T]:
     return _decode_optional
 
 
-def try_functions(*funcs: Callable[[Any], T]) -> Callable[[Any], Union[T, Any]]:
+def try_decoding_functions(funcs: List[Callable[[Any], T]],
+                           types: List[T]) -> Callable[[Any], Union[T, Any]]:
     """Tries to use the functions in succession, else returns the same value unchanged."""
 
-    def _try_functions(val: Any) -> Union[T, Any]:
-        for func in funcs:
+    assert len(funcs) == len(types), 'Each decoding function must have a unique type.'
+
+    def _try_decoding_functions(val: Any) -> Union[T, Any]:
+        error_messages: List[str] = []
+        for func, t in zip(funcs, types):
             try:
                 return func(val)
-            except Exception:
+            except Exception as e:
+                error_message = f"-> Failed to parse as class {t.__name__}: {add_tab_to_new_lines(format_error(e))}"
+                error_messages.append(error_message)
                 continue
         # If no function worked, raise an exception
-        raise TypeError(f"No valid parsing for value {val}")
+        class_names = [t.__name__ for t in types]
+        exception_message = f"Failed to parse value for multiple classes: " \
+                            f"value={val}, classes={class_names}.\n" \
+                            f"Got parsing errors:"
+        for error_message in error_messages:
+            exception_message += f'\n{error_message}'
+        raise TypeError(add_tab_to_new_lines(exception_message))
 
-    return _try_functions
+    return _try_decoding_functions
 
 
 def decode_union(*types: Type[T]) -> Callable[[Any], Union[T, Any]]:
@@ -217,11 +230,14 @@ def decode_union(*types: Type[T]) -> Callable[[Any], Union[T, Any]]:
     while type(None) in types:
         types.remove(type(None))
 
+    if len(types) == 1:  # There is a single not None class
+        return decode_optional(types[0])
+
     decoding_fns: List[Callable[[Any], T]] = [
         decode_optional(t) if optional else get_decoding_fn(t) for t in types
     ]
     # Try using each of the non-None types, in succession. Worst case, return the value.
-    return try_functions(*decoding_fns)
+    return try_decoding_functions(funcs=decoding_fns, types=types)
 
 
 def decode_list(t: Type[T]) -> Callable[[List[Any]], List[T]]:
@@ -314,11 +330,10 @@ def no_op(v: T) -> T:
 
 def try_constructor(t: Type[T]) -> Callable[[Any], Union[T, Any]]:
     """ Tries to use the type as a constructor. If that fails, returns the value as-is. """
-    return try_functions(lambda val: t(**val), lambda val: t(val))
+    funcs = [lambda val: t(**val), lambda val: t(val)]
+    return try_decoding_functions(funcs=funcs, types=[t, t])
 
 
 from pathlib import Path
 
 decode.register(Path, Path)
-
-

pyrallis/utils.py

@@ -298,6 +298,10 @@ def format_error(e: Exception):
         return f'Exception: {e}'
 
 
+def add_tab_to_new_lines(text: str):
+    return text.replace('\n', '\n\t')
+
+
 def is_generic_arg(arg):
     try:
         return arg.__name__ in ['KT', 'VT', 'T']

tests/test_decoding.py

@@ -3,6 +3,7 @@
 
 import yaml
 import json
+import toml
 
 from pyrallis.utils import PyrallisException
 from .testutils import *
@@ -49,7 +50,7 @@ class SomeClass:
     new_b = pyrallis.parse(config_class=SomeClass, config_path=tmp_file, args="")
     assert new_b == b
 
-    arguments = shlex.split(f"--config_path {tmp_file}")
+    arguments = ['--config_path', str(tmp_file)]
     new_b = pyrallis.parse(config_class=SomeClass, args=arguments)
     assert new_b == b
 
@@ -98,7 +99,7 @@ class SomeClass:
 
     new_b = pyrallis.parse(config_class=SomeClass, config_path=tmp_file, args="")
     assert new_b == b
-    arguments = shlex.split(f"--config_path {tmp_file}")
+    arguments = ['--config_path', str(tmp_file)]
     new_b = pyrallis.parse(config_class=SomeClass, args=arguments)
     assert new_b == b