Add pushback iter.

diff --git a/iter_utils.py b/iter_utils.py
index 977cf1df2b7e103f140757a71316339522d6c3ff..b1a137c33a7c83c0c221492511240b803a1684ad 100644 (file)
--- a/iter_utils.py
+++ b/iter_utils.py
@@ -54,6 +54,48 @@ class PeekingIterator(Iterator):
             return None
 
 
+class PushbackIterator(Iterator):
+    """An iterator that allows you to push items back
+    onto the front of the sequence.  e.g.
+
+    >>> i = PushbackIterator(iter(range(3)))
+    >>> i.__next__()
+    0
+    >>> i.push_back(99)
+    >>> i.push_back(98)
+    >>> i.__next__()
+    98
+    >>> i.__next__()
+    99
+    >>> i.__next__()
+    1
+    >>> i.__next__()
+    2
+    >>> i.push_back(100)
+    >>> i.__next__()
+    100
+    >>> i.__next__()
+    Traceback (most recent call last):
+      ...
+    StopIteration
+    """
+
+    def __init__(self, source_iter: Iterator):
+        self.source_iter = source_iter
+        self.pushed_back: List[Any] = []
+
+    def __iter__(self) -> Iterator:
+        return self
+
+    def __next__(self) -> Any:
+        if len(self.pushed_back):
+            return self.pushed_back.pop()
+        return self.source_iter.__next__()
+
+    def push_back(self, item: Any):
+        self.pushed_back.append(item)
+
+
 class SamplingIterator(Iterator):
     """An iterator that simply echoes what source_iter produces but also
     collects a random sample (of size sample_size) of the stream that can