[KevinH]

@BeckyEbook

Here is a proof of concept adapting your approach to using just an environment variable to determine the full absolute path to the desired log file.

diff --git a/src/Resource_Files/python3lib/functionrep.py b/src/Resource_Files/python3lib/functionrep.py
index 3a0aca2ca..d61f85e3a 100644
--- a/src/Resource_Files/python3lib/functionrep.py
+++ b/src/Resource_Files/python3lib/functionrep.py
@@ -1,4 +1,5 @@
 import re
+import os
 
 from fr_utils import unescapeit, toUpper, toLower, swapcase, capitalize
 
@@ -114,3 +115,10 @@ def replace_titlecase_ignore_tags(match, number, file_name, metadata, data, *arg
 def replace_swapcase_ignore_tags(match, number, file_name, metadata, data, *args, **kwargs):
     '''Swap the case of the matched text, ignoring the text inside tag definitions.'''
     return apply_func_to_html_text(match, swapcase)
+
+
+def replace_debug_log(message):
+    logfile = os.environ.get('SIGIL_FUNCTION_REPLACE_LOG_FILE', None);
+    if logfile:
+        with open(logfile, "a", encoding="utf-8") as f:
+            f.write(message)
diff --git a/src/Resource_Files/python3lib/functionsearch.py b/src/Resource_Files/python3lib/functionsearch.py
index 958b0787f..104211341 100644
--- a/src/Resource_Files/python3lib/functionsearch.py
+++ b/src/Resource_Files/python3lib/functionsearch.py
@@ -124,6 +124,7 @@ class FunctionSearch(object):
         global replace_capitalize_ignore_tags
         global replace_titlecase_ignore_tags
         global replace_swapcase_ignore_tags
+        global replace_debug_log
 
         self.metadataxml = metadataxml
         self.function_name = function_name
@@ -157,6 +158,7 @@ class FunctionSearch(object):
             try:
                 result=self.replace(match,self.number,self.bookpath,self.metadataxml,self.funcData)
             except Exception as e:
+                replace_debug_log(str(e))
                 print(e)
             return result
 
@@ -175,6 +177,7 @@ class FunctionSearch(object):
         try:
             result=self.replace(match,self.number,self.bookpath,self.metadataxml,self.funcData)
         except Exception as e:
+            replace_debug_log(str(e))
             print(e)
         return result

Then to use it inside your replace function first set the environment variable:

export SIGIL_FUNCTION_REPLACE_LOG_FILE=/Users/kbhend/Desktop/junk.txt

Then in your replace routine you can now use the replace_debug_log routine.

def replace(match, number, file_name, metadata, data):
    if match:
        replace_debug_log("I am here with number " + str(number) + "\n")
        return match.group(0)

Please give this a try in your own build of Sigil. If that works, I will try to refine this approach using redirecting of sys.stdout and sys.stderr behind the scenes if I can.