fix(generator): fall back to the login link on cp1252 consoles

Redirected stdout on Windows uses the ANSI code page, which cannot
encode the half-block characters print_ascii emits, so the generator
crashed with UnicodeEncodeError before showing any login path (#150).
Render the QR art only when stdout can encode it, otherwise print the
tg:// login link, and reconfigure stdio with errors=replace so no other
print site (account labels, error text) can abort a login mid-flight.

Author: MohammedAlkindi

session_string_generator.py

@@ -67,6 +67,32 @@ def _check_installation() -> None:
         sys.exit(1)
 
 
+def _stream_can_encode(text: str, stream) -> bool:
+    """True if *stream* can write *text* without raising UnicodeEncodeError."""
+    encoding = getattr(stream, "encoding", None)
+    if not encoding:
+        return True
+    try:
+        text.encode(encoding)
+    except (LookupError, UnicodeEncodeError):
+        return False
+    return True
+
+
+def _harden_stdio() -> None:
+    """Replace unencodable output instead of crashing mid-login.
+
+    On Windows, redirected stdout falls back to the legacy ANSI code page
+    (e.g. cp1252), which cannot encode the QR block characters or non-latin
+    text, so a bare print() raises UnicodeEncodeError and aborts the login.
+    """
+    for stream in (sys.stdout, sys.stderr):
+        try:
+            stream.reconfigure(errors="replace")
+        except (AttributeError, ValueError):
+            pass
+
+
 def _render_qr(qr) -> None:
     import qrcode
 
@@ -77,11 +103,18 @@ def _render_qr(qr) -> None:
     qr_obj.make(fit=True)
     f = io.StringIO()
     qr_obj.print_ascii(out=f, invert=True)
-    print(f.getvalue())
+    art = f.getvalue()
+
+    if _stream_can_encode(art, sys.stdout):
+        print(art)
+        print("Scan the QR code above with your Telegram app:")
+        print("  Open Telegram > Settings > Devices > Link Desktop Device\n")
+        print(f"Or open this link on a device where you're logged in:\n  {qr.url}\n")
+    else:
+        encoding = getattr(sys.stdout, "encoding", None) or "unknown"
+        print(f"This console ({encoding}) cannot draw the QR code.")
+        print(f"Open this link on a device where you're logged in instead:\n  {qr.url}\n")
 
-    print("Scan the QR code above with your Telegram app:")
-    print("  Open Telegram > Settings > Devices > Link Desktop Device\n")
-    print(f"Or open this link on a device where you're logged in:\n  {qr.url}\n")
     print(f"Expires at: {qr.expires.strftime('%H:%M:%S')}")
     print("Waiting for you to scan...")
 
@@ -150,6 +183,7 @@ def _phone_login(client: TelegramClient) -> None:
 
 
 def main() -> None:
+    _harden_stdio()
     args = _parse_args()
     _check_installation()
 

tests/test_session_string_generator.py

@@ -1,4 +1,6 @@
 import asyncio
+import io
+import sys
 from datetime import datetime, timedelta, timezone
 
 import pytest
@@ -162,6 +164,52 @@ def sign_in(self, *args, **kwargs):
     assert client.sign_in_calls == ["secret"]
 
 
+def _bytes_stdout(encoding):
+    """A strict TextIOWrapper standing in for a redirected console stream."""
+    return io.TextIOWrapper(io.BytesIO(), encoding=encoding, errors="strict", write_through=True)
+
+
+def test_render_qr_prints_ascii_art_when_console_can_encode(monkeypatch):
+    qr = _FakeQR([])
+    stdout = _bytes_stdout("utf-8")
+    monkeypatch.setattr(sys, "stdout", stdout)
+
+    session_string_generator._render_qr(qr)
+
+    out = stdout.buffer.getvalue().decode("utf-8")
+    assert "▄" in out  # the half-block characters of the QR art
+    assert qr.url in out
+
+
+def test_render_qr_falls_back_to_link_when_console_cannot_encode(monkeypatch):
+    # Redirected stdout on Windows uses the ANSI code page (cp1252), which
+    # cannot encode the QR block characters and used to crash the generator.
+    qr = _FakeQR([])
+    stdout = _bytes_stdout("cp1252")
+    monkeypatch.setattr(sys, "stdout", stdout)
+
+    session_string_generator._render_qr(qr)
+
+    out = stdout.buffer.getvalue().decode("cp1252")
+    assert "cannot draw the QR code" in out
+    assert qr.url in out
+    assert "Waiting for you to scan..." in out
+
+
+def test_harden_stdio_replaces_unencodable_output_instead_of_crashing(monkeypatch):
+    stdout = _bytes_stdout("cp1252")
+    stderr = _bytes_stdout("cp1252")
+    monkeypatch.setattr(sys, "stdout", stdout)
+    monkeypatch.setattr(sys, "stderr", stderr)
+
+    session_string_generator._harden_stdio()
+    print("session for █Ж█ ready")  # raises under strict cp1252
+
+    out = stdout.buffer.getvalue().decode("cp1252")
+    assert "session for" in out
+    assert "ready" in out
+
+
 def test_parse_args_qr_selects_qr_login(monkeypatch):
     monkeypatch.setattr("sys.argv", ["session_string_generator.py", "--qr"])