Fix for nested errors are not correctly resolved

When an exception is thrown within a WCI, the last frame is used as
context to add the input lines. This does not work if the exception is
thrown within a function within WCI. Therefore we now iterate through
all traceback frames to find the one corresponding to WCI.

The corresponding PR in WCI osscar-org/widget-code-input#26
to solve it there.

src/scwidgets/code/_widget_code_input.py

@@ -1,9 +1,18 @@
 import inspect
 import re
+import sys
+import traceback
 import types
+import warnings
+from functools import wraps
 from typing import List, Optional
 
 from widget_code_input import WidgetCodeInput
+from widget_code_input.utils import (
+    CodeValidationError,
+    format_syntax_error_msg,
+    is_valid_variable_name,
+)
 
 from ..check import Check
 
@@ -127,3 +136,104 @@ def get_code(func: types.FunctionType) -> str:
             )
 
         return source
+
+    def get_function_object(self):
+        """
+        Return the compiled function object.
+
+        This can be assigned to a variable and then called, for instance::
+
+          func = widget.get_function_object() # This can raise a SyntaxError
+          retval = func(parameters)
+
+        :raise SyntaxError: if the function code has syntax errors (or if
+          the function name is not a valid identifier)
+        """
+        globals_dict = {
+            "__builtins__": globals()["__builtins__"],
+            "__name__": "__main__",
+            "__doc__": None,
+            "__package__": None,
+        }
+
+        if not is_valid_variable_name(self.function_name):
+            raise SyntaxError("Invalid function name '{}'".format(self.function_name))
+
+        # Optionally one could do a ast.parse here already, to check syntax
+        # before execution
+        try:
+            exec(
+                compile(self.full_function_code, __name__, "exec", dont_inherit=True),
+                globals_dict,
+            )
+        except SyntaxError as exc:
+            raise CodeValidationError(
+                format_syntax_error_msg(exc), orig_exc=exc
+            ) from exc
+
+        function_object = globals_dict[self.function_name]
+
+        def catch_exceptions(func):
+            @wraps(func)
+            def wrapper(*args, **kwargs):
+                """Wrap and check exceptions to return a longer and clearer
+                exception."""
+
+                try:
+                    return func(*args, **kwargs)
+                except Exception as exc:
+                    err_msg = format_generic_error_msg(exc, code_widget=self)
+                    raise CodeValidationError(err_msg, orig_exc=exc) from exc
+
+            return wrapper
+
+        return catch_exceptions(function_object)
+
+
+# Temporary fix until https://github.com/osscar-org/widget-code-input/pull/26
+# is merged
+def format_generic_error_msg(exc, code_widget):
+    """
+    Return a string reproducing the traceback of a typical error.
+    This includes line numbers, as well as neighboring lines.
+
+    It will require also the code_widget instance, to get the actual source code.
+
+    :note: this must be called from withou the exception, as it will get the
+        current traceback state.
+
+    :param exc: The exception that is being processed.
+    :param code_widget: the instance of the code widget with the code that
+        raised the exception.
+    """
+    error_class, _, tb = sys.exc_info()
+    frame_summaries = traceback.extract_tb(tb)
+    # The correct frame summary corresponding to widget_code_intput is not
+    # always at the end therefore we loop through all of them
+    wci_frame_summary = None
+    for frame_summary in frame_summaries:
+        if frame_summary.filename == "widget_code_input":
+            wci_frame_summary = frame_summary
+    if wci_frame_summary is None:
+        warnings.warn(
+            "Could not find traceback frame corresponding to "
+            "widget_code_input, we output whole error message.",
+            stacklevel=2,
+        )
+
+        return exc
+    line_number = wci_frame_summary[1]
+    code_lines = code_widget.full_function_code.splitlines()
+
+    err_msg = f"{error_class.__name__} in code input: {str(exc)}\n"
+    if line_number > 2:
+        err_msg += f"     {line_number - 2:4d} {code_lines[line_number - 3]}\n"
+    if line_number > 1:
+        err_msg += f"     {line_number - 1:4d} {code_lines[line_number - 2]}\n"
+    err_msg += f"---> {line_number:4d} {code_lines[line_number - 1]}\n"
+    if line_number < len(code_lines):
+        err_msg += f"     {line_number + 1:4d} {code_lines[line_number]}\n"
+    if line_number < len(code_lines) - 1:
+        err_msg += f"     {line_number + 2:4d} {code_lines[line_number + 1]}\n"
+
+    return err_msg