std: use SIGWINCH signal to update the size

SIGWINCH is a signal send to the process when the host terminal is
resized, note that it's only available on Unix platforms. Catching this
signal allows us to dynamically adapt the number of columns at a very
low cost compared to `dynamic_ncols`. Indeed, as this approach is signal
based, there is no need to poll the terminal size on each iteration.

tqdm/std.py

@@ -7,6 +7,7 @@
 >>> for i in trange(10):
 ...     ...
 """
+import signal
 import sys
 from collections import OrderedDict, defaultdict
 from contextlib import contextmanager
@@ -367,6 +368,8 @@ class tqdm(Comparable):
     monitor = None
     _instances = WeakSet()
 
+    registered_classes = set()
+
     @staticmethod
     def format_sizeof(num, suffix='', divisor=1000):
         """
@@ -665,6 +668,7 @@ def format_meter(n, total, elapsed, ncols=None, prefix='', ascii=False, unit='it
 
     def __new__(cls, *_, **__):
         instance = object.__new__(cls)
+        tqdm.registered_classes.add(cls)
         with cls.get_lock():  # also constructs lock if non-existent
             cls._instances.add(instance)
             # create monitoring thread
@@ -1008,17 +1012,18 @@ def __init__(self, iterable=None, desc=None, total=None, leave=True, file=None,
                 TqdmKeyError("Unknown argument(s): " + str(kwargs)))
 
         # Preprocess the arguments
+        keep_original_size = ncols is not None, nrows is not None
+        force_dynamic_ncols_update = dynamic_ncols
         if (
             (ncols is None or nrows is None) and (file in (sys.stderr, sys.stdout))
-        ) or dynamic_ncols:  # pragma: no cover
-            if dynamic_ncols:
-                dynamic_ncols = _screen_shape_wrapper()
-                if dynamic_ncols:
-                    ncols, nrows = dynamic_ncols(file)
+        ) or force_dynamic_ncols_update:  # pragma: no cover
+            dynamic_ncols = _screen_shape_wrapper()
+            if force_dynamic_ncols_update and dynamic_ncols:
+                keep_original_size = False, False
+                ncols, nrows = dynamic_ncols(file)
             else:
-                _dynamic_ncols = _screen_shape_wrapper()
-                if _dynamic_ncols:
-                    _ncols, _nrows = _dynamic_ncols(file)
+                if dynamic_ncols:
+                    _ncols, _nrows = dynamic_ncols(file)
                     if ncols is None:
                         ncols = _ncols
                     if nrows is None:
@@ -1054,6 +1059,7 @@ def __init__(self, iterable=None, desc=None, total=None, leave=True, file=None,
         self.fp = file
         self.ncols = ncols
         self.nrows = nrows
+        self.keep_original_size = keep_original_size
         self.mininterval = mininterval
         self.maxinterval = maxinterval
         self.miniters = miniters
@@ -1067,6 +1073,7 @@ def __init__(self, iterable=None, desc=None, total=None, leave=True, file=None,
         self.lock_args = lock_args
         self.delay = delay
         self.gui = gui
+        self.force_dynamic_ncols_update = force_dynamic_ncols_update
         self.dynamic_ncols = dynamic_ncols
         self.smoothing = smoothing
         self._ema_dn = EMA(smoothing)
@@ -1450,7 +1457,7 @@ def format_dict(self):
         if self.disable and not hasattr(self, 'unit'):
             return defaultdict(lambda: None, {
                 'n': self.n, 'total': self.total, 'elapsed': 0, 'unit': 'it'})
-        if self.dynamic_ncols:
+        if self.force_dynamic_ncols_update and self.dynamic_ncols:
             self.ncols, self.nrows = self.dynamic_ncols(self.fp)
         return {
             'n': self.n, 'total': self.total,
@@ -1523,3 +1530,21 @@ def wrapattr(cls, stream, method, total=None, bytes=True, **tqdm_kwargs):
 def trange(*args, **kwargs):
     """Shortcut for tqdm(range(*args), **kwargs)."""
     return tqdm(range(*args), **kwargs)
+
+
+def resize_signal_handler(signalnum, frame):
+    for cls in tqdm.registered_classes:
+        with cls.get_lock():
+            for instance in cls._instances:
+                if instance.dynamic_ncols:
+                    ncols, nrows = instance.dynamic_ncols(instance.fp)
+                    if not instance.keep_original_size[0]:
+                        instance.ncols = ncols
+                    if not instance.keep_original_size[1]:
+                        instance.nrows = nrows
+
+
+try:
+    signal.signal(signal.SIGWINCH, resize_signal_handler)
+except AttributeError:
+    pass  # Some systems, like Windows, do not have SIGWINCH