make sure to flush stdout/stderr when calling readline()/raw_input(). It helps to mitigates this pytest issue:

pytest-dev/pytest#5134

Prior to this fix, pdb++ failed to display the output when you run it under
the conditions described in the issue above.

Author: antocuni

pyrepl/readline.py

@@ -181,11 +181,19 @@ class _ReadlineWrapper(object):
     saved_history_length = -1
     startup_hook = None
     config = ReadlineConfig()
+    stdin = None
+    stdout = None
+    stderr = None
 
     def __init__(self, f_in=None, f_out=None):
         self.f_in = f_in if f_in is not None else os.dup(0)
         self.f_out = f_out if f_out is not None else os.dup(1)
 
+    def setup_std_streams(self, stdin, stdout, stderr):
+        self.stdin = stdin
+        self.stdout = stdout
+        self.stderr = stderr
+
     def get_reader(self):
         if self.reader is None:
             console = UnixConsole(self.f_in, self.f_out, encoding=ENCODING)
@@ -198,7 +206,18 @@ def raw_input(self, prompt=''):
             reader = self.get_reader()
         except _error:
             return _old_raw_input(prompt)
-        reader.ps1 = prompt
+
+        # the builtin raw_input calls PyOS_StdioReadline, which flushes
+        # stdout/stderr before displaying the prompt. Try to mimic this
+        # behavior: it seems to be the correct thing to do, and moreover it
+        # mitigates this pytest issue:
+        # https://github.com/pytest-dev/pytest/issues/5134
+        if self.stdout and hasattr(self.stdout, 'flush'):
+            self.stdout.flush()
+        if self.stderr and hasattr(self.stderr, 'flush'):
+            self.stderr.flush()
+
+        reader.ps1 = prompt            
         return reader.readline(startup_hook=self.startup_hook)
 
     def multiline_input(self, more_lines, ps1, ps2, returns_unicode=False):
@@ -405,6 +424,7 @@ def _setup():
 
     _wrapper.f_in = f_in
     _wrapper.f_out = f_out
+    _wrapper.setup_std_streams(sys.stdin, sys.stdout, sys.stderr)
 
     if '__pypy__' in sys.builtin_module_names:    # PyPy