put-back-iterator

A drop-in replacement for iter(iterable) that allows putting items back after consuming them. Ideal for parsing, lexing, and other scenarios requiring lookahead or backtracking.

Why Use This?

Python's built-in iterators don't support pushing items back, forcing workarounds like:

• Rebuilding iterators with itertools.chain
• Using lists/deques as buffers
• Complex state tracking

This package solves it with:

• ✅ Simple API: .put_back(item) and .has_next()
• ✅ Compatibility with iter(iterable): Works with any iterable (str, list, generators, etc.)
• ✅ Python 2/3 compatible

Quick Start

# coding=utf-8
from __future__ import print_function
from put_back_iterator import PutBackIterator

# Wrap any iterable
it = PutBackIterator([1, 2, 3])

# Consume an item
print(next(it))  # 1

# Put it back for later
it.put_back(1)

# Check if more items exist (without consuming)
print(it.has_next())  # True

# Iterate as usual
print(list(it))  # [1, 2, 3]

Use Cases

1. Parsing/Tokenizing

tokens = PutBackIterator(tokenize(source_code))
while tokens.has_next():
    token = next(tokens)
    if token == 'IF':
        # Peek ahead without consuming
        if tokens.has_next() and next(tokens) == 'EOF':
            tokens.put_back('EOF')
            handle_incomplete_if()

2. Backtracking Algorithms

def backtrack(it):
    item = next(it)
    if not is_valid(item):
        it.put_back(item)  # Try alternative path
        return fallback()

3. Stream Processing

for chunk in PutBackIterator(stream):
    if needs_retry(chunk):
        chunk = modify(chunk)
        it.put_back(chunk)  # Reprocess

API Reference

PutBackIterator(iterable) Wraps an iterable.,Compatible with iter(iterable).

Methods

• .put_back(item: T) -> None Pushes an item back into the iterator (LIFO order).
• .has_next() -> bool Returns True if more items exist (peeks without consuming).
• .__iter__() and .__next__() / .next() Compatible with Python 2/3 iterator protocol.

Contributing

Contributions are welcome! Please submit pull requests or open issues on the GitHub repository.

License

This project is licensed under the MIT License.