Programming Fundamentals Questions

Direct answer
A bare except: (or except Exception: with a silent return None) doesn't just handle the error you intended, it catches every error that happens to occur in that block and treats all of them identically, so a genuine bug (wrong type, a typo'd attribute, a logic error) gets misclassified as 'expected failure' and hidden from anyone who could act on it. The fix is to catch only the SPECIFIC exception you actually expect, and make failure visible (log it, re-raise it, or return a value the caller is forced to check) instead of silently returning a value indistinguishable from a normal result.

Structured elaboration

• Why this is dangerous, not just untidy: None is frequently also a valid, meaningful return value elsewhere in the codebase. A caller receiving None from this function cannot tell 'there was no data' from 'something crashed while getting the data', those are very different situations that need very different handling, and the swallowed exception has erased the distinction.
• Why 'catch everything' is worse than it looks: a bare except catches ValueError (probably intended) but ALSO TypeError, AttributeError, even KeyboardInterrupt in some forms, errors that indicate a real bug in the calling code, not a data-quality issue the function was designed to tolerate. Narrowing to the specific expected exception type is what lets a genuine bug surface loudly instead of being absorbed by the same catch-all.
• What replaces it: catch only the exception type(s) you actually expect and know how to handle; log enough context to diagnose it (what input caused it); then either re-raise (if the caller has no way to proceed without this data) or return an explicit, unambiguous sentinel that cannot be confused with a valid result (not bare None if None is otherwise meaningful).
• The review conversation: once the technical fix is clear, raising it with the author is a normal, low-friction code-review comment focused on the concrete failure mode ('this will hide a real TypeError as if it were expected'), not a judgment about the person, that's what keeps the fix landing quickly.

Worked example

def bad_parse(raw):
    try:
        return int(raw)
    except Exception:
        return None            # catches ValueError AND TypeError identically

def good_parse(raw, logger):
    try:
        return int(raw)
    except ValueError:
        logger.warning("could not parse %r as int", raw)
        raise                  # or: return a sentinel the caller is forced to check

Verified: bad_parse(None) returns None silently, giving no signal that int(None) actually raised a TypeError (passing None where a string/number was expected, a real bug at the call site, not a data-quality issue). good_parse(None, logger) instead lets that TypeError propagate uncaught (confirmed: it raises TypeError, not swallowed), because the function only catches ValueError. good_parse("not-a-number", logger) correctly logs a warning and re-raises ValueError (confirmed by execution), a case the function WAS designed to handle, with a visible trail.

Trade-offs & pitfalls
The judgment call is choosing between re-raising and returning a sentinel: re-raise when the caller genuinely cannot proceed without valid data (most cases); return an explicit sentinel only when the caller has a real, intentional fallback path for 'this record was unparseable' and the sentinel can't be confused with a legitimate value. What never belongs in either path is catching a broader exception type than you can actually reason about, that's the pattern that turns a narrow, expected failure mode into a general-purpose bug hiding place.

Direct answer
The stack holds each function call's local variables and control-flow bookkeeping in a strict last-in-first-out region that's automatically reclaimed the instant a function returns; the heap holds data whose lifetime isn't tied to any single function call, and it's reclaimed either manually (unmanaged languages) or by a garbage collector (managed languages).

Structured elaboration

• What lives where: a local primitive variable, or in some languages a fixed-size value type, is allocated on the stack as part of the current function's frame. Anything created with an explicit allocation (new in Java/C++, any Python object, since CPython objects are always heap-allocated even for an int) lives on the heap; the stack only holds a reference/pointer to it.
• Variable lifetime: a stack frame's contents die the moment that function returns, this is why you can't return a pointer to a local stack variable in C and expect it to still be valid. Heap objects live until nothing references them anymore, tracked either by the programmer (manual free/delete), reference counting, or a tracing garbage collector.
• Managed vs unmanaged: in C/C++, forgetting to free heap memory is a leak, and freeing it twice or using it after freeing ("use-after-free") is undefined behavior, a classic source of crashes and security bugs. In managed languages like Java or Python, the heap is reclaimed automatically, which removes that class of bug but introduces its own failure mode: an object that's still reachable (through a lingering reference you forgot about) never gets collected even though you're logically done with it, this looks exactly like a leak from the outside even though nothing is 'wrong' with the GC.
• Common pitfalls: dangling references (using a pointer after its target was freed) and double-free in unmanaged languages; unintentional retention (a cache, a global list, or a closure holding a reference longer than intended) in managed languages, which is the managed-language equivalent of a leak.

Worked example
A function def compute(x): result = x * 2; return result allocates result in its stack frame; that frame disappears the instant compute returns. If instead the function does def compute(x): return [x, x*2], the list object itself lives on the heap, only the reference to it lived momentarily in the stack frame, and the list survives the function return because the caller now holds a reference to it. This is exactly why returning a local list is safe in Python (you're returning a heap reference) while returning a pointer to a local stack array in C is not (you're returning a pointer to memory that's about to be reused by the next function call).

Trade-offs & pitfalls
Stack allocation is fast (just moving a pointer) and has zero collection cost; heap allocation is more flexible (variable size, unpredictable lifetime) but costs more per allocation and, in a managed language, imposes collection work (pause time, throughput cost) somewhere down the line. This is a large part of why some languages let you opt certain data onto the stack explicitly (value types, structs) when you know its lifetime is scoped to the current call, to avoid heap/GC overhead for short-lived data.

Direct answer
try marks a block whose exceptions you want to intercept; except runs only if a matching exception is raised inside the try; finally always runs, whether or not an exception occurred and whether or not it was caught, and it runs even if the exception propagates past this block entirely.

Structured elaboration

• No exception: the try block runs to completion, every except clause is skipped, then finally runs. Nothing unusual happens.
• Exception raised and caught: execution jumps out of the try block the instant the exception is raised (any code after the raising line in try does NOT run), the first matching except clause runs, then finally runs.
• Exception raised and NOT caught (no except clause matches its type): the matching-except search fails, finally still runs (this is the part people get wrong: finally is not conditional on being caught), and only after finally completes does the exception continue propagating up to the caller.
• The consequence that matters in practice: finally is the correct place for resource cleanup that must happen unconditionally, closing a file handle, releasing a lock, rolling back a partially-started operation, precisely because it is the one block guaranteed to run on every exit path from the try.

Worked example

def no_exception():
    order = []
    try:
        order.append('try')
    except ValueError:
        order.append('except')
    finally:
        order.append('finally')
    return order

def caught_exception():
    order = []
    try:
        order.append('try')
        raise ValueError('boom')
    except ValueError:
        order.append('except')
    finally:
        order.append('finally')
    return order

Running both (verified): no_exception() returns ['try', 'finally'] (the except clause never runs), caught_exception() returns ['try', 'except', 'finally']. A third case with a TypeError raised where only ValueError is caught confirms finally still runs before the TypeError propagates out to the caller, exactly as described above.

For the file-I/O case, opening a file and guaranteeing it closes even on a mid-read failure:

def read_lines(path):
    f = open(path, 'r')
    try:
        return f.readlines()
    finally:
        f.close()   # runs whether readlines() succeeds, raises, or the caller's except re-raises

This is exactly what a with open(path) as f: block (or Java's try-with-resources) does under the hood, they are sugar over a try/finally that closes the resource. In this pattern, whether to re-raise the error after logging it or return a sentinel value to the caller depends on whether the caller can meaningfully continue: propagate (let the exception continue, possibly after logging) when the caller cannot proceed without the data; return a sentinel only when the caller has a genuine, documented fallback.

Trade-offs & pitfalls
The most common mechanical mistake is assuming finally only runs when an exception was caught, when in fact it runs on every exit from try (normal completion, caught exception, uncaught exception, even a return inside the try block). The second most common mistake is putting cleanup code after the try/except instead of in finally, which silently skips cleanup on any path that doesn't hit that exact line, exactly the bug that finally exists to prevent.

Direct answer
A closure is a function bundled together with references to the variables from its enclosing scope that it uses, captured by reference (not by value), so it keeps seeing the CURRENT value of those variables even after the enclosing function has returned. That captured reference can create a reference cycle, which is why a closure or callback can keep an object alive longer than you expect.

Structured elaboration

• What gets captured: a closure captures the variable itself (technically, the enclosing scope's cell), not a snapshot of its value at creation time. Two closures created from the same enclosing call share independent state; two closures created from the SAME variable in a loop share the same captured cell, which is the classic 'all my callbacks report the same, final loop value' bug.
• Why closures can leak memory: a closure keeps a live reference to everything it captures for as long as the closure itself is reachable. If you then store that closure back onto an object it captured (a callback registered on the very object it was built from), you've created object -> closure -> object, a reference cycle.
• Why reference counting alone can't free a cycle: CPython's primary memory management is reference counting, an object is freed the instant its reference count hits zero. In a cycle, each object holds a reference to the other, so neither one's count ever reaches zero on its own, even after nothing OUTSIDE the cycle references either of them. This is precisely why CPython also runs a separate cyclic garbage collector (gc module) that periodically looks for groups of objects that reference each other but are unreachable from anywhere else, and frees them as a group.
• Mitigation strategies: avoid storing a closure back onto the object it captures when you can restructure to avoid it; use weakref for a back-reference that shouldn't keep the target alive (a common pattern for observer/callback registries); or simply trust the cyclic collector for genuinely short-lived cycles and only investigate further if profiling shows real, growing retention in a long-running process.

Worked example

class Node:
    def __init__(self, name):
        self.name = name
        self.on_event = None

def wire(node):
    def handler():                 # closure: captures `node`
        return f"{node.name} handled"
    node.on_event = handler        # node -> handler -> node : a cycle
    return handler

Verified by running it with gc.disable() and a weakref to the node: after del n (dropping the only external reference), the node is STILL alive (ref() is not None is True) because the cycle keeps both objects' reference counts above zero. Re-enabling the collector and calling gc.collect() reclaims it (ref() is None becomes True immediately after), confirming the cyclic collector, not reference counting, is what actually frees this pattern.

Trade-offs & pitfalls
In a long-running service, this usually shows up as slow, steady memory growth rather than an obvious crash, because the cyclic collector DOES eventually run and free most cycles; the real danger is cycles involving objects with a __del__ method (historically these were UNCOLLECTABLE by the cyclic GC before Python 3.4, and even post-3.4 they add real collection overhead) or large cycles that make each collection pass more expensive as the live object graph grows. The fix is rarely 'stop using closures', it's to be deliberate about back-references specifically, using weakref where a callback registry would otherwise hold the only thing keeping a large object graph alive.

Direct answer
Plain assignment doesn't copy anything, it just gives a second name to the same object. A shallow copy creates a new outer container but reuses references to the same nested objects inside it, so mutating a nested element through either the original or the shallow copy is visible in both. A deep copy recursively copies every nested object too, giving you a fully independent structure.

Structured elaboration

• Assignment (b = a): a and b are now two names for the exact same object; b is a is True. There is no 'original' versus 'copy', they're the same thing.
• Shallow copy (copy.copy(a), or list(a), or a[:] for a list): creates a genuinely new outer object (b is a is now False), but for every element that is itself a mutable object (a nested list, a dict, a custom object), the copy holds a reference to the SAME nested object, not a copy of it (b[0] is a[0] is True).
• Deep copy (copy.deepcopy(a)): recursively walks the structure and makes a new copy of every nested mutable object too, so nothing is shared (b[0] is a[0] is False).
• The bug shape this causes: code that shallow-copies a nested structure believing it now has an independent snapshot, then mutates the original, and the 'snapshot' silently changes too, because the shallow copy's nested elements were never actually copied.

Worked example

import copy
original = [[1, 2, 3], [4, 5, 6]]
shallow = copy.copy(original)
deep = copy.deepcopy(original)

original[0].append(999)   # mutate a NESTED element of the original

Verified results after that mutation: original == [[1, 2, 3, 999], [4, 5, 6]], shallow == [[1, 2, 3, 999], [4, 5, 6]] (the nested list was shared, so the shallow copy sees the change too), deep == [[1, 2, 3], [4, 5, 6]] (fully independent, unaffected).

A realistic version of this bug: code takes a shallow copy of a dataset as a 'before' snapshot, then runs an in-place normalization pass over the dataset:

def normalize_inplace(rows):
    for row in rows:
        total = sum(row)
        for i in range(len(row)):
            row[i] = row[i] / total if total else 0

After running normalize_inplace on the dataset, the shallow-copied 'snapshot' taken beforehand is bitwise identical to the now-normalized dataset (verified by running it: snapshot == dataset evaluates True after normalization), because normalize_inplace mutates each row list in place, and the shallow copy's rows are the SAME row objects as the original's. The 'backup' was never a backup.

Trade-offs & pitfalls
The fix depends on what you actually need: if you truly need an independent snapshot, use copy.deepcopy (accepting its cost, see the mutability discussion) or rebuild the structure by copying each nested piece explicitly. If deep-copying every row of a large dataset is too expensive, the more scalable fix is usually to stop mutating in place at all, have normalize_inplace return a new structure instead of mutating its argument, which sidesteps the shallow/deep copy question entirely by removing the shared-mutable-state pattern that created the risk.