document format_dict overriding

- fully:
    + fixes #562, fixes #482, fixes #494, fixes #656

README.rst

@@ -536,6 +536,26 @@ Points to remember when using ``{postfix[...]}`` in the ``bar_format`` string:
   object. To prevent this behaviour, insert an extra item into the dictionary
   where the key is not a string.
 
+Additional ``bar_format`` parameters may also be defined by overriding
+``format_dict``:
+
+.. code:: python
+
+    from tqdm import tqdm
+    class TqdmExtraFormat(tqdm):
+        """Provides a `total_time` format parameter"""
+        @property
+        def format_dict(self):
+            d = super(TqdmExtraFormat, self).format_dict
+            total_time = d["elapsed"] * (d["total"] or 0) / max(d["n"], 1)
+            d.update(total_time=self.format_interval(total_time) + " in total")
+            return d
+
+    for i in TqdmExtraFormat(
+          range(10),
+          bar_format="{total_time}: {percentage:.0f}%|{bar}{r_bar}"):
+        pass
+
 Nested progress bars
 ~~~~~~~~~~~~~~~~~~~~
 

tqdm/tests/tests_tqdm.py

@@ -1033,10 +1033,7 @@ class TqdmExtraFormat(tqdm):
         @property
         def format_dict(self):
             d = super(TqdmExtraFormat, self).format_dict
-            try:
-                total_time = d["elapsed"] * d["total"] / d["n"]
-            except ZeroDivisionError:
-                total_time = 0
+            total_time = d["elapsed"] * (d["total"] or 0) / max(d["n"], 1)
             d.update(total_time=self.format_interval(total_time) + " in total")
             return d