From 2d793336ff3b6f4b4f9139cd0f2045e9d92f6861 Mon Sep 17 00:00:00 2001
From: Alce Ops <alceops@users.noreply.github.com>
Date: Tue, 28 Apr 2026 14:18:45 +0000
Subject: [PATCH] fix: parse tracebacks after stderr prefixes

---
 index.ts                                | 11 +++++++----
 test/python/error_with_stderr_prefix.py | 10 ++++++++++
 test/test-python-shell.ts               |  9 +++++++++
 3 files changed, 26 insertions(+), 4 deletions(-)
 create mode 100644 test/python/error_with_stderr_prefix.py

diff --git a/index.ts b/index.ts
index 9569354..4a8bd1e 100644
--- a/index.ts
+++ b/index.ts
@@ -395,13 +395,16 @@ export class PythonShell extends EventEmitter {
   private parseError(data: string | Buffer) {
     let text = '' + data;
     let error: PythonShellError;
+    const tracebackHeader = 'Traceback (most recent call last):';
+    const tracebackStart = text.indexOf(tracebackHeader);
 
-    if (/^Traceback/.test(text)) {
-      // traceback data is available
-      let lines = text.trim().split(newline);
+    if (tracebackStart >= 0) {
+      // Traceback can be prefixed by stderr logs, so parse from the traceback header.
+      const tracebackText = text.slice(tracebackStart).trim();
+      let lines = tracebackText.split(newline);
       let exception = lines.pop();
       error = new PythonShellError(exception);
-      error.traceback = data;
+      error.traceback = tracebackText;
       // extend stack trace
       error.stack +=
         newline + '    ----- Python Traceback -----' + newline + '  ';
diff --git a/test/python/error_with_stderr_prefix.py b/test/python/error_with_stderr_prefix.py
new file mode 100644
index 0000000..457d543
--- /dev/null
+++ b/test/python/error_with_stderr_prefix.py
@@ -0,0 +1,10 @@
+import sys
+
+sys.stderr.write('prefix log before traceback\n')
+
+
+def fail():
+    raise Exception('Error sample')
+
+
+fail()
diff --git a/test/test-python-shell.ts b/test/test-python-shell.ts
index 3d5de81..bd7c8bf 100644
--- a/test/test-python-shell.ts
+++ b/test/test-python-shell.ts
@@ -614,6 +614,15 @@ describe('PythonShell', function () {
         done();
       });
     });
+    it('should parse traceback when stderr has prefixed logs', function (done) {
+      let pyshell = new PythonShell('error_with_stderr_prefix.py');
+      pyshell.on('pythonError', function (err) {
+        err.message.should.be.equal('Exception: Error sample');
+        err.should.have.property('traceback');
+        err.traceback.should.containEql('Traceback (most recent call last)');
+        done();
+      });
+    });
   });
 
   describe('.kill()', function () {