What does the functools module offer? | Interview Question

Q: What does the functools module offer?

**Target Audience:** Python developers who write decorators, sort objects or cache expensive calls; candidates asked the open-ended "what is in functools?". **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3). > **How to read this doc:** Each idea is explained in plain words first, then tagged with `📌 Interview term:` — that is the exact vocabulary an interviewer expects to hear. Every output shown was printed by a real run; the **Verified** section quotes it word for word. ## 1. Why This Even Matters — A Story First A kitchen has its chefs (your functions) and its gadgets: a pre-set timer you only need to start (partial), a label maker that puts the right name on every container (wraps), a fridge that keeps yesterday's sauce so you do not cook it again (the caches), and a sorting station that sends each delivery to the right chef depending on what it is (single dispatch). None of the gadgets cooks

The problem

Question presented to candidate: "Tell me about functools. Which parts of it do you actually use, and what problem does each one solve?"

What a strong answer should cover:

functools holds higher-order functions: tools that take functions and return functions (or callable objects), for adapting, caching and dispatching.
Adapting: partial / partialmethod freeze arguments; wraps (via update_wrapper) copies __name__, __doc__ and sets __wrapped__ on decorator wrappers; reduce folds a sequence; cmp_to_key adapts old compare functions for sorted.
Caching: lru_cache(maxsize=...) memoises with LRU eviction and cache_info(); cache (3.9) is the unbounded version; cached_property (3.8) computes once per instance and stores the result in the instance __dict__.
Classes and dispatch: total_ordering fills in missing comparison methods from __eq__ plus one of them; singledispatch / singledispatchmethod choose an implementation by the type of the first argument.
Trade-offs: cached arguments must be hashable, caches keep references alive (memory leaks on methods), cached_property needs a __dict__, partial is preferable to a lambda because it is introspectable and picklable.

Code / implementation expected: Optional — a partial, a wraps-based decorator, or an lru_cache with cache_info().

functools

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Solution

Target Audience: Python developers who write decorators, sort objects or cache expensive calls; candidates asked the open-ended "what is in functools?". Difficulty: Medium Python version: Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3).

How to read this doc: Each idea is explained in plain words first, then tagged with 📌 Interview term: — that is the exact vocabulary an interviewer expects to hear. Every output shown was printed by a real run; the Verified section quotes it word for word.

1. Why This Even Matters — A Story First

A kitchen has its chefs (your functions) and its gadgets: a pre-set timer you only need to start (partial), a label maker that puts the right name on every container (wraps), a fridge that keeps yesterday's sauce so you do not cook it again (the caches), and a sorting station that sends e

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11 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

Pythonpartial, partialmethod, reduce, wraps, total_ordering and cmp_to_key

Practice

Pythonlru_cache, cache, cached_property and singledispatch

Practice

Discussion

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