RepetoPython 3.15: How does nargs='*' differ from nargs='+'?
It accepts zero or more values rather than requiring at least one.
Python 3.15: standard library essentials
360 questions in English on files, collections, text, data and concurrency in Python 3.15.
- 360 cards
- By Repeto
- Free
What the cards look like
Python 3.15: Why should code avoid tempfile.mktemp for creating a temporary file?
It returns an unused name without creating the file, leaving a race before creation.
Python 3.15: For cursors created after connection.row_factory = sqlite3.Row, how can fetched rows be accessed?
Access by column name as well as by positional index.
Python 3.15: Does the zoneinfo module itself bundle the full IANA time-zone database?
No. It uses system data or the separately installed tzdata package when available.
Python 3.15: What is bisect.bisect_right([1, 3, 3, 5], 3)?
3, the insertion index after the existing equal values.
Python 3.15: When code looks up a function as client.fetch, where should a test normally patch it?
Patch client.fetch, the name used by the code under test, rather than only its original definition elsewhere.
All 360 cards
Every card in the deck, in study order.
Question Answer
Python 3.15: How does nargs='*' differ from nargs='+'? It accepts zero or more values rather than requiring at least one.
Python 3.15: Why should code avoid tempfile.mktemp for creating a temporary file? It returns an unused name without creating the file, leaving a race before creation.
Python 3.15: For cursors created after connection.row_factory = sqlite3.Row, how can fetched rows be accessed? Access by column name as well as by positional index.
Python 3.15: Does the zoneinfo module itself bundle the full IANA time-zone database? No. It uses system data or the separately installed tzdata package when available.
Python 3.15: What is bisect.bisect_right([1, 3, 3, 5], 3)? 3, the insertion index after the existing equal values.
Python 3.15: When code looks up a function as client.fetch, where should a test normally patch it? Patch client.fetch, the name used by the code under test, rather than only its original definition elsewhere.
Python 3.15: What three items does each os.walk iteration yield? The current directory path, a list of subdirectory names and a list of file names.
Python 3.15: How can a TypedDict mark a key as allowed to be absent while keeping its value type int when present? Annotate the key as typing.NotRequired[int].
Python 3.15: What indentation does textwrap.dedent remove? Whitespace common to the beginning of every nonblank line.
Python 3.15: What does re.MULTILINE change about ^ and $? They can match the start and end of individual lines, not only the whole string's boundaries.
Python 3.15: What type does csv.reader normally return for each field? str; ordinary numeric-looking fields are not automatically converted.
Python 3.15: Which contextlib stack can manage async context managers and async cleanup callbacks? AsyncExitStack.
Python 3.15: What is the root logger's default severity threshold before configuration? WARNING.
Python 3.15: What does collections.defaultdict(list) create when an absent key is accessed with []? A new empty list, inserted under that key.
Python 3.15: Which zlib function reverses zlib.compress for ordinary zlib-format data? zlib.decompress(data).
Python 3.15: Does random.sample(population, k) sample with replacement? No. It selects distinct population positions; equal-valued entries may still appear if the population contains duplicates.
Python 3.15: Does subprocess.run(..., cwd=folder) change the parent's working directory? No. It sets the child process's working directory.
Python 3.15: What does ssl.create_default_context provide for ordinary client TLS connections? A context configured to verify server certificates and hostnames using trusted certificate authorities.
Python 3.15: How can an argparse argument restrict input to 'fast' or 'safe'? Pass choices=['fast', 'safe'] to add_argument.
Python 3.15: How can decimal precision be changed temporarily without leaving the caller's context altered? Use with decimal.localcontext() as ctx and set ctx.prec inside that context.
Python 3.15: Which ordinary Python type does sqlite3 use for a retrieved BLOB value? bytes.
Python 3.15: Why place cleanup in a finally block around a contextmanager generator's yield? So it runs whether the with-body returns normally or raises an exception.
Python 3.15: What bytes does struct.pack('!H', 513) produce? b'\x02\x01'.
Python 3.15: How can shutil.move handle a move across filesystems? It can copy the source and then remove it, rather than performing one atomic rename.
Python 3.15: What type is result.stdout after subprocess.run(..., capture_output=True) with the default stream settings? bytes.
Python 3.15: What data type should be passed to hashlib.sha256 for a text message? A bytes-like value, typically obtained with text.encode('utf-8').
Python 3.15: How can warnings.warn mark a warning as a DeprecationWarning? Pass category=DeprecationWarning.
Python 3.15: What does ensure_ascii=False change when encoding non-ASCII text with json.dumps? It allows non-ASCII characters to appear directly in the returned string.
Python 3.15: What does os.fspath do with an object implementing __fspath__? It obtains the object's filesystem representation as str or bytes.
Python 3.15: With autocommit=False, what does a connection's with context do on a clean exit? It commits the open transaction; an exception instead triggers rollback.
Python 3.15: What is the parent of pathlib.PurePosixPath('/srv/data/report.csv')? PurePosixPath('/srv/data').
Python 3.15: What does datetime's fold flag distinguish during a repeated local clock interval? The earlier occurrence with fold=0 and the later occurrence with fold=1.
Python 3.15: Which io class provides an in-memory text stream? io.StringIO.
Python 3.15: What does leaving a with ThreadPoolExecutor(...) block normally wait for? Completion of pending work during executor shutdown.
Python 3.15: Before a queue is shut down, what does q.get_nowait() raise if no item is available? queue.Empty.
Python 3.15: Which sqlite3.connect database name creates a private in-memory database? ':memory:'.
Python 3.15: How can a Mock simulate raising OSError when called? Set side_effect to OSError or an OSError instance.
Python 3.15: Which text-encoding mode becomes the default in Python 3.15? UTF-8 Mode. Explicit encoding remains useful when a file uses a different encoding.
Python 3.15: What does sys.argv[0] normally identify? The script name or invocation-dependent program indicator, not the first user argument.
Python 3.15: What is list(itertools.islice(range(10), 2, 7, 2))? [2, 4, 6].
Python 3.15: Is unpickling data from an untrusted source safe? No. Unpickling can execute arbitrary code; do not treat pickle as an untrusted-data interchange format.
Python 3.15: What cleanup does tempfile.TemporaryDirectory perform when its context exits? It removes the temporary directory and its contents.
Python 3.15: What is list(itertools.accumulate([2, 3, 4]))? [2, 5, 9], the running totals.
Python 3.15: What is math.gcd(18, 24)? 6.
Python 3.15: Which hashlib function derives a key from a password, salt and iteration count using HMAC? hashlib.pbkdf2_hmac(...).
Python 3.15: Will uuid.uuid5 return the same UUID for the same namespace and name? Yes; it is deterministic.
Python 3.15: For d = timedelta(days=1, seconds=30), how does d.total_seconds() differ from d.seconds? total_seconds() is 86430.0; seconds is only the within-day remainder, 30.
Python 3.15: What is bisect.bisect_left([1, 3, 3, 5], 3)? 1, the insertion index before the existing equal values.
Python 3.15: What is Decimal('2.345').quantize(Decimal('0.01'), rounding=ROUND_HALF_UP)? Decimal('2.35').
Python 3.15: Can an absolute attacker-controlled second argument to urljoin replace the base URL's host? Yes. Validate or constrain that argument before relying on the base host.
Python 3.15: Can random.randint(2, 8) return both 2 and 8? Yes. Both endpoints are included.
Python 3.15: Which re function prepares arbitrary literal text for use inside a regex pattern? re.escape(text); it is not intended to escape replacement strings.
Python 3.15: Does logging.basicConfig normally reconfigure a root logger that already has handlers? No. It normally does nothing unless force=True is used.
Python 3.15: Which statistics function returns all equally most frequent values instead of choosing one? statistics.multimode(data), in first-encounter order.
Python 3.15: What order results from deque([1, 2, 3]).rotate(1)? [3, 1, 2]; positive rotation moves items to the right.
Python 3.15: Why pass logger.info('Loaded %s rows', count) rather than building the message eagerly? Logging can defer message interpolation until the record is actually formatted.
Python 3.15: What is functools.reduce(operator.add, [], 10)? 10; the initializer is returned for an empty input.
Python 3.15: Which shutil function recursively removes a directory tree? shutil.rmtree(path).
Python 3.15: Which common zipfile constant requests DEFLATE compression rather than uncompressed storage? zipfile.ZIP_DEFLATED, which requires zlib support.
Python 3.15: What does the ! prefix mean in a struct format? Network byte order, meaning big-endian, with standard sizes and no alignment padding.
Show the other 300 cards
Python 3.15: Why protect process-spawning entry code with if __name__ == '__main__'? Importing the module in a child should not recursively start new worker processes.
Python 3.15: How does quote_plus encode a space differently from quote? quote_plus uses +; quote uses %20.
Python 3.15: Which urlencode option turns {'tag': ['a', 'b']} into repeated tag parameters? doseq=True.
Python 3.15: What is list(itertools.pairwise([2, 4, 8]))? [(2, 4), (4, 8)].
Python 3.15: Why is caching a function whose result depends on changing external state potentially incorrect? The same arguments may return an old cached result without re-reading that state.
Python 3.15: What does re.search return when the pattern does not match? None.
Python 3.15: Which datetime.datetime method parses a string according to an explicit format? datetime.datetime.strptime(text, format_string).
Python 3.15: What is math.prod([2, 3, 4])? 24.
Python 3.15: What is the return type of gzip.compress(b'hello')? bytes containing a gzip-compressed stream.
Python 3.15: What happens when Future.result() is called after the worker raised an exception? It raises that worker exception in the caller.
Python 3.15: How does json.load differ from json.loads? load reads a file-like object; loads parses an in-memory JSON string, bytes or bytearray.
Python 3.15: How can os.getenv read APP_MODE while returning 'dev' when it is absent? os.getenv('APP_MODE', 'dev').
Python 3.15: What does Fraction.limit_denominator(max_denominator) find? A nearby rational value whose denominator does not exceed the supplied limit.
Python 3.15: Why can itertools.cycle use growing memory while first traversing a large input? It saves elements so that it can replay them on later cycles.
Python 3.15: What does nargs='+' require for an argparse argument? One or more values, collected into a list.
Python 3.15: How should a variable value be supplied to sqlite3 instead of formatting it into SQL text? Use a placeholder and a separate parameter sequence or mapping.
Python 3.15: Which argparse class constructs a command-line argument parser? argparse.ArgumentParser.
Python 3.15: Can random.randrange(2, 8) return 8? No. Its upper bound is excluded.
Python 3.15: How do you declare a required positional argument named filename with argparse? parser.add_argument('filename').
Python 3.15: In which order does ExitStack run registered exit callbacks? Last registered, first called.
Python 3.15: What constraint do ProcessPoolExecutor functions, arguments and results normally need to satisfy? They must be picklable, and workers must be able to import the required definitions.
Python 3.15: What time-zone information is missing from a naive datetime? It has no usable UTC-offset information identifying its time zone.
Python 3.15: How do you open a text file with UTF-8 decoding regardless of ambient encoding settings? open(path, encoding='utf-8').
Python 3.15: Can Future.cancel normally stop a task that is already running? No. Cancellation succeeds only before execution has started.
Python 3.15: Does typing.cast(int, value) convert value into an integer? No. It returns value unchanged and supplies a hint to the type checker.
Python 3.15: Which tarfile extraction filter is the default starting with Python 3.14, including 3.15? 'data', which rejects several dangerous path and link cases but does not eliminate every risk.
Python 3.15: Why is shell=True risky with an interpolated untrusted argument? The shell can interpret metacharacters as commands rather than literal argument data.
Python 3.15: Which standard-library function escapes <, > and & for ordinary HTML text content? html.escape(text); quote=True also escapes quote characters.
Python 3.15: Why should async code normally re-raise CancelledError after cleanup? Swallowing it can break cancellation and structured-concurrency behavior.
Python 3.15: Where does traceback.print_exc write by default? sys.stderr.
Python 3.15: How many bytes does hashlib.sha256(data).digest() return? 32 bytes.
Python 3.15: Can a ? placeholder safely stand in for an SQL table name? No. Placeholders bind values, not SQL identifiers; choose identifiers from a controlled set.
Python 3.15: What two values does tempfile.mkstemp return? An open file descriptor and an absolute pathname; the caller must close and remove the file.
Python 3.15: Which standard logging level is more severe than ERROR? CRITICAL.
Python 3.15: What does urllib.parse.parse_qs('x=1&x=2') return? {'x': ['1', '2']}.
Python 3.15: Which Path method removes a file or symbolic link rather than a directory tree? path.unlink(); missing_ok=True tolerates a missing path.
Python 3.15: What is json.dumps's default response to an unsupported custom object? It raises TypeError unless a suitable default handler or encoder is supplied.
Python 3.15: Does Executor.map yield results in task completion order? No. It yields them in input order, even when later tasks finish first.
Python 3.15: Does shutil.copy2 guarantee copying every kind of filesystem metadata? No. It attempts metadata preservation, but platform-specific metadata may be lost.
Python 3.15: What Python type does a binary stream's read() normally return? bytes; binary streams do not perform text decoding.
Python 3.15: Can a handler's higher level discard a record that passed the logger's level? Yes. The handler applies its own level threshold.
Python 3.15: Why can the same log message appear twice when handlers are attached to a logger and an ancestor? Propagation can deliver the record to handlers at both levels.
Python 3.15: What is the default outcome when strict UTF-8 decoding meets invalid bytes? A UnicodeDecodeError is raised.
Python 3.15: Which copytree option permits copying into already existing destination directories? dirs_exist_ok=True; matching destination files can be overwritten.
Python 3.15: Which datetime method formats a value using directives such as %Y-%m-%d? strftime(format_string).
Python 3.15: Where is an exception raised inside a with block delivered to a generator context manager? At the generator's yield expression.
Python 3.15: How can re.sub replace only the first occurrence of a pattern? Pass count=1.
Python 3.15: Which hmac helper avoids ordinary equality's content-dependent early exit when comparing authentication tags? hmac.compare_digest(a, b); supply compatible types.
Python 3.15: How should a dataclass field obtain a separate empty list for each instance? Use field(default_factory=list).
Python 3.15: How do you configure a Mock to return 42 when called? Set mock.return_value = 42, or construct Mock(return_value=42).
Python 3.15: What does TestCase.subTest provide inside a loop of related cases? Separate reporting of each parameterized block while allowing later iterations to run after a failure.
Python 3.15: What does asyncio.run(coro) provide for a top-level coroutine? It manages an event loop and runs the coroutine to completion, then performs loop cleanup.
Python 3.15: What does stack.enter_context(cm) return? The value returned by cm.__enter__(), while registering its exit method for cleanup.
Python 3.15: What does shutil.which('python3') return when no matching executable is found? None.
Python 3.15: How do you retrieve the Color member whose value equals 7? Color(7).
Python 3.15: Why does ipaddress.ip_network('192.0.2.5/24') raise ValueError by default? Host bits are set; strict=False allows them and normalizes to the network address.
Python 3.15: Which attribute does functools.wraps set to reference the original callable? __wrapped__.
Python 3.15: What does cursor.fetchall return when there are no remaining rows? An empty list.
Python 3.15: What remains after d = deque([1, 2], maxlen=2) and d.append(3)? deque([2, 3], maxlen=2); appending discards the oldest item from the opposite end.
Python 3.15: Does dt.replace(tzinfo=zone) convert dt's clock time to that zone? No. It changes the attached zone information without adjusting the clock fields.
Python 3.15: Which mapping binds :name in execute('SELECT :name', ...)? {'name': 'Ada'}, using the parameter name without the colon as the key.
Python 3.15: Is shlex.quote a general escaping solution for Windows cmd.exe arguments? No. It is designed for Unix/POSIX-style shells.
Python 3.15: Which argparse action makes --verbose set a value to True when present? action='store_true'.
Python 3.15: Does os.chdir change the working directory only for the calling function? No. It changes the process working directory, affecting other code using relative paths.
Python 3.15: What does typing.Optional[int] permit beyond int? None; it does not mean that a function argument may be omitted.
Python 3.15: Which ArgumentParser method creates a group of options that cannot be supplied together? add_mutually_exclusive_group().
Python 3.15: Which decorator helper preserves a wrapped function's name and documentation on a wrapper? functools.wraps(wrapped_function).
Python 3.15: Does dataclasses.asdict perform only a shallow extraction of field references? No. It recursively converts nested dataclasses and containers and deep-copies other values.
Python 3.15: What does os.scandir yield instead of bare file-name strings? DirEntry objects, which provide names, paths and file-type information.
Python 3.15: Which flag makes the dot wildcard match newline characters too? re.DOTALL, also available as re.S.
Python 3.15: Does Path.iterdir recursively visit a directory's descendants? No. It yields paths for the directory's immediate children, in arbitrary order.
Python 3.15: Which urllib.parse.quote argument also percent-encodes slash characters? safe=''.
Python 3.15: Why should a groupby group's values be consumed or saved before advancing the outer iterator? Groups share the underlying iterator, so advancing it makes earlier group data unavailable.
Python 3.15: What does base64.b64encode(b'hi') return? b'aGk=', a bytes value.
Python 3.15: For class Color(Enum): RED = 7, what does Color.RED.value return? 7.
Python 3.15: What object does ArgumentParser.parse_args normally return? An argparse.Namespace containing attributes for the parsed arguments.
Python 3.15: Does time.sleep pause only an asyncio task or the calling operating-system thread? It blocks the calling thread; use await asyncio.sleep(...) for cooperative async waiting.
Python 3.15: What does heapq.heappushpop([2, 5, 7], 1) return? 1; the pushed item is the smallest, so it is returned immediately.
Python 3.15: What does pathlib.PurePosixPath('notes.txt').with_suffix('.md') return? PurePosixPath('notes.md'); it constructs a new path without renaming a file.
Python 3.15: Which newline setting should be used when opening text files for csv.reader or csv.writer? newline='', allowing the csv module to handle line endings itself.
Python 3.15: What date is datetime.date(2024, 2, 28) + datetime.timedelta(days=1)? 2024-02-29; 2024 is a leap year.
Python 3.15: Why are raw string literals often used for regular expression patterns? They prevent Python string-literal escapes from consuming backslashes intended for the regex parser.
Python 3.15: How does functools.cache differ from a size-limited lru_cache? It is an unbounded memoization cache, equivalent to lru_cache(maxsize=None).
Python 3.15: With autocommit=False, which connection method saves the current transaction's changes? connection.commit().
Python 3.15: What does random.shuffle(items) return after shuffling a mutable sequence? None; it shuffles the sequence in place.
Python 3.15: Which high-level subprocess function runs a command, waits for completion and returns a CompletedProcess? subprocess.run(...).
Python 3.15: What is statistics.mean([2, 4, 9])? 5.
Python 3.15: Which re flag enables case-insensitive matching? re.IGNORECASE, also available as re.I.
Python 3.15: What does with contextlib.suppress(FileNotFoundError) suppress? Only matching FileNotFoundError exceptions from the block, not all exceptions.
Python 3.15: Which concurrent.futures helper yields futures as they finish? as_completed(futures).
Python 3.15: Which parse_qs option preserves a query parameter whose value is empty? keep_blank_values=True.
Python 3.15: Does itertools.groupby combine equal keys that occur in separated parts of an unsorted input? No. It groups consecutive equal keys; sort first when all equal keys must be grouped together.
Python 3.15: What does cursor.fetchone return when no row remains? None.
Python 3.15: How can unittest verify that an operation raises ValueError? Run the operation inside with self.assertRaises(ValueError):.
Python 3.15: Do runtime checks against @runtime_checkable Protocol verify complete method signatures? No. They check required attribute presence, not full type signatures.
Python 3.15: Why can TestCase.addCleanup be safer than relying only on tearDown for acquired resources? Registered cleanup functions run even if setUp subsequently fails.
Python 3.15: Which statistics function uses the sample-variance denominator n − 1? statistics.variance(data).
Python 3.15: Which Python value corresponds to SQL NULL in sqlite3 bindings and results? None.
Python 3.15: What happens to a threading.Lock acquired through with lock when the block raises? The lock is released during context exit.
Python 3.15: Which unicodedata operation converts canonically equivalent text to NFC form? unicodedata.normalize('NFC', text).
Python 3.15: Does leaving with connection automatically close a sqlite3 connection? No. Close it explicitly or use a separate closing context manager.
Python 3.15: How do you move an in-memory stream back to its beginning? Call stream.seek(0).
Python 3.15: Which logging component controls a record's textual layout? A Formatter.
Python 3.15: What does struct.unpack('!H', b'\x02\x01') return? (513,), a one-element tuple.
Python 3.15: What happens to integer dictionary keys after a JSON encode/decode round trip? They become strings, because JSON object member names are strings.
Python 3.15: Which b64decode option rejects non-alphabet characters instead of discarding them by default? validate=True.
Python 3.15: What does shlex.split('tool "two words"') return in its default POSIX mode? ['tool', 'two words'].
Python 3.15: Does constructing a TypedDict produce an object that enforces its declared value types at runtime? No. It produces an ordinary dictionary; the declared structure is for type checking.
Python 3.15: For Point = collections.namedtuple('Point', 'x y'), what does Point(2, 5).y return? 5.
Python 3.15: Can asyncio.run be called in a thread that already has a running event loop? No. Await the coroutine from the existing loop instead.
Python 3.15: How do you retrieve the named group 'code' from a successful match object m? m.group('code').
Python 3.15: Which unittest.mock class models an asynchronous callable whose result can be awaited? AsyncMock.
Python 3.15: How do you prevent os.walk(topdown=True) from descending into a listed subdirectory? Remove its name from the yielded dirnames list in place.
Python 3.15: What does math.comb(5, 2) count? The 10 unordered selections of two items from five distinct items.
Python 3.15: Which subprocess.run argument sends data to the child's standard input? input=...; use str in text mode and bytes in binary mode.
Python 3.15: Which json.loads argument lets JSON fractional numbers become Decimal objects? parse_float=decimal.Decimal.
Python 3.15: Which statistics function computes population variance with denominator n? statistics.pvariance(data).
Python 3.15: What exception can subprocess.run raise if the child exceeds timeout? subprocess.TimeoutExpired, after killing and waiting for the child process.
Python 3.15: When gather propagates one child's exception by default, are all other children automatically cancelled? No. Other children generally continue running; this differs from TaskGroup failure handling.
Python 3.15: What does a hash object's copy() method let you reuse? The current hashing state, so different suffixes can share an already processed prefix.
Python 3.15: What Python type does a text stream's read() normally return? str; decoding converts stored bytes into text.
Python 3.15: What does b'' from a connected TCP socket's recv usually indicate? The peer has closed its sending side and no more data remains to read.
Python 3.15: Does calling an async def function immediately run its body to completion? No. It normally creates a coroutine object, which must be awaited or scheduled.
Python 3.15: Why is contextlib.redirect_stdout unsafe as a general solution for concurrent threads? It changes process-global sys.stdout, so other threads can be affected.
Python 3.15: Should random.choice generate a password-reset token? No. Use secrets, which is intended for security-sensitive randomness.
Python 3.15: How can a simulation use its own reproducible random state without reseeding the module-global generator? Create a separate random.Random(seed) instance.
Python 3.15: If setUp succeeds, does tearDown normally run when the test method fails? Yes. It still runs after the test method's failure.
Python 3.15: After heapq.heapify(values), which element is stored at values[0] if the heap is nonempty? The smallest element.
Python 3.15: Does Queue.join mean that the worker threads themselves have exited? No. It waits for the unfinished-task count to reach zero.
Python 3.15: Which threading primitive lets one thread signal an event that other threads can wait for? threading.Event.
Python 3.15: Which math function offers more accurate summation of floating-point values than a naive running total? math.fsum(iterable).
Python 3.15: What does operator.itemgetter('name')({'name': 'Ada'}) return? 'Ada'.
Python 3.15: What does an absent key return from a Counter without inserting a count? 0.
Python 3.15: What does pathlib.PurePosixPath('logs') / 'today.txt' represent? The relative path logs/today.txt; / joins path components.
Python 3.15: What can await asyncio.sleep(0) do in a long-running coroutine? Yield control so other ready tasks can run.
Python 3.15: What is fractions.Fraction(6, 8)? Fraction(3, 4), reduced to lowest terms.
Python 3.15: What does asyncio.wait_for normally do to its awaited task on timeout? It cancels the task and raises TimeoutError; waiting for cancellation cleanup may exceed the nominal timeout.
Python 3.15: Does typing.Final prevent assignment at runtime by itself? No. It communicates reassignment restrictions to type checkers.
Python 3.15: Why protect a shared read-modify-write operation with a threading.Lock rather than relying on the GIL? The operation is not guaranteed to be atomic; free-threaded builds also do not provide GIL-based serialization.
Python 3.15: Does PurePath.is_relative_to prove that a path stays inside a directory after resolving symlinks? No. It is a lexical path test, not a filesystem containment check.
Python 3.15: After s = io.StringIO('abc') and s.read(1), what does the next s.read() return? 'bc'; the stream position advanced past the first character.
Python 3.15: Does Fraction(0.1) necessarily equal Fraction(1, 10)? No. Converting the float preserves its exact binary floating-point value.
Python 3.15: What does queue.Queue.get() do by default when the queue is empty? It blocks until an item is available, unless interrupted by an applicable shutdown or exception.
Python 3.15: Do pickle.dumps and pickle.loads serialize to and consume a text string format? No. Pickle uses a binary byte-stream representation.
Python 3.15: Which time clock is appropriate for elapsed deadlines because it cannot go backward with wall-clock adjustments? time.monotonic().
Python 3.15: What is pathlib.PurePosixPath('/srv/report.csv').name? The string 'report.csv', containing the final path component.
Python 3.15: What static-checking behavior distinguishes typing.Any from object? Any permits unchecked operations and compatibility with other types; object still requires operations valid for an arbitrary object.
Python 3.15: What extra filesystem work does Path.resolve perform compared with merely joining path components? It makes the path absolute and resolves symbolic links and '..' components.
Python 3.15: Does secrets.token_urlsafe(16) promise a token of exactly 16 text characters? No. 16 specifies random bytes before URL-safe Base64 encoding.
Python 3.15: Does asyncio.gather return successful results in completion order or argument order? Argument order.
Python 3.15: What does contextlib.aclosing(obj) await on async context exit? obj.aclose().
Python 3.15: Does shutil.copyfile preserve a source file's permission bits and timestamps? No. It copies file contents; use copy2 for an attempt to preserve metadata too.
Python 3.15: Which fields participate in generated dataclass equality by default? All fields marked compare=True, which is the default.
Python 3.15: What exception does sys.exit raise rather than forcibly terminating all code immediately? SystemExit; normal exception unwinding still runs applicable finally blocks.
Python 3.15: What is struct.calcsize('!I')? 4 bytes.
Python 3.15: How do you obtain an aware current UTC datetime without using deprecated utcnow()? datetime.datetime.now(datetime.UTC).
Python 3.15: Which datetime method converts an aware timestamp to another time zone while preserving the instant? dt.astimezone(target_zone).
Python 3.15: With autocommit=False, which method discards pending transaction changes? connection.rollback().
Python 3.15: What kind of identifier does zoneinfo.ZoneInfo('Europe/Paris') use? An IANA time-zone key, whose rules can include daylight-saving changes.
Python 3.15: Why does deepcopy keep a memo dictionary during a copy operation? It handles recursive structures and avoids repeatedly copying the same encountered object.
Python 3.15: What does secrets.token_bytes(16) return? 16 cryptographically strong random bytes.
Python 3.15: Which json function parses a JSON string into Python objects? json.loads(text).
Python 3.15: What does an asyncio.TaskGroup context do before normal exit? It waits for all tasks created within the group to finish.
Python 3.15: Which random function samples a list of k elements with replacement and can use weights? random.choices(population, weights=..., k=k).
Python 3.15: With shell=False, does subprocess.run(['echo', '*.csv']) ask a shell to expand the wildcard? No. '*.csv' is passed as a literal argument.
Python 3.15: What type does ZipFile.read('notes.txt') return? bytes, even if the member contains text.
Python 3.15: Which enum decorator rejects duplicate member values rather than accepting aliases? @enum.unique.
Python 3.15: Which Path.mkdir options create missing parent directories and tolerate an existing directory? parents=True and exist_ok=True; an existing non-directory can still cause an error.
Python 3.15: Which datetime class method parses '2026-04-09' into a date? datetime.date.fromisoformat('2026-04-09').
Python 3.15: What is collections.Counter('aba')['a']? 2.
Python 3.15: How can logger.error include the current exception traceback? Pass exc_info=True while handling the exception.
Python 3.15: What does csv.DictReader use as field names when fieldnames is not supplied? The first row of the input, which is then omitted from the data rows.
Python 3.15: How does socket.sendall differ from send for a byte string? sendall keeps sending until all data is sent or an error occurs; send may write only part of it.
Python 3.15: What is list(itertools.zip_longest([1, 2], ['a'], fillvalue=None))? [(1, 'a'), (2, None)].
Python 3.15: Which logging component sends a record to a destination such as a file or stream? A Handler.
Python 3.15: Is urllib.error.HTTPError also a kind of URLError? Yes. Catch HTTPError first if it needs separate handling.
Python 3.15: What must a consumer call once for each retrieved item when using a queue.Queue instance q with q.join for completion tracking? q.task_done() after finishing that item's work.
Python 3.15: Which Logger method logs an error and includes the active exception traceback by default? logger.exception(...), normally called inside an except block.
Python 3.15: What does cursor.executemany(sql, rows) do with parameterized DML? It executes the statement repeatedly with each supplied parameter set.
Python 3.15: Which deque method removes and returns the leftmost element? popleft().
Python 3.15: Can a keyword passed when calling a partial override a keyword stored in that partial? Yes. Call-time keyword arguments take precedence.
Python 3.15: Why does Decimal('0.1') better express an exact decimal tenth than Decimal(0.1)? The string specifies the decimal value directly; the float version preserves the float's binary approximation.
Python 3.15: Which argument's type determines dispatch for functools.singledispatch? The first positional argument's type.
Python 3.15: What does subprocess.run(..., check=True) do when the child returns a nonzero status? It raises subprocess.CalledProcessError.
Python 3.15: What does typing.Annotated[int, 'positive'] add to int? Metadata for tools; Python does not automatically enforce positivity.
Python 3.15: Which math predicate tests whether a float is NaN? math.isnan(value). Comparing NaN to itself with == is false.
Python 3.15: What does doctest test in documentation strings? Interactive Python examples and their expected displayed output.
Python 3.15: What does ThreadPoolExecutor.submit(fn, *args) return immediately? A Future representing the submitted call; it may already be complete.
Python 3.15: What does urllib.parse.urlencode({'q': 'red blue'}) produce? 'q=red+blue'.
Python 3.15: Which re function yields match objects lazily for non-overlapping matches? re.finditer(pattern, text).
Python 3.15: Which ZipFile method returns a list of member names? namelist().
Python 3.15: What does operator.attrgetter('real')(3 + 4j) return? 3.0, the object's real attribute.
Python 3.15: What type does urllib.request.urlopen(...).read() normally return? bytes; decode using the response's appropriate character encoding when text is needed.
Python 3.15: How does re.search differ from re.prefixmatch in where it looks for a match? search scans anywhere; prefixmatch starts at the beginning. prefixmatch was added in 3.15 as the preferred name for the older match API.
Python 3.15: Which unittest.TestCase assertion compares values for equality and reports useful differences? assertEqual(actual, expected).
Python 3.15: Which add_argument parameter supplies a value when an optional argument is absent? The default parameter.
Python 3.15: How do you open a gzip-compressed file for UTF-8 text reading? gzip.open(path, 'rt', encoding='utf-8').
Python 3.15: Which dataclass hook is called after the generated __init__ assigns the fields? __post_init__().
Python 3.15: Do ordinary Python type annotations automatically reject arguments of the wrong type at runtime? No. Type hints primarily support static checking and tooling; runtime validation must be implemented separately.
Python 3.15: What does HMAC add to an ordinary hash for message authentication? A secret key shared by the parties creating and verifying the authentication tag.
Python 3.15: What operation does an Iterator provide in addition to being iterable? __next__(), which produces the next item or raises StopIteration.
Python 3.15: Which asyncio helper can run a blocking I/O function in a worker thread? await asyncio.to_thread(fn, *args).
Python 3.15: What does operator.methodcaller('replace', 'a', 'x')('banana') return? 'bxnxnx'.
Python 3.15: Which contextlib helper can stand in for a with block when no resource management is needed? contextlib.nullcontext(), optionally carrying an enter_result.
Python 3.15: Which file-opening mode appends text without truncating existing contents? 'a'; use an explicit text encoding when appropriate.
Python 3.15: What relationship can a single TypeVar express in a function's input and return annotations? That the chosen input type and return type are linked, rather than independently arbitrary.
Python 3.15: Why is validating member paths alone insufficient when processing an untrusted archive? Its contents can still consume excessive storage, memory or processing time.
Python 3.15: Does textwrap.fill return a list of lines or a single string? A single string containing wrapped lines separated by newline characters.
Python 3.15: What commonly written method does @dataclass generate from annotated instance fields by default? __init__, along with other configured methods such as __repr__ and __eq__.
Python 3.15: What does traceback.format_exc return while handling an exception? A formatted traceback string for the currently handled exception.
Python 3.15: What does contextlib.closing(obj) call on context exit? obj.close().
Python 3.15: Does itertools.count(5) stop after a fixed number of values? No. It is unbounded unless the consumer stops taking values.
Python 3.15: Which Mock method verifies one call with specified arguments and no additional calls? assert_called_once_with(...).
Python 3.15: Does Base64 encode data confidentially? No. It is a reversible binary-to-text encoding, not encryption.
Python 3.15: Which operator function returns the truth value of an object as a bool? operator.truth(obj).
Python 3.15: Which method clears an lru_cache-decorated function's stored results? cached_function.cache_clear().
Python 3.15: What constraint does lru_cache place on arguments used as cache keys? They must be hashable.
Python 3.15: Which math.isclose tolerance is usually needed when comparing a tiny value with zero? A suitable abs_tol; relative tolerance alone does not make a nonzero value close to zero.
Python 3.15: What usually happens to sibling tasks when a TaskGroup child raises a non-cancellation exception? They are cancelled; after cleanup, failures are raised together in an exception group.
Python 3.15: Which subprocess.run option captures both stdout and stderr separately? capture_output=True.
Python 3.15: Where does assigning cm['x'] = 4 store the value in a ChainMap? In the first mapping only.
Python 3.15: Can a SHA-256 digest be decrypted with a key to recover the original data? No. A hash is not reversible encryption.
Python 3.15: After a = [[1]] and b = copy.deepcopy(a), does b[0].append(2) change a[0]? No. The nested list was copied as well.
Python 3.15: Which asyncio context manager limits how long a block of async work may take? asyncio.timeout(delay), used with async with.
Python 3.15: Which asyncio function schedules a coroutine as a Task on the running loop? asyncio.create_task(coro).
Python 3.15: How can argparse convert --count to an integer during parsing? parser.add_argument('--count', type=int).
Python 3.15: Does urllib.parse.urlsplit guarantee that a parsed URL is safe or semantically valid for an application? No. The application must validate the components it intends to trust.
Python 3.15: Which pathlib class family performs lexical path manipulation without filesystem access? PurePath and its platform-specific subclasses, PurePosixPath and PureWindowsPath.
Python 3.15: Does @dataclass(frozen=True) make nested mutable objects deeply immutable? No. It blocks normal field assignment, but nested mutable values can still change.
Python 3.15: How does dataclasses.replace(obj, x=3) affect obj? It constructs a new instance with the changed field; the original is unchanged.
Python 3.15: What is pathlib.PurePosixPath('backup.tar.gz').suffixes? The list ['.tar', '.gz'].
Python 3.15: What is list(itertools.repeat('x', 3))? ['x', 'x', 'x'].
Python 3.15: Which itertools tool lazily flattens a stream of iterables by one level? itertools.chain.from_iterable(iterables).
Python 3.15: Does ExitStack.pop_all immediately execute the registered cleanup callbacks? No. It transfers them to a new ExitStack, which owns the remaining cleanup.
Python 3.15: How can tar.extractall explicitly request the data extraction filter? Pass filter='data'.
Python 3.15: Which sys attribute normally gives the absolute path of the running Python interpreter? sys.executable; it can be empty or None if Python cannot determine it.
Python 3.15: What happens to an existing file when Path.write_text writes new text to it? Its contents are overwritten, not appended.
Python 3.15: What types must keys and values assigned to os.environ have? They must be strings.
Python 3.15: Which standard stream should a command-line tool normally use for diagnostics kept separate from its normal output? sys.stderr.
Python 3.15: What can create_autospec catch that an unrestricted Mock may accept? Calls incompatible with the real object's signature or attributes.
Python 3.15: Why use await asyncio.sleep(delay) instead of time.sleep(delay) inside an async task? It suspends the task without blocking the event-loop thread.
Python 3.15: What resource-management benefit does using TemporaryFile as a context manager provide? The file is closed and normally removed when the context exits.
Python 3.15: Why can itertools.tee require substantial memory when one consumer runs far ahead? It buffers values until the slower copies consume them.
Python 3.15: How can pathlib.Path read a UTF-8 text file with an explicit encoding? Call path.read_text(encoding='utf-8').
Python 3.15: Is asyncio.Lock intended to synchronize operating-system threads? No. It coordinates asyncio tasks and is not thread-safe.
Python 3.15: Which contextlib class manages a dynamically determined number of synchronous context managers? ExitStack.
Python 3.15: What does leaving a unittest.mock.patch context do to the patched name? It restores the original object, including when the block raises.
Python 3.15: Why should csv.writer be used instead of joining fields with commas? It handles delimiters, quotes and embedded newlines according to the CSV dialect.
Python 3.15: Which re function requires the entire string to match a pattern? re.fullmatch(pattern, text).
Python 3.15: What is math.isqrt(20)? 4, the floor of the exact square root.
Python 3.15: What does typing.Callable[[str], int] describe? A callable accepting a string argument and returning an integer.
Python 3.15: When does a TestCase's setUp run? Before each test method, not just once for the whole class.
Python 3.15: Does decimal context precision count fractional places or significant digits in arithmetic results? Significant digits, not a fixed number of fractional places.
Python 3.15: What is list(itertools.chain([1, 2], [3]))? [1, 2, 3].
Python 3.15: What does @abc.abstractmethod prevent when applied in an ABC with unimplemented abstract methods? Instantiating a subclass that has not implemented all required abstract methods.
Python 3.15: What does math.isfinite(float('inf')) return? False; infinities and NaNs are not finite.
Python 3.15: What is list(itertools.product([1, 2], ['x']))? [(1, 'x'), (2, 'x')].
Python 3.15: Which pathlib call searches below a directory for matching .csv paths recursively? path.rglob('*.csv'). It yields matches lazily; check is_file() if only files are wanted.
Python 3.15: Why use zipfile.ZipFile in a with block when creating an archive? It ensures the archive is closed and essential directory records are written.
Python 3.15: How many hexadecimal characters does a SHA-256 hexdigest contain? 64.
Python 3.15: Does the env argument to subprocess.run automatically merge its mapping with the parent's environment? No. It supplies the child's environment mapping; merge with os.environ explicitly if needed.
Python 3.15: Does updating a hash with b'ab' then b'cd' match hashing b'abcd' at once? Yes. update calls append bytes to the same logical input stream.
Python 3.15: Which DictWriter method writes its configured field names as a header row? writeheader().
Python 3.15: How can subprocess.run decode captured output explicitly as UTF-8 text? Pass encoding='utf-8', which also enables text mode.
Python 3.15: What does maxsize=128 limit in functools.lru_cache? The number of recently used argument-result entries retained.
Python 3.15: Which urlsplit result attribute contains the path without the query string? path.
Python 3.15: Which os function returns the current working directory as a string? os.getcwd().
Python 3.15: Can a function registered with ExitStack.callback suppress an exception by returning True? No. That callback is not passed the exception details and its return value does not suppress it.
Python 3.15: What distinguishes uuid.uuid4 from a name-based UUID? It creates a random UUID rather than deriving it from a namespace and name.
Python 3.15: Does defaultdict.get('missing') invoke the default factory? No. The factory is invoked by missing-key [] access, not get().
Python 3.15: What does list(itertools.batched([1, 2, 3, 4, 5], 2)) return by default? [(1, 2), (3, 4), (5,)].
Python 3.15: What Python type does json.dumps({'x': 1}) return? str, not bytes.
Python 3.15: After a = [[1]] and b = copy.copy(a), are a[0] and b[0] the same list? Yes. A shallow copy creates a new outer container while sharing nested objects.
Python 3.15: How many times must a generator decorated with contextlib.contextmanager yield in normal use? Exactly once: setup precedes the yield and cleanup follows it.
Python 3.15: What does arcname control in ZipFile.write(filename, arcname=...)? The member name stored inside the archive, independently of the source filesystem path.
Python 3.15: What logging call conventionally creates a logger named after the current module? logging.getLogger(__name__).
Python 3.15: Which json.dumps option rejects NaN and infinity rather than emitting nonstandard JSON tokens? allow_nan=False.
Python 3.15: What does contextlib.ContextDecorator let a context manager do for an ordinary synchronous function in addition to supporting with? Wrap the function as a decorator, applying the context around each call.
Python 3.15: Which os operation replaces a destination path with a source path, atomically if the rename succeeds? os.replace(src, dst); moving across filesystems can fail.
Python 3.15: What is math.floor(-2.7)? -3.
Python 3.15: What does typing.Literal['read', 'write'] express to a type checker? A value restricted to the two specified string literals.
Python 3.15: Does a class have to inherit from a Protocol to satisfy its static interface? No. Structural typing checks whether the required members are present with compatible types.
Python 3.15: Which method-name prefix does unittest's default loader recognize as a test? test.
Python 3.15: Which open mode fails if the target file already exists? 'x' for exclusive text creation, or 'xb' for binary creation.
Python 3.15: After f = functools.partial(pow, 2), what does f(5) return? 32; the first argument to pow is fixed at 2.
Python 3.15: Which time function provides a high-resolution performance counter for measuring short elapsed intervals? time.perf_counter(); subtract two readings rather than interpreting one as a calendar time.
Python 3.15: For an Enum member Color.RED, which attribute gives its declared name as a string? Color.RED.name.
Python 3.15: What does Counter('abb').most_common(1) return? [('b', 2)].
Python 3.15: What is pathlib.PurePosixPath('backup.tar.gz').stem? The string 'backup.tar'; only the final suffix is removed.
Python 3.15: Which ArgumentParser method adds support for commands such as 'build' and 'clean'? add_subparsers(), followed by add_parser() for each subcommand.
Python 3.15: What does itertools.batched(data, 3, strict=True) do when the final batch is incomplete? It raises ValueError when that incomplete batch is reached.
Python 3.15: Which io class provides an in-memory binary stream? io.BytesIO.
Python 3.15: Which mapping supplies a key that appears in several maps of a ChainMap? The first mapping containing the key.
Python 3.15: How do you retrieve the Color member named 'RED'? Color['RED'].
Python 3.15: Which CompletedProcess attribute contains the child process's exit status? returncode.
Python 3.15: Why use Popen.communicate rather than independently reading one pipe while the child also writes heavily to another? communicate handles the streams together, avoiding the common full-pipe deadlock pattern.
Python 3.15: What is math.ceil(-2.7)? -2.
Python 3.15: What sequence should accompany execute('SELECT ?', ...) to bind the string 'Ada' as one value? ('Ada',), with the trailing comma that makes a one-element tuple.
Python 3.15: Does warnings.warn always interrupt execution like raising an exception? No. Warning filters control whether it is shown, ignored or converted into an exception.
Python 3.15: What is statistics.median([1, 3, 7, 9])? 5.0, the mean of the two central values.
How you study it in Repeto
- Install the deck free. It lands in your library on the web, iPhone, iPad and Mac.
- Answer from memory. See the front, recall the answer, then flip and rate how it went.
- Review at the right time. FSRS spaced repetition brings each card back just before you would forget it, so a few minutes a day is enough.
Sources and license
Original Repeto study-card text. Educational diagrams and museum images retain the source, creator and license attribution recorded on each card and in the documented source manifest.