Merge existing sorted iterables without first joining every item into one list.
What You Will Learn
heapq.merge combines already-sorted streams lazily, making it suitable for ordered logs and chunked files. It does not sort each input for you. If you ask for every result and store them in a list, the result list still uses memory proportional to all items.
Time complexity assumes a specific input structure. Include setup work such as sorting or building a lookup table when comparing complete solutions.
Step-by-Step Example
The example below makes the behavior visible with small input values that you can change safely.
from heapq import merge
from itertools import islice
east = [1, 5, 9, 14]
west = [2, 4, 8, 16]
combined = merge(east, west)
print(list(islice(combined, 5)))How the Example Works
- The imports provide the standard-library tools used in the demonstration: heapq, itertools.
- The variable east holds the result of [1, 5, 9, 14]. Later lines use that value or inspect how it changes.
- The variable west holds the result of [2, 4, 8, 16]. Later lines use that value or inspect how it changes.
- The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.
For the chosen input, the expression list(islice(combined, 5)) prints [1, 2, 4, 5, 8]. Compare that result with the rule above before you change the argument.
Expected output from the example:
[1, 2, 4, 5, 8]When You Would Use This
This technique matters when a program handles larger queues, sorted records, or repeated lookups. Choose the data structure based on actual operations and the preconditions the algorithm requires. Verify boundary inputs before optimizing for speed.
Practice: Modify the Code
Merge three short timestamp lists and fetch only the earliest four times.
Verify the changed output against the behavior described above, including the relevant invalid or boundary case.
For complete parameters and version details, consult the official Python documentation.
Common Beginner Mistakes
- Feeding unsorted lists to heapq.merge and expecting it to fix their order.
- Using list() on a large merged stream and giving up the streaming memory benefit.
Key Takeaways
- Merge existing sorted iterables without first joining every item into one list.
- heapq.merge combines already-sorted streams lazily, making it suitable for ordered logs and chunked files.
- Apply this operation only after checking the assumptions and input conditions shown in the example.