Prepare for Python interviews

is = identity (same object); == = value equality via __eq__. Use is only for singletons like None.

__new__ creates and returns the instance; __init__ initialises the already-created instance. __new__ matters for immutables and singletons.

A mutex letting one thread run CPython bytecode at a time, protecting reference counts. Limits CPU-bound threads, not I/O.

Reference counting frees objects at count 0; a generational GC collects cycles; pymalloc handles small objects.

Shallow copies the container but shares nested objects; deep recursively copies everything (copy.deepcopy).

Defaults are evaluated once, so a mutable default is shared across calls. Use None and create inside.

An inner function captures enclosing variables via shared cell objects that outlive the outer call.

Closures read captured variables at call time, not definition time — the loop-lambda gotcha.

Name lookup order: Local → Enclosing → Global → Built-in.

@dec rebinds name = dec(func); the wrapper (a closure) adds behaviour around the call.

yield suspends the frame, keeping locals and the instruction pointer alive until the next next().

Delegates iteration (and send/throw) to a sub-generator, flattening nested generators.

__enter__ sets up, __exit__ tears down even on exception; with guarantees cleanup.

C3 linearisation orders base classes; super() follows the MRO, enabling cooperative multiple inheritance.

Objects with __get__/__set__ that control attribute access — the machinery behind property and methods.

The class of a class; type is the default. They customise class creation (registries, validation).

Coroutines suspend at await and yield to a loop that runs other ready tasks and resumes them on I/O completion.

Threads: shared memory, GIL-limited CPU (good for I/O). Processes: own GIL/memory, true CPU parallelism.

Defining __call__ — instances then work like functions.

Keys hash to buckets for average O(1) access; collisions resolved by probing + equality.

The hash locates the bucket; a changing hash would lose the entry, so keys must be immutable/hashable.

Layered packages (router/service/repository/domain), config at the edges, DI, a clean public API.