From c2b7b02c0b5bb132eb3a4110f53a99a723aabd84 Mon Sep 17 00:00:00 2001
From: Paulo Costa <me@paulo.costa.nom.br>
Date: Thu, 19 Sep 2019 02:44:48 -0300
Subject: [PATCH] Avoid flickering

StdRedirectMixin makes the bar erase itself before writing the new update.
This works fine and is invisible most of the time.

Most of the time -- If the refreshes are very fast, flickering becomes visible

This PR first checks if there is something buffered to be written in stdout/stderr, and doesn't erase the current line otherwise
---
 examples.py          | 13 +++++++++++++
 progressbar/bar.py   |  7 ++++---
 progressbar/utils.py | 12 ++++++++++++
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/examples.py b/examples.py
index 05099efc..cf8b7e51 100644
--- a/examples.py
+++ b/examples.py
@@ -31,6 +31,19 @@ def wrapped():
     return wrapped
 
 
+@example
+def fast_example():
+    ''' Updates bar really quickly to cause flickering '''
+    with progressbar.ProgressBar(max_value=1, widgets=[progressbar.Bar()]) as bar:
+        start = time.time()
+        while True:
+            elapsed = time.time() - start
+            if elapsed > 1:
+                break
+            else:
+                bar.update(elapsed, force=True)
+
+
 @example
 def shortcut_example():
     for i in progressbar.progressbar(range(10)):
diff --git a/progressbar/bar.py b/progressbar/bar.py
index fe945282..1cfcf4af 100644
--- a/progressbar/bar.py
+++ b/progressbar/bar.py
@@ -180,9 +180,10 @@ def start(self, *args, **kwargs):
         DefaultFdMixin.start(self, *args, **kwargs)
 
     def update(self, value=None):
-        if not self.line_breaks:
-            self.fd.write('\r' + ' ' * self.term_width + '\r')
-        utils.streams.flush()
+        if utils.streams.needs_flush():
+            if not self.line_breaks:
+                self.fd.write('\r' + ' ' * self.term_width + '\r')
+            utils.streams.flush()
         DefaultFdMixin.update(self, value=value)
 
     def finish(self, end='\n'):
diff --git a/progressbar/utils.py b/progressbar/utils.py
index aba7477c..aa130cab 100644
--- a/progressbar/utils.py
+++ b/progressbar/utils.py
@@ -135,6 +135,9 @@ def write(self, value):
     def flush(self):
         self.buffer.flush()
 
+    def _needs_flush(self):
+        return bool(self.buffer.getvalue())
+
     def _flush(self):
         value = self.buffer.getvalue()
         if value:
@@ -249,6 +252,15 @@ def unwrap_stderr(self):
             sys.stderr = self.original_stderr
             self.wrapped_stderr = 0
 
+    def needs_flush(self):
+        if self.wrapped_stdout:
+            if self.stdout._needs_flush():
+                return True
+        if self.wrapped_stderr:
+            if self.stderr._needs_flush():
+              return True
+        return False
+
     def flush(self):
         if self.wrapped_stdout:  # pragma: no branch
             try: