From efeb55ba367a01118cd50c9b70ac1595c0402ef7 Mon Sep 17 00:00:00 2001
From: Aditya Jain <adityaj0714@gmail.com>
Date: Wed, 19 Aug 2026 23:39:36 -0700
Subject: [PATCH] Fix UnboundLocalError in
 ConfigFileWriter._update_subattributes for empty nested sections

current_indent (and the loop index i) were only assigned when a
sub-option line matched OPTION_REGEX. Updating a nested section that
currently has no sub-keys (e.g. `s3 =` immediately followed by another
[section] header, or as the last line of the file) left these
variables unbound, raising UnboundLocalError instead of writing the
value.

Initialize current_indent to None and i to the pre-loop index so both
previously-crashing cases insert the new values correctly, matching
the behavior of the equivalent non-empty-section cases.

Fixes #10587
---
 .../next-release/bugfix-configure-10587.json  |  5 ++++
 awscli/customizations/configure/writer.py     |  2 ++
 .../customizations/configure/test_writer.py   | 28 +++++++++++++++++++
 3 files changed, 35 insertions(+)
 create mode 100644 .changes/next-release/bugfix-configure-10587.json

diff --git a/.changes/next-release/bugfix-configure-10587.json b/.changes/next-release/bugfix-configure-10587.json
new file mode 100644
index 000000000000..5497115a7c46
--- /dev/null
+++ b/.changes/next-release/bugfix-configure-10587.json
@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "configure",
+  "description": "Fixed an `UnboundLocalError` raised by `aws configure set` when updating a nested config section (e.g. `s3`, `s3api`) that currently has no sub-keys under it."
+}
diff --git a/awscli/customizations/configure/writer.py b/awscli/customizations/configure/writer.py
index 83825a53dab6..6b5f4be7c315 100644
--- a/awscli/customizations/configure/writer.py
+++ b/awscli/customizations/configure/writer.py
@@ -215,6 +215,8 @@ def _update_section_contents(self, contents, section_name, new_values):
 
     def _update_subattributes(self, index, contents, values, starting_indent):
         index += 1
+        current_indent = None
+        i = index - 1
         for i in range(index, len(contents)):
             line = contents[i]
             match = self.OPTION_REGEX.search(line)
diff --git a/tests/unit/customizations/configure/test_writer.py b/tests/unit/customizations/configure/test_writer.py
index 9c85516762ef..cf662ab33299 100644
--- a/tests/unit/customizations/configure/test_writer.py
+++ b/tests/unit/customizations/configure/test_writer.py
@@ -326,6 +326,34 @@ def test_updated_nested_attribute_new_section(self):
             '[profile foo]\n'
             'foo = bar\n')
 
+    def test_add_to_empty_nested_stanza_followed_by_section(self):
+        original = (
+            '[default]\n'
+            's3 =\n'
+            '[profile foo]\n'
+            'foo = bar\n'
+        )
+        self.assert_update_config(
+            original, {'__section__': 'default',
+                       's3': {'signature_version': 'newval'}},
+            '[default]\n'
+            's3 =\n'
+            '    signature_version = newval\n'
+            '[profile foo]\n'
+            'foo = bar\n')
+
+    def test_add_to_empty_nested_stanza_at_eof(self):
+        original = (
+            '[default]\n'
+            's3 =\n'
+        )
+        self.assert_update_config(
+            original, {'__section__': 'default',
+                       's3': {'signature_version': 'newval'}},
+            '[default]\n'
+            's3 =\n'
+            '    signature_version = newval\n')
+
     def test_update_nested_attr_no_prior_nesting(self):
         original = (
             '[default]\n'