allow using module-function-combinations as dynamic field defaults

src/yamlhttpforms/form.py

@@ -1,4 +1,5 @@
 from pathlib import Path
+from importlib import import_module
 from typing import Dict, Callable, Union, Optional, Any, TYPE_CHECKING
 
 from yaml import safe_load
@@ -9,14 +10,26 @@ if TYPE_CHECKING:
 
 PathT = Union[Path, str]
 CallableDefaultT = Callable[[], Optional[str]]
-DefaultValueT = Union[str, CallableDefaultT]
+DefaultInitT = Union[str, Dict[str, str]]
 OptionsT = Dict[str, str]
 
 
 class FormField:
-    def __init__(self, name: str, default: DefaultValueT = None, options: OptionsT = None, required: bool = False):
+    def __init__(self, name: str, default: DefaultInitT = None, options: OptionsT = None, required: bool = False):
         self.name: str = name
-        self._default: Optional[DefaultValueT] = default
+        self._default: Union[str, CallableDefaultT, None]
+        if isinstance(default, dict):
+            try:
+                module, function = default['module'], default['function']
+            except KeyError:
+                raise TypeError(f"Default for field '{name}' is invalid. The default must be either a string or a "
+                                f"dictionary with the special keys 'module' and 'function'.")
+            obj = import_module(module)
+            for attr in function.split('.'):
+                obj = getattr(obj, attr)
+            self._default = obj
+        else:
+            self._default = default
         self.options: Optional[OptionsT] = options
         self.required: bool = required
 
@@ -215,3 +228,6 @@ def load_form(*def_paths: PathT, dir_sort_key: Callable[[PathT], Any] = None, di
         full_payload=full_payload,
         url=url
     )
+
+form = load_form('../../foo/a.yaml')
+print(form.get_payload(mandatory_field='a'))