Issue #20058: sys.stdin.readline() in IDLE now always returns only one line.

Lib/idlelib/PyShell.py

@@ -1393,6 +1393,9 @@ class PseudoInputFile(PseudoFile):
         line = self._line_buffer or self.shell.readline()
         if size < 0:
             size = len(line)
+        eol = line.find('\n', 0, size)
+        if eol >= 0:
+            size = eol + 1
         self._line_buffer = line[size:]
         return line[:size]
 

Misc/NEWS

@@ -118,6 +118,8 @@ Library
 IDLE
 ----
 
+- Issue #20058: sys.stdin.readline() in IDLE now always returns only one line.
+
 - Issue #19481: print() of unicode, str or bytearray subclass instance in IDLE
   no more hangs.